Materials have an important effect on exhaust system design, and there are normally four choices here if we are talking about basic materials groups: steel, stainless steels, superalloys and titanium.

Of these, titanium is the easiest to deal with because it is used mainly for applications where the exhaust system is well supported – that is, with multiple supports along the length of the system – and this is generally the case on motorcycles where, as a minimum, systems are supported at the cylinder head and towards the rear of the machine. Full titanium systems are expensive, but are found on road and racing motorcycles. Titanium is relatively strong and light, but can suffer from brittleness when used at high temperature as oxygen diffuses into the surface. Ideally it should be welded in an inert atmosphere to prevent the material forming oxides and nitrides.

Steel, stainless steel and superalloys could all be used in many applications, but there are important differences in properties.

Steel is the cheapest option. It is easily available and is the cheapest of the three. It bends and forms nicely, so is easy to work with, and it requires no special processes for welding. Its weaknesses lie in the lack of strength at temperature and its inherent lack of corrosion resistance. The combination of these leads to systems that can lack durability, and to make up for loss of strength, it is necessary to use greater wall thicknesses than would be possible in other types of material.

Stainless steel is the next step up from steel, and in exchange for improved high-temperature mechanical properties, there are other disadvantages to contend with. Stainless is more costly than steel, and can be harder to form and weld. There are special stainless steels for welding that have elements added to prevent internal corrosion of the welds. Slip joints can become seized together occasionally, as stainless has a strong tendency to gall and fret. Compared to steel, the high-temperature properties of stainless allow systems to be made in thinner wall sections for the same level of durability.

Superalloys are the most expensive materials. They have been developed specifically for high-temperature service, and possess a very desirable combination of strength and corrosion resistance. They are composed of elements that are generally expensive to buy and, owing to their melting points, are also expensive to process. The superalloy sheet or tube material is consequently expensive as well.

Superalloys have to be welded in an inert atmosphere, otherwise the alloy will react with oxygen and nitrogen, detracting from the strength of the weld. Their room-temperature strength and stiffness make them more difficult to form than other types of materials, but they are used where maximum durability or minimum weight is needed. No other type of material has yet come close to being able to match it in these respects. Wall thicknesses of less than 0.4 mm have commonly been used for 10 years or more on car systems that are supported only at the cylinder head.

Written by Wayne Ward

Many of the national championships and the World Touring Car series are based on cars that are available with a 2.0 litre turbocharged engine. As this is a popular choice for buyers of family cars in Europe, this is a sensible choice. In Australia, for example, their ‘touring car’ is a 5.0 litre naturally aspirated V8, while in Germany, the national motor manufacturers fight it out among themselves with 4.0 litre naturally aspirated V8 engines. In Japan, there have also been cars with V8s, some of which were bespoke V8 race engines based on endurance engines competing at Le Mans. The Vemac from the early 2000s, for example, used a Zytek V8.

The different race series have had very differing demands in terms of exhausts, not only in terms of layout but also such aspects as the physical size and operating temperatures. The turbocharged engines which are used in WTCC and Japanese SuperGT, for example, will run higher exhaust gas temperatures, for example, than those in naturally aspirated engines.

Even where engine regulations are superficially similar, there may be large disparities in engine output and therefore large differences in the effect on exhaust systems. Many touring car series are powered by engines that are limited in terms of power by intake air restrictors or fuel flow meters. Both devices have the same effect – they limit power by limiting engine speed. The Japanese SuperGT championship, which uses a fuel-flow limit, has 2.0 litre turbocharged engines producing around 500 bhp, while the WTCC has an air intake restrictor applied to the same type of engine, but with the resulting power output thought to be around 350 bhp.

The difference in maximum exhaust gas mass flow is proportional to the power output, so there may be noticeable differences in exhaust design in order to maintain performance. In particular, packaging the exhaust so as to avoid flow loss through flow separation from the ‘inside’ of bends in pipes might require different pipe bend centreline radii to be used. Apart from this difference, it wouldn’t be a surprise to find that different primary pipe diameters are used. To improve transient engine response, it is likely that any series using turbocharged engines will probably see teams running some form of thermal insulation on the primary pipes, which may be anything from a ceramic coating to a formed metal enclosure.

Such thermal insulation around the exhaust primary pipes is not necessary for naturally aspirated engines, although some form of exhaust insulation might be required in order to reduce heat transfer to adjacent components or to the driver compartment.

Written by Wayne Ward

Attracting less attention than the car category but no less competitive is the motorcycle category. The motorcycles have evolved over the years in a number of ways. In the early days, the key criteria for a motorcycle to be successful were reliability, plenty of ground clearance and a big fuel tank. The requirements for ground clearance and a large-capacity fuel tank made the motorcycles quite ponderous initially, and they remained vulnerable to mechanical damage in the event of any accident. As competition grew, speeds increased and it became more important to make the motorcycles tolerant of any day-to-day damage caused during riding as well as minor accidents.

The BMW R80 was often the machine of choice for Dakar rally entrants, the flat-twin engine offering the necessary ground clearance and the basic motorcycle being renowned for reliability. The exhausts on the early motorcycles were particularly vulnerable to damage, with the unprotected primary pipes exiting forward from the cylinder heads and being swept underneath the head before exiting in a low-level silencer. Later variants swept the exhaust upwards, with a high-level silencer being less vulnerable in the case of the rider falling off and when crossing rivers.

Over the years, more manufacturers were drawn to the Dakar, and the basic engine configuration of single- and twin-cylinder engines with front-exiting exhausts meant the first part of the primary pipes were less prone to damage from a collision, but perhaps more likely to being damaged by stones thrown up by the front wheel. Following the example of the later BMW R80 machines, rally raid bikes now all have high-level silencers, although they are sometimes only upswept just before the silencer entry. Having the silencer exit at a high level at least protects the engine from the ingress of water via the exhaust if the machine were to come to a halt in the middle of a river crossing.

What has happened is that the primary pipes are now very carefully shielded from impact damage and are very often not visible for most of their length, particularly where they are routed low along the side of the bike. Metallic as well as carbon composite materials have been used to protect the exhaust system from stone impact damage and from the results of the rider falling off the machine.

In recent years, the trend for motorcycles has been towards smaller-capacity machines that are more agile and easier for the rider to handle. From 2014, only single- and twin-cylinder machines with a maximum capacity of 450 cc are eligible, with the result that the manufacturers who produce 450 cc motocross and enduro bikes have had a sound basis for their rally raid machines. The entry from Yamaha, for example, is a single-cylinder machine, although this has a forward-facing inlet and a rear-facing exhaust. In this case, the primary pipe is much better protected from damage than some rival machines owing to the fact that the rest of the engine acts as a combined stone and bash guard for the pipe.

The exhausts are better supported than on many road and competition bikes too, with extra supports between the cylinder head and the silencer bracket.

Written by Wayne Ward

Audi needs to remain competitive in the face of new competition, and its latest diesel engine seems to be a match for the gasoline competition, although Porsche is surprisingly quick in its latest foray into the top division at Le Mans.

We last covered diesel exhaust systems specifically in 2010, when the focus was on diesel particulate filters. In that article we explained why such filters were required, how they work, how they affect noise and so on. The press pictures of the latest Audi R18 diesel do not clearly show where a particulate filter might be, but comments by Audi engine chief Ulrich Baretzky about being able to eliminate a second particulate filter seem to point to the fact that Audi still needs to use one to prevent visible smoke.

Why one filter instead of two? There is no magic in this – the R18 engine is based on a V6 block with a wide-angle vee, and the inlet system feeds the heads from the outside of the vee. The exhaust system is therefore in the centre of the vee, and the exhaust manifolds on each bank feed into a single twin-inlet turbocharger. One exit pipe, one particulate filter.

This exhaust system architecture makes sense for an engine with a single turbo, and is possible because of engine rules that are not too restrictive. Formula One, on the other hand, has a single turbo mounted to a V6 engine, and the exhaust routing is far less elegant because the rules mandate that the inlet system is in the centre of the vee and the exhausts on the outside.

From the available ‘spy’ pictures, there does appear to be a ‘racetrack-shaped’ particulate filter housing – that is, a block with round ends – immediately following the turbocharger. There has been speculation that the filter is housed within the single cylindrical exit pipe, but the pictures of the engine from other angles tell the true story.

It would be possible to make such a small-diameter long filter, but that would present too great a restriction to flow and an unacceptable pressure loss that would give reduced efficiency and increased fuel consumption. It is unlikely that there would be significant blockage of the R18’s filter by unburned soot owing to the high exhaust gas temperatures because of the high load.

During extended safety car running this might be a concern, but the carbon should be burned off quickly, either when returning to racing speeds or by using a controlled regeneration cycle that increases exhaust gas temperature in order to achieve the same effect (thanks to Paul Cole for his comments on my previous online article).

The racetrack-shaped filter elements are more expensive than the cylindrical types to manufacture and to house, but these are often used when packaging requirements won’t allow a sufficiently large filter face area with a cylindrical form.

Written by Wayne Ward

I loved two-stroke Grand Prix motorcycle racing, especially the top 500 cc class. The bikes were light and difficult to ride, and I never had any illusions that I would be able to tame one of them. The four-stroke class has brought us machines with an incredible amount of power, and engines that are more closely related to those in their road-going cousins. As such, the bike manufacturers turned away from two-strokes largely because of emissions.

The four-stroke engines are more complicated than the two-strokes. There are lots of valves with camshafts and springs that need to be controlled, which is not conducive to a cheap engine formula for motorsport. It was perhaps inevitable therefore, especially during the global economic downturn, that engine numbers would be limited. This year, that limit is five. Use of further engines brings with it penalties, so preserving the engine in all circumstances is very important.

Anyone who closely follows MotoGP (which is the top class in the latest four-stroke era) will be aware of the complaints about tyres this year. Some very good riders are really struggling to understand the limits of their tyres, and so are making more mistakes than normal. The result is lots of crashes, and whether these are of the frantic cartwheeling variety, with riders being tossed about like dishcloths, or simply sliding off into a corner, the motorcycle gets damaged.

There is no allowance for crashes – whether they are due to tyres, weather or unwarranted ambition on behalf of the rider – on the number of engines allotted to a rider, so the engine needs to be protected in such circumstances. Damage can come in many forms, and the obvious cause of impact damage is probably the easiest to counteract. However, the gravel trap, where many motorcycles come to rest after unseating their rider, presents a particular hazard to an engine, as it can ingest large chunks of hard debris through the inlet and exhaust.

Now that engine life in MotoGP needs to be preserved at all costs, what we have seen is for the end of the exhaust to be equipped with a ‘screen’ which is designed to prevent gravel going into the exhaust system. One has to remember that there are some pretty strong pressure waves moving in both directions along the exhaust, and it is not certain that gravel in the exhaust will always be ejected from the tailpipe.

While the protective screen does its job, it will also inevitably interact with the exhaust pulses, necessarily weakening them and affecting the nature of the reflected wave. A compression wave meeting the open end of a pipe is reflected as a rarefaction wave, while one meeting the closed end is reflected as a compression wave. A compression wave meeting a partly open end of a pipe is likely to still reflected as a rarefaction wave, but this is probably much weaker in magnitude than for a fully open pipe. Consequently its action in helping the gas exchange process is diminished.

Written by Wayne Ward

There have been a lot of changes to the NASCAR Sprint Cup regulations, but the exhaust routing and construction remains a challenge, especially in light of the drive for reduced exhaust system mass. Preserving reliability is a real challenge.

Of course, there is always a drive for performance improvement, and the exhaust system is one area that can deliver such a gain. With relatively open engine regulations allowing changes to the more traditional areas for finding improved performance (such as valve lift profiles and porting), exhaust development can easily be forgotten, but there are certainly performance benefits to be had in this area.

Traditionally, lowering the pressure losses from the exhaust valve to the exhaust exit has proven fruitful, but the NASCAR packaging constraints and regulations mean the exit section and the flattened portion of the exhaust system that comes before it contribute more pressure loss than a traditional cylindrical system. We saw some flattened exhaust exits in Formula One in 2012, but they were designed for a very specific application – namely ‘blowing’ the underside of the car floor to induce extra downforce. Such considerations are not applicable to NASCAR, and the flattened exit would certainly have harmed the performance of the Formula One engine.

Flattened sections of pipe, if they only maintain the same cross-section as the round tube that feeds them, will have vastly increased pressure losses due to a drastic reduction in hydraulic diameter (see the RET-Monitor article published two years ago for more details). The flattened portion of the exhaust for a NASCAR is long, although from the occasional picture that we see published on the internet, it seems that care has been taken to increase the cross-sectional area of the exhaust system.

This is especially important, as the exhaust exit section is braced between the top and bottom of the exit, effectively dividing the flattened tube into two or three smaller tubes. That can reduce the overall hydraulic diameter by a huge amount; a 50% reduction is not unreasonable. Each time there is an effective or actual change in flow diameter – whether this is an increasing or decreasing area – there is an associated pressure loss, which is impossible to account for by simple calculation. Techniques such as actual measurement or CFD need to be used. For example, where the round section of the exhaust changes to the flat section, there is a tapered section that gradually increases the area of the duct.

If this increase in area is too sudden, the flow can separate from the walls of the exhaust system, causing areas of recirculating flow which cause a pressure loss. Sections of the exhaust that have diverging tapers can have high pressure loss due to separation, and converging sections naturally increase pressure losses due to increasing flow velocities. By optimising each change, we can minimise pressure losses and increase performance.

Written by Wayne Ward

In recent years, many of the performance increases have come from exhausts and the associated mapping and engine control changes that have accompanied them. We have known about the mass flow and energy contained in exhaust flows for a long time. Blown diffusers were used sporadically in the past, but in recent years we saw a return to the use of exhaust gases for aerodynamic benefit, with blown floors and rear wings and the use of the Coanda effect.

The regulations for 2014 inhibit such developments for two main reasons. The first is a result of the drastic changes to the basic engine type and its use. The 2.4 litre V8s are replaced with a 1.6 litre V6 which is turbocharged and which has a fuel flow limit that imposes both a maximum instantaneous mass flow rate and a maximum total fuel use in a race. The limit on fuel mass flow rate limits the exhaust mass flow rate and therefore the amount of work the exhaust gas can do on a rear wing, for example.

The rules also encourage teams to use as much of the energy in the exhaust gas as possible to generate electrical energy to be used for propulsion; the less energetic the flow exiting the tailpipe, the less work can be extracted from it. There will always be a temptation to try to exploit the exhaust gas to do something useful, but it will be much less effective than in the past.

The second reason is that the exhaust flow is going to be less effective in improving car performance, because the exhaust exit position, direction, shape and cross-sectional area are more tightly controlled, giving less latitude for doing something clever to gain performance. For instance, the tailpipe diameter must be between 97.7 and 138mm. Given a reduced mass flow rate compared to 2013, the resulting cross-sectional areas could give a tenfold reduction in exhaust gas velocity, making any extraction of useful work much harder.

The previous incarnation of the rules prevented the weird and wonderful exhausts that were used to ‘blow the floor’, but gave us exhaust exits that were submerged into the sidepods and angled downwards, still providing significant gains. The tailpipe diameters and lengths were not chosen to optimise engine performance, but to provide aerodynamic benefits, probably coming with a small engine performance penalty.

In addition to the significant ‘blunting’ of a whole area of car performance, the exhaust will be very different in its general architecture. The exhausts will be joined to a single turbocharger assembly that will be linked to a turbo-generator which will provide energy, not only for propulsion but to accelerate the turbocharger itself. Gone will be the days of turbo-lag: the reshaping of Formula One regulations will give us turbocharged engines with the transient response that we might expect from a naturally aspirated engine, and markedly improved efficiency.

Written by Wayne Ward

Rob McElnea scored a third place for the Padgett’s race team in the 1990 Hungarian World Superbike round on a Yamaha OW-01. I was very taken with the OW-01, so when I was a bit older (and a bit richer) I bought one. One of the first jobs was to replace the original silencer with a Yamaha race-kit item. There were several kilos’ difference in mass in the silencer alone; the silencer can was made of aluminium rather than steel, and was physically smaller, and the pipe that forms part of the silencer was made from a thin carbon steel rather than the more substantially proportioned road item. Eventually the standard stainless steel front pipes, although nicely made, were replaced by thinner gauge race-kit items made from carbon steel. In 1990, when this bike was current, race-kit items were aluminium and steel.

Fast-forward less than five years and we started to see the introduction of titanium to a limited extent for exhaust systems, and carbon fibre reinforced polymer (CFRP) exhaust cans. The early ‘carbon cans’ had a tendency to shake themselves to pieces and so were either expensive, light and lacked durability or were expensive, heavy and durable. While development soon made them more durable, carbon cans never became universal, although many exhaust companies supplying non-factory teams liked to use them – they sold lots of exhausts to people who ran sports bikes on the road.

Another 20 years on and we see that most World Superbikes have a full titanium system. Some superbike teams have experimented with superalloys such as Inconel but it has not been taken up widely. The higher engine speeds used and the tendency to have the engine tuned for higher speeds in order to improve power have in general seen the exhaust systems become shorter. In the early 1990s, the end of the silencer on the inline four-cylinder bikes was very close to the rear of the motorcycle, and now many of the equivalent machines have a short and stubby silencer, the rearmost part of which is not far behind the rider’s boot.

There is a packaging and weight distribution advantage to having the exhaust cans low on the machine and closer to the centre of gravity. Even Ducati, which pioneered underseat exhaust cans for its road motorcycles, has relented and packaged its silencers at the bottom of the fairing, even though it appears to have made pipe routing quite tortuous. Yamaha meanwhile, which has made only a token effort in support of its World Superbike entrants in 2013 after being very strong only a few years ago, still retains its underseat silencers. Superbike racing is less important to Yamaha than it once was, and this might explain why its road bikes no longer reflect the latest trends in superbike racing

Fig. 1 - The Ducati Panigale race bike had underslung silencers and tortuous pipe routing (Courtesy of Team Alstare) 

Written by Wayne Ward

These are, after all, road-going cars – you are meant to be so impressed that you rush out to buy one – but it would be wrong to assume that these are simply stripped-out shopping cars; studying their exhausts alone makes that obvious.

It is highly unlikely that a passenger car exhaust would last a full season or more of racing at WTCC level. However the bespoke racing systems that replace them are expected to achieve this target. Replacing exhausts on a regular basis owing to fatigue is not top of the list, especially when race weekends are often taken up with repairing more obvious external damage!

Some parts will need to be changed due to damage sustained during racing, but the systems are designed to isolate damage, thus protecting the rest of the system. For cars with rear-exit systems, it was common to wreck or at least damage almost every part of them if the car was hit from behind. The whole crash load was transferred through every component in the exhaust line, and much of the load was reacted by the primaries, requiring a very expensive exhaust system and much time-consuming work on the car.

Exhaust systems have been developed which incorporate a collapsible section, similar in principle to a passenger car’s crumple zone or to the tubes used to retard a vehicle in a crash test. This dissipates most of the energy involved and therefore isolates the expensive forward components from damage. Another cause of impact damage can be heavy use of the kerbs; this can damage exhausts on some cars where the exhaust routing leaves them exposed.

BTCC cars have to conform to strict exhaust noise regulations. Despite having to use a turbocharger (which attenuates exhaust noise) and a catalyst, which again removes some energy, a silencer is also required. These are packed with a noise-insulating material which, over time, loses its effectiveness as it becomes contaminated and compressed. The silencer is designed to be serviced though, during which the exhaust packing is renewed.

The use of a catalytic convertor in BTCC is mandated, and to prevent anyone turning this to their advantage an homologated catalyst has to be used.

Exhaust heat needs to be managed carefully. If you look at the Dynojet Toyota BTCC car [Fig. 1], you’ll see that the primary pipes have a thermal barrier coating. Retaining heat in the exhaust helps transient engine response under acceleration. Not only is heat management for performance a concern, it is also a consideration for driver comfort. The exhaust systems of touring cars are often coated or wrapped with insulating materials in order to limit the heat transfer to the driver compartment – this is discussed in more detail in this month’s article on coatings.

Despite the very competitive nature of touring car racing, the materials used for exhaust manufacture aren’t the most expensive available – austenitic stainless steels are still used in preference to more exotic choices such as Inconel or titanium.

Fig. 1 - The BTCC Dynojet Toyota uses thermal barrier coatings on its exhaust primaries (Courtesy of Dynojet Racing)

Written by Wayne Ward

GT racing has always been the backbone of national and international endurance car racing. In the good times there is the money to support big-budget Prototypes, but there is always a very healthy, well supported base of GT teams that offer excitement and close racing, even thought the sometimes less competitive ‘top class’ often gets the bulk of TV coverage. The ‘strength in depth’ of GT racing is illustrated by the number of big name drivers it now attracts. Such competitive drivers, and the very level playing field in the GT classes, means the cars are now driven very hard for the full duration of the event. No longer does the field simply string out into a procession.

JRM Motorsport builds the Nissan GT-R cars that run in GT3 competition. Its chief engineer, Nigel Stepney, offered some insight into what the company is looking for from an exhaust system. I asked what the particular difficulties are in specifying a system for long-distance racing. Stepney says, “Achieving the correct balance between the weight of the system while retaining the reliability and performance are the fundamental aspects to consider for the exhaust system on any endurance race car, and no less so on a GT3 car”.

Packaging of exhausts is always a compromise – small-diameter pipes and tight radii are good for packaging, but can hurt performance owing to flow separation. It is important to try to package the components in a way that leads to the best possible performance, rather than simply find parts that fit easily but which can lead to slower lap times.

In terms of packaging the exhaust systems for GT racing versus Prototype racing, one extra item to consider is the requirement for catalytic convertors. These are not only bulky but can also be quite heavy, owing to the construction of the catalyst itself. Their physical bulk makes them awkward to package, and their mass means they exert forces on the rest of the system, which can reduce fatigue life under conditions of vibration, making the exhaust designer and fabricator think harder about the best solution for reliability.

Owing to the homologation aspect of GT racing, it is necessary to produce a single exhaust system design; different system designs are not allowed for Le Mans, for example, with its high proportion of straights. Given this constraint, compromises have to be made to reach the targets of sufficient durability, ease of maintenance and acceptable costs. It is important for a GT team running a tight budget not to have to replace exhaust systems before reaching the mileage target, and especially important that failures don’t happen in a race.

The choice of materials is one aspect of the exhaust specification that needs to be looked at carefully. Steel or stainless steel offer relatively cheap exhaust systems, and in many cases these can prove adequate. However, with turbocharged engines such as those in the GT-R, the high temperatures can significantly reduce exhaust component life. JRM therefore specifies Inconel, a well known high-temperature material that has found favour in the top levels of racing.

Formula One and NASCAR both have high duty cycles, and the exhaust temperatures are also very high, as is the case with a turbocharged GT car. So it should come as no surprise that all three turn to the same kind of materials to meet their durability targets, which for the Nissan GT-R is 5000 km, although parts will regularly be run further in testing. Running the parts further gives useful statistical information on failures, and is a good predictor of where beneficial design changes can be made.

Fig. 1 - The JRM Nissan GT-R is a turbocharged GT3 car. It uses Inconel to achieve its durability target of greater than 5000 km (Courtesy of JRM)

Written by Wayne Ward

Of course, the motorsport environment is a graduated scale of harshness, and close to the top of this scale must be rallying. Each part of the car is subjected to real abuse, and the exhaust is no exception.

I asked Grahame Smith of JRM Racing about the exhausts for his team’s rally car, which is used for a range of rally competition from tarmac to the very harsh African rallies. His opinion is that the terrain presents the real challenge. On tarmac, the only real hazard is if the exhaust makes contact with the ground; for such stages the car is set up as low as possible to the ground to lower centre-of-gravity height and improve stage time. Fig. 1 shows the car in the air on a tarmac stage. Over-exuberance on the driver’s part might subject the exhaust to ground contact (and possibly much worse) but the team will probably know the hazards in a given stage and, with experience, can set the ride height and suspension to cope with such jumps.

By contrast, on an African rally stage, large ruts, holes, gravel and rocks are the order of the day, and even though there may be no contact between exhaust and the ground, damage is accumulated, “When conditions are rough, the exhaust is constantly being damaged, and if this becomes too severe then the performance of the engine will be affected,” Smith said.

The design of the exhaust needs to take into account the type of use – on rough stages for example, it will the ease of replacing damaged sections of the exhaust system. The design needs to incorporate the best combination of performance, durability and ease of servicing. Performance encompasses both design for best engine performance and the lowest system mass. Durability may involve using heavier materials or thicker sections, and ease of servicing can involve more joints in the system, or routing the exhaust in a way that might adversely affect performance. This is really important, as it may be necessary to replace exhaust components during a relatively short service interval.

An exhaust used for tarmac rallies may incorporate thin-walled material and still last for a number of years. Titanium sections in the exhaust are not out of the question, and can survive well without additional shielding. The other extreme of exhaust durability is found when the car is used on very rough stages. Even though the exhaust system is far stronger and better protected, it may be necessary to replace parts of the exhaust every day or even in the short services between stages. It is normal to use thick-section tube in order to strengthen the exposed sections of the exhaust, with bespoke guards fitted to minimise the likelihood of impact damage from rocks, gravel or raised sections between ruts.

Fig. 1 - Jumps can present problems for exhausts on tarmac stages, even though they are relatively kind compared to rough gravel stages (Courtesy of JRM Racing)

Written by Wayne Ward

In the case of an endurance bike, the most exposed engine-related component is undoubtedly the exhaust system. As you may be able to see in the photo below, the exhaust system on this Suzuki has already suffered a mishap – the silencer has certainly seen much better days.

What we might find surprising though is that much of the exhaust system is titanium, as shown by the colour of the pipes, owing to the thin oxide films that form on the material when they operate at high temperatures. We might have expected it to be made from steel, stainless steel or perhaps a material such as Inconel. However, most motorcycle exhaust systems benefit from the fact that they are relatively short and are well supported. As can be seen from the picture, this system is typical of many motorcycle exhausts in that it is connected to the chassis via a bracket attached to the silencer.

There is something unusual about this particular exhaust though, in that the ‘lug’ on the silencer which attaches to the bracket is not a welded lug but is riveted to the exhaust can. The problem with having the bracket welded is that the weld geometry is far from ideal. The two sheets of titanium from which a welded bracket would be made, are folded with a 90º bend and welded around their periphery to the silencer. In doing so the design has effectively manufactured a lug with an inbuilt crack, as it is impossible (unless using diffusion bonding) to weld over the whole contact area.

Providing the rivets are of a suitable size, this should be a reliable method of attaching the lug. The rivets are in a relatively cool area; the silencer is of the perforated tube type, with the volume between the inner perforated tube and the wall of the silencer packed with fibrous material which acts to attenuate exhaust noise.

There are further advantages with this method compared to a welded joint. The design of the lug becomes completely free of the need to design it as a sheet metal component. It may be designed with any type of machined feature in order to reduce stress concentration to a minimum, and can be manufactured from any material. In the case of this exhaust, the lug has large machined radii where the ‘upright’ meets the flat plate which is riveted to the silencer wall. Additionally, with welded lugs, we are constrained to making the lug from the same type of material as the part to which it is welded and in a compatible alloy.

Fig. 1 - The SERT Suzuki endurance team has raced at Le Mans for more than 30 years; during that time it has helped to develop exhausts that can remain reliable for 24-hour racing (Courtesy of www.suzuki-racing.com)

Written by Wayne Ward

With the increased engine displacement came a new rule limiting the engines to an 81 mm maximum bore. In pursuit of performance it is likely that all the engines will be taking advantage of the full 81 mm allowance in order to keep the stroke as short as possible. The short stroke allows the highest possible engine speed and therefore, if efficient breathing is maintained and friction kept under control, the maximum power output.

The 800 cc machines were not limited in terms of bore size or number of cylinders, so they had higher engine speeds. One result of the change to 1000 cc, and the lower engine speed, is a change in exhaust tuning. The time taken for a pressure wave to travel from the exhaust valve to the end of a given exhaust system or junction in the system is, for set parameters such as temperature and gas composition, a fixed quantity. The 1000 cc machines will therefore need an exhaust system that is tuned to suit the lower speed range. If we wish to increase the time taken for a wave to reach the exhaust system end (or junction) to be reflected and to make its way back to the exhaust valves then the length of the exhaust system (again assuming fixed gas temperatures and so on) has to increase, as do the distances from the exhaust valve to any junctions in the exhaust.

With an increase in exhaust length comes an increase in mass and a different challenge in terms of packaging. There may be difficulties in  packaging the required system length on the bike without resorting to design features such as adding extra curved sections. The Honda RC211V (the original 990 cc five-cylinder bike from before the 800 cc era) and the RC213V (the new four-cylinder 1000 cc bike) notably struggled to accommodate the required pipe length from the rear bank of cylinders. The pipe is therefore contorted to accommodate the extra length, much like a single coil from a helical spring, there being a complete 360° ‘ring’ in the underseat section of their exhaust systems. This type of design feature may be specific to the bikes with vee engines that use an underseat exhaust exit on the rear bank of cylinders, as the underseat exit leaves little room for sufficient pipe length. At present only Honda and Ducati have vee engines, but in the past the Ilmor and Suzuki bikes were also V4s.

Having to resort to adding extra curves in the exhaust will lead to increased exhaust back-pressure and a possible small loss of performance. If the curves are too tight then the flow can separate from the walls of the system, giving rise to a very significant increase in pressure loss and performance.

Written by Wayne Ward

Before the CoT, chassis manufacture was less consistent, and this often meant that each individual chassis would require its own bespoke tailpipes. Bearing in mind that any one driver may have had up to 20 chassis at his disposal throughout the year, the outlay for tailpipes alone would be considerable. With the CoT, however, came a reduced tailpipe inventory.

Something else that came with the CoT was a heavier chassis, and many teams struggled to reach the minimum weight limit for the car. This led the teams to consider better quality stainless steels and more exotic materials, specifically superalloys such as Inconel. The excellent high-temperature strength of superalloys allows exhaust systems to be far lighter than if they were made from stainless for the same level of reliability.

Having struggled to meet the minimum weight limit for the initial CoT, the next challenge is to meet the 2013 weight limit, which has seen a 160 lb/73 kg reduction compared to 2012. This will perhaps force teams to look for further weight savings in the exhaust system. Certainly those who are still running stainless will find significant savings by looking at other alloys.

The past few years have been difficult for many people, and motorsport has not been immune from the ravages of austerity. Durability and cost have assumed a new level of importance, and when it comes to exhausts, NASCAR teams have tended to monitor the condition of their exhausts more carefully. In conjunction with improved materials this improved monitoring has been responsible for an increase in the typical life of an exhaust system.

A lot of importance is placed on the routing of the exhaust system, although this is not primarily for engine performance reasons. A well-routed exhaust system may allow improved control arm geometry and/or better underfloor aerodynamics, both of which may be quite potent in terms of reducing lap times.

The recent change in fuel systems for NASCAR Sprint Cup from carburettors to fuel-injected engines gave teams and engine suppliers a lot to think about in 2012. Specifically, they needed to work out how to manage the new electronic fuel injection and how to maintain reliability. With the year of learning over, and a new challenge to tackle in the form of the lighter 2013 cars, we are likely to see a renewed focus on exhaust design. We may even see increased exhaust system development for performance reasons.

Fig. 1 - The new ‘Generation 6’ Sprint Cup cars are much lighter in 2013, which may lead teams to consider developing lower-mass exhausts

Written by Wayne Ward

The new FIA regulation for the 2012 season stipulated that the exhaust system exit must be on the top deck of the car. A year ago I wrote that “there is no reason why the engineers involved won’t try to do something clever with the exhaust mass flow with aerodynamic surfaces on the top deck of the car, or to influence the flow over the rear wing”, although I wouldn’t have predicted that the pace of exhaust development would be as swift as it turned out to be.

The buzzword this year in terms of exhausts was ‘Coanda’. Put simply, the Coanda effect is the tendency of a fluid flow to be attracted’ and to ‘stick’ to a solid surface. In general, a stream of gas tends to entrain and mix with its surroundings. When a solid surface is brought close to the moving stream of gas, entrainment is limited from that direction; in reaction to this the jet accelerates and moves towards the surface. Once attached, the flow will tend to adhere to the form of the surface, even if it curves away from the original flow direction.

Its industrial uses are varied and it finds application in aircraft, air conditioning and fluid separators. However, we might claim, with some justification, that the Coanda effect is most ‘at home’ when applied to exhaust exits. Very aptly, the Coanda effect is named after an aircraft engineer who noted the effect of adjacent surfaces on the flow of gas from an engine he had designed more than a century ago. Coanda designed, and possibly briefly flew, the first aircraft not deriving its thrust from a propeller but from a jet of air.

In terms of the 2012 Formula One season, a number of teams designed exhaust exits that encouraged the exhaust flow to adhere to the top surface of the sidepods of the car which then sloped downwards toward the diffuser. As noted elsewhere, the deflection of the exhaust flow is due to a combination of the Coanda effect and ‘downwash’, where the upstream flow of air over the car’s sidepods is already well adhered to the bodywork surfaces and its direction of flow relative to the car has a downward component.

The Coanda exhausts were designed into some cars from the beginning of the season, with others adopting the technique later in the season with varying degrees of success. Playing catch-up is never easy and requires a lot of effort, with additional programmes of mid-season development consuming time and money.

Written by Wayne Ward

The Honda World Superbike Team has campaigned a pair of Fireblades in the 2012 World Championship, ridden by Hiroshi Aoyama and Jonathan Rea. The team’s technical coordinator Pieter Breddels answered some questions regarding the Honda’s exhaust requirements. In terms of performance development when working toward a new exhaust system design, Breddels stated that consistency is important. Keeping pipe lengths as equal as possible means the tuning behaviour for each cylinder is the same. Experimenting with different pipe diameters and testing what is best for performance is also important. Exhaust packaging, whether for a car or bike, is often a compromise, since fitting in the pipe sizes and bend radii that you would like to use is not always possible.

Exhaust system mass is an important consideration and, in common with many race motorcycle exhausts, the team uses a full titanium system. Bike exhausts are well supported, so titanium can provide sufficient longevity. Breddels comments, “We think a life of 1500 km is acceptable.” Previously, stainless steel had been used for the headers (primary pipes) but the weight of the system was deemed to be “a problem”. The aim of using stainless steel in the past was to improve reliability, but with careful design and manufacture, the longevity of the titanium system has improved to the point where the heavier material is no longer required.

There are strict regulations in World Superbike concerning noise, and the titanium silencers need to be replaced before the 1500 km limit, but this is only for the silencer (muffler) to be re-packed to maintain its noise attenuation. It is typical for a silencer with packing to become noisier over time; some of the material is lost, escaping through the perforations via which the pressure waves ‘percolate’ into the packing. The rest either degrades, becomes compressed or contaminated.

While many superbike aftermarket exhaust systems use ‘link pipes’ or ‘balance pipes’ to link primary pipes to each other, the Honda World Superbike Team finds no requirement for these, with the exhaust system working well enough over the working range without them. Such link-tubes can help fill in holes in the torque curve or smooth power delivery, although this is often to the detriment of the power curve elsewhere. They also represent a reliability risk, as the slip joints, if not perfectly aligned, put stress into the exhaust system and in particular the welds.

The Honda system is a 4-2-1 configuration, with pairs of primaries joining together before the resulting two pipes are then joined ahead of the silencer.

In recent years there has been a real trend for silencers to be placed lower on the bike and for the system to be shorter. Gone are the days of large, high-mounted silencers alongside the rear wheel. The silencer on the Honda begins only just rearward of the swing-arm pivot and, as can be seen from Fig. 1, starts inside the fairing.

Fig. 1 - The Honda World Superbike Team achieves a winning combination of low system mass, longevity and performance from its titanium exhaust system (Courtesy of Honda)

Written by Wayne Ward

In the BTCC, competitors have the option of tuning their own engine, sourcing one through an engine supplier, or running the series' spec engine, which has proven to be a competitive option. The spec engine is a four-cylinder turbocharged unitand the exhaust manifold is part of the engine as supplied to the teams. In developing its own exhaust, Thorney had to design and manufacture the link between the exhaust manifold and the turbocharger, plus all the pipework beyond the turbine.

The manifold supplied with the engine is a ceramic-coated component. As has been discussed in Race Engine Technology magazine and various coatings articles for RET-Monitor , ceramic coatings provide a useful thermal barrier, protecting components that are in to the engine, preventing excessive heat radiation back into the engine and cooling fluids, and keeping the heat in the exhaust gas flow, improving turbocharger transient response.

Thorney Motorsport's John Thorne says the exhaust system was designed with various goals in mind. Low mass was stated as being an important aim, as was ensuring sufficient performance in terms of maximum power but also peak and mid-range torque. Reliability and impact resistance were also mentioned.

It is widely held that superalloy materials such as Inconel provide the lightest exhaust; this has been shown to be true through its use at the highest levels of motorsport. However, it does require some very specialised welding techniques, such as welding in an inert atmosphere, with the pipes also purged with an inert gas, so they do not lend themselves to trackside repairs. So an austenitic stainless steel (type 304) is specified, with on-site welding being an important consideration. The longevity of the system, if there is no impact damage, is expected to be at least one racing season. The teams do not expect to have to carry lots of exhaust spares, and so trackside repairs in the event of an unexpected impact have to be considered.

There are other considerations in the design of the exhaust system. The mandated single side exit and the series-spec sills mean the exhaust system has to pass through the sills. Careful design and installation ensures that the exhaust system does not make contact with the inside of the glass-fibre composites sills, as this can easily cause a fire.

Thorney has designed a clever system with some compliant elements that allow the car to suffer side impacts without permanent damage to the exhaust supports. While other teams are often sidelined through exhaust system failure following an impact, the Thorney-supplied cars have benefited from some creative thinking and are relatively immune from such problems.

Written by Wayne Ward

GT racing is characterised by long races, and 24-hour events are a popular format. The old racing adage that "to finish first, you must first finish" is one that emphasises reliability, and endurance racing often rewards reliability as much as pure speed. In this regard, the production car exhaust systems are replaced with items that are designed both to enhance performance and improve durability. Such systems are often much lighter than the production car version too.

The basic functions of an exhaust system are to duct burnt gases to somewhere convenient and to provide a degree of tuning to help improve engine efficiency. Roadcar exhaust systems are very often not simply a collection of tubes; evermore stringent legislation dictates complex silencing arrangements and emissions control. The natural desire of the car manufacturer to provide their GT cars with stupefying levels of performance means that the exhaust systems are sometimes equipped with valve arrangements to control noise and driveability.

This technology has been with us for a long time. Yamaha first used its EXUP valve on road motorcycles in 1987, and it continues to supply engines thus equipped. The valves in the exhaust system restrict flow and reduce noise at low engine speeds and part throttle, and then the valve opens at higher engine speeds and throttle openings. This means that road-legal GT cars can be driven relatively quietly in urban areas.

However, emissions control and driving refinement do not carry the same priority for the GT race team, so such systems are often dispensed with in the interests of mass reduction and reliability. The silencers required to meet a 110 dB noise limit for racing are far less restrictive than those required to comply with roadcar legislation, and large, heavy catalyst monoliths ('bricks') designed to improve exhaust gas emissions are also superfluous. Valves in the exhaust system to control noise and improve driveability are simply seen as excess weight and providing an unnecessary risk to reliability.

The Ferrari 458 Italia and the Aston Martin Vantage - beautiful examples of the current crop of cars used with success in GT racing - have catalysts and valves in the standard exhaust system on the production car. Catalysts often negate the requirement for further silencing, so in removing the catalyst bricks it is usually necessary to introduce a silencer/silencers that weren't originally part of the exhaust.

The effect of removing such items is to make the exhaust system much lighter, and this in itself provides opportunity for further lightening. By removing unwanted mass and through the use of improved materials for construction, the exhaust system can be made from thinner tubes without compromising reliability.

Fig. 1 - The Aston Martin Vantage GT racer is based on a roadcar with a complex exhaust system (Courtesy of Aston Martin)

Written by Wayne Ward

Turbocharged engines have many advantages. They are compact and, as such, can produce high performance from a physically small and light package. They are typically slightly more efficient than a naturally aspirated engine. In racing they are used widely, in rallying and sportscars. IndyCar has a V6 turbo, and Formula One will follow a couple of years later.

Rallying in particular requires good transient response, as not only the acceleration of the car is affected by poor throttle response. Rally driving techniques use the throttle to control the direction of the car - rally drivers are the originators of the 'drift' driving technique.

For a rally car, immediate and predictable throttle response is more important than in most other forms of motorsport. This requirement gave rise to the development of a number of innovative anti-lag systems in rallying, the aim of which is to provide the level of throttle response that would normally be expected of a naturally aspirated engine from a turbocharged engine.

A number of these systems relied on combustion within the exhaust system, the aim being to keep the turbocharger spinning fast because of high mass flow rates of hot gases. With constant boost available, the engine is therefore ready and able to provide good response as soon as the driver opens the throttle, even from low engine speed. Engines with a good anti-lag system can produce very high torque from low speed, where a turbocharged engine with no anti-lag often compares poorly to a naturally aspirated engine.

One such system involves passing very rich mixtures through the engine when off-throttle, and bypassing air from the inlet straight to the exhaust. When the unburned fuel-rich mixture and the air are recombined in the exhaust, combustion takes place. In a multi-cylinder manifold this can lead to almost continuous combustion. An alternative system is to pass a very lean mixture through the engine and inject fuel into the exhaust. Both systems treat the exhaust as a combustion chamber, much as is the case with a jet engine where fuel is introduced to a flow of compressed air.

The first of these methods requires additional hardware in order to provide 'bypass air' to the exhaust, while the second requires extra injection equipment, a supply of fuel and spare capacity within the control software in order to carry out the strategy successfully.

In addition to the naturally hotter exhaust gas temperatures associated with a turbocharged engine, combustion within the exhaust is a further reliability challenge, giving the exhaust little relief from high temperature. Normally, when a 'conventional' turbocharged engine is on closed throttle, there is some respite from high exhaust temperature. Anti-lag equipped engines are often characterised by loud 'bangs' coming from the exhaust as combustion takes place.

Written by Wayne Ward

In terms of mechanical stress and strain, the load cycles just keep coming. The mechanical aspect of motorsport is typically concerned with making the lightest piece of equipment suitable for performing its task and which won't break before the end of the event. Endurance racing also has to take account of unexpected failures (they can only be found out through repeated 24-hour testing) and to make the car or motorcycle easy to work on; service items such as tyres and brakes need to be replaced quickly, and more onerous tasks require the car or motorcycle to come apart and go back together easily.

Where the teams put real effort into this, it is very impressive. The big-budget car teams, Audi in particular, are masterful in this regard, but the next best teams I have seen have been the motorcycle teams. The 24-hour races at Le Mans and the 24 Hour Bol D'Or events (sadly no longer held at Paul Ricard) are full of motorcycles with imaginative solutions to make the bikes easy to work on.

Exhaust systems are a case in point. Owing to their necessary curvature, the exhausts are made up of several sections of pipe, often with each held to the adjacent section using springs rather than solid plates as is common in car racing. With a simple spring-pulling tool these exhausts can be removed and replaced quickly. One problem though with using springs on the joints is their tendency to resonate, and working out whether your motorcycle engine excites such resonance isn't an easy matter for the typical privateer team.

However, that is not to say they are destined for a life full of broken exhaust springs; teams have devised a couple of low-cost methods of damping the spring motion. The first, and cheapest, is to run a bead of silicone sealer along the length of the spring. While the exhaust is hot, the spring generally has enough air around it for the silicone to survive.

The second method is to use 'heatshrink'. This is a product more usually used for the outer sleeving of wiring looms, and is available in a variety of (sometimes hideous) colours and materials. Good-quality electrical heatshrink sleeving is generally black and made from a fluorocarbon rubber called Viton, which has high-temperature capabilities (for a rubber). The grade used for heatshrink is more like a normal solid polymer than a conventional flexible rubber. It requires plastic behaviour (in the mechanical sense) in order to have the ability to shrink when heat is applied.

A small-bore tube of the material is expanded to give a permanent deformation (that is, it deforms plastically). This can be slid over the exhaust spring, and when heat is applied via a 'hot air gun' the plastic deformation is partly recovered and the sleeve shrinks, encapsulating the spring but not with enough force to prevent it being fitted.

Fig. 1 - The Le Mans 24 Hour race for motorcycles has shown that ingenuity is not limited to big-budget car teams. This is a 1990s Kawasaki ZXR-7, when the top bikes were beautiful prototypes

Written by Wayne Ward

The Honda in particular has strayed furthest from what everyone perhaps expected these engines to be. It has the airbox and inlet on the front of the engine and the exhaust port pointing backwards in the bike, a layout not commonly used for motorcycles. One might expect that the exhaust pipe would exit straight towards the rear of the motorcycle, coming out underneath and at the rear of the seat unit. However, Honda sweeps the exhaust forwards in the motorcycle, and then down, allowing a silencer to sit just behind and below the engine.

By comparison, the KTM and Ioda engines are much more akin to a conventional motorcycle layout, with the inlet on the rear of the engine and the exhaust port on the front. As per conventional practice, the KTM and Ioda engines bring the exhaust down the front of the engine. We will have to wait to see what GE chooses to do here.

Why does the Honda bring the exhaust around the front of the engine, having gone to the effort of designing an engine with the exhaust port on the rear of the cylinder head? Well, it is probably because it is an easy way to package the necessary exhaust length. The straight-run approach from the exhaust port to the back of the bike may not have given enough length for the exhaust to tune properly without having to design a contorted pipe. If the engines require a silencer, then having this mass low on the bike is probably another reason not to use an underseat exhaust exit.

It seems that the exhaust packaging could be more easily achieved if the cylinder orientation was conventional. If we consider the exhaust alone, this is probably true. However, the small cylinder requires only a small airbox, and this can easily be housed above and ahead of the engine. Giving the inlet a 'straight run' into the airbox will help maximise pressure at the inlet port, which is good for engine performance. It is probably the case that the design of the exhaust and the orientation of the cylinder are dictated by considerations of engine breathing.

Fig. 1 - The KTM Moto3 engine has a conventional cylinder and exhaust layout

Written by Wayne Ward

There is also specific guidance for the exits of Sprint Cup exhausts regarding height, which should be a minimum of 2 in (50.8 mm) except for exhausts used at Daytona and Talladega. So, we can expect to see a very flattened exit to the exhaust. This flattened exhaust system is desirable as it allows the car to run low.

So, what are the implications of running flattened sections of pipe in the car? The pressure losses in a length of straight pipe are dictated by the fluid within, its flow rate, Reynolds number and the geometry of the pipe. The minimum pressure loss in the pipe is achieved where the cross-section of the pipe is circular. Take the example of a 4 in diameter pipe; this has a cross-sectional area of 12.56 sq in . If we turn this into a simple rectangular section of 2 in by 6.28 in, the pressure losses in the pipe are the same as would be found in a circular pipe of just over 3 in.

The concept that gives us this relationship between pipe geometry and effective diameter is that of 'hydraulic diameter'. The hydraulic diameter of any duct is defined as four times the cross-sectional area divided by the perimeter.

Where:
dh = hydraulic or effective diameter
A = cross-sectional area
p = perimeter

For a circular section, the diameter and the hydraulic diameter are equal.

For a given height or section, say 2 in, the increase in hydraulic diameter with increasing section width is a matter of diminishing returns. If we were able to run an exit twice as wide - that is, 2 in x 12.56 in - the hydraulic diameter is increased to only 3.45 sq in, an increase of only 15% for a doubling in area. If, however, we keep the 6.28 in side and increase the height to 4 in, we find that the hydraulic diameter increases to 4.88 sq in.

The pattern here is that there is a definite advantage to running rectangular pipes of certain aspect ratio, as in the ratio of the lengths of the sides. Within the rules that NASCAR mandates, and within the packaging constraints of the car, there is much scope for experiment. One problem with rectangular sections with two short sides and two long sides is that they often require 'bracing' from top to bottom. In terms of hydraulic diameter, this has some interesting effects. Sharpen your pencil and see what you find.

Fig. 1 - In NASCAR Sprint Cup, the exhaust exits between the front and rear wheels. We can't see the exhaust exit directly, but we can be sure that it has been well engineered

Written by Wayne Ward

There have been pictures in the press of some very weird and wonderful exhaust exits; at first sight these would appear to be just an incongruous design that is likely to present a flow restriction. Some exhaust systems are clearly very much longer than would be optimal for engine performance taken in isolation, which is basically how they were developed until a few years ago. Now, having started to realise the full potential on car performance - that is, working towards the minimum lap time rather than simply the best peak power - exhaust systems have been designed as an aid to aerodynamic performance. The teams that benefit from this type of development are naturally those whose relationship with their engine supplier is strongest, and those teams who have the budget to support such development and testing.

The rules were changed mid-season to restrict the amount of 'hot blowing' allowed. Hot blowing is a strategy where the mass flow through the engine is managed by controlling engine operating parameters independently of driver input. For example, if the idea is to generate downforce through a corner by having exhaust flow under the car, the throttles will be opened, fuel may be injected, ignition timing changed and so on, so that the car isn't pushed along against the driver's wishes, but the mass flow is increased even though the driver may be 'off the gas'. The more throttle opening that can be used without 'pushing' the car in corners, the greater the additional exhaust mass flow that can be used for aerodynamic gain.

For 2012, the practice of 'blowing the floor' is to be consigned to the history books, after a season of frantic development, joining a list of similar development 'hot potatoes' such as double diffusers, f-ducts and spring-mass dampers, which have been banned after a short but fast-moving development arms race.

We ought not to think this will mean the end of trying to exploit the potent effect of exhaust mass flow to improve the performance of the car. Blown diffusers were once used to good effect, and there is no reason why the engineers involved won't try to do something clever with the exhaust mass flow with aerodynamic surfaces on the top deck of the car, or to influence the flow over the rear wing. Of course, there is some simple gain to be had from the thrust available from the exhaust, and people have been very deliberately using this to advantage for many decades.

Fig. 1 - The exhaust the Formula One cars of 2012 will be restricted to in terms of development for aerodynamic gain compared to 2011

Written by Wayne Ward

The racing kit, which was seen as being necessary to be competitive, was supplied by the factory, and often included a 'kit' exhaust. Today, the road-going motorcycles on which the racers are based are, in general, much more reasonably priced, and it is much more likely that the bikes will have a race kit that includes parts made in partnership with technical experts outside the company. It is now rare, even for the factory-supported teams, to have an unbadged 'works' exhaust.

It is also common for individual teams to have development projects running with an exhaust manufacturer. This model of independent exhaust development - either in conjunction with a manufacturer or an individual team - is a real boon for the exhaust makers, with race fans being able to buy similar products for their road-going machinery. The teams benefit from an independent source of development, which can be carried out at a reduced rate because there is a source of income derived from sales.

The exhaust systems used vary in design according to the machinery used. Yamaha and Suzuki both race with inline four-cylinder engines, but these have different firing orders. The Yamaha, in common with the MotoGP engine, uses a cruciform crankshaft rather than the more traditional flat-plane design.

This crankshaft configuration is discussed in Ian Cramp's recent RET-Monitor article. The system used by Yamaha is a basic 4-2-1 configuration, but after the final collector, the pipe splits into two before the under-seat exit, so it can be said to be a 4-2-1-2. The primary pipes are of a tapered header design and are linked in pairs, with cylinders one and two joined and cylinders three and four joined. The company producing the systems is known for its hydroforming technology, and this is likely to be used on the Superbike systems.

The Suzuki uses a conventional flat-plane crankshaft, but still joins the same pairs of primary pipes together, as does the cruciform Yamaha, and again splits into two silencers, but which exit at the sides of the bike. The Kawasaki again pairs the same cylinders in its exhaust system. However, some of the inline four-cylinder systems also use link pipes between adjacent primaries to help tune the exhaust system to work at certain speeds.

The Aprilia, being a V4, is more limited in scope for joining different pipes together; predictably this machine runs cylinders paired as per the architecture of the engine, with the two secondary pipes joining into a single collector before the silencer.

Fig. 1 - Link pipes on exhaust systems developed in World Superbike competition

Written by Wayne Ward

The NGTC rules allow the use of a 2 litre turbocharged engine in comparison to the naturally aspirated S2000 engines which have been used by the existing entrants. It is easy to imagine that there are going to be lots of differences in the design and manufacture of the exhausts for the cars that compete under the different sets of regulations.

The cars with the NGTC turbocharged engines have to 'hang' a specified turbocharger unit on the primaries of the engine; this is in addition to the fact that there are much higher exhaust gas temperatures for the turbocharged engines. While 'basic' materials would almost certainly suffice for the S2000 cars, it is likely that the turbocharged entrants will have to look to more exotic materials. Certainly stainless steel will be the very minimum specification of material that is likely to prove durable, but it is more likely that high-temperature materials such as Inconel will be the norm. For 2013 and beyond, when all of the cars will be operating to NGTC specifications, there is a planned performance boost from the current 300 hp; this will increase exhaust gas temperatures further.

The efficiency of a turbocharger is affected by the temperature of the incoming flow as it enters the turbine, and the ideal situation (thermodynamically speaking) is for this to be as high as possible. This is one reason why some of the exhausts for the turbocharged NGTC cars are ceramic coated in order to insulate the exhaust and retain heat within the exhaust gas. In the case of the S2000 cars, retention of heat within the exhaust has no engine performance gain.

Exhaust insulation coatings and heat shields might prove effective for both types of engine in protecting adjacent components from the effects of radiated heat. It can also prove worthwhile to prevent radiated heat from reaching the engine or gearbox structure, in order to minimise the heat transfer to coolant.

The design of the S2000 exhausts, with respect to the choice of pipe lengths and diameters, will be driven by the effects this has on the tuning of the engine; the turbocharged NGTC exhausts will be designed with the transient response of the engine in mind. People have found good success with turbocharger response by having the turbo mounted on a short exhaust.

Fig. 1 - British Touring Car racing operates two specifications of engines; turbocharged runners such as this Toyota have different exhaust requirements from the naturally aspirated runners

Written by Wayne Ward

The MP4 is powered by a 3.8-litre V8 turbocharged engine. I spoke to the chief engineer on the GT project, Andy Thorby, about the exhaust installation for the new car. In previous articles on the subject of exhausts, the demands placed on the exhaust system by turbocharged engines have been discussed. Despite these, Thorby makes some points about the exhaust system for the twin-turbo V8 (known as the M838T engine).

First, because the turbine in the turbocharger removes a lot of energy from the exhaust, the noise is attenuated, and therefore the car requires less silencing. I have recently heard a turbocharger referred to as a 'rotary silencer'. There is no tuning advantage to doing anything clever with the exhausts after the turbocharger, so there are, in effect, two separate exhaust systems on the car, Some naturally aspirated V8 engines make best performance when the exhausts from each bank are joined, but this is not the case with the M838T.

The hot turbos and exhausts radiate a lot of heat, so these are encased in heat shields. There is an aluminium subframe that requires protection, but there are other advantages to using heat shields. If heat was free to radiate outwards, a portion of it would naturally be transferred to the engine and gearbox, causing them to heat up. The result of this radiation is seen in increased heat rejection to the lubrication and coolant circuits (oil and water) which would therefore need greater cooling capacity to achieve the required fluid inlet temperatures. The mitigation of engine and gearbox heat rejection requirements is important in reducing cooling requirements to a minimum, this benefiting the speed of the vehicle by reducing the aerodynamic penalty of large coolers.

The cast exhaust manifold from the production car is retained for the GT racer. In order to minimise turbo lag, the exhaust manifold is designed such that the distance from the turbine to the cylinder head is kept to a minimum. While this approach is known to lead to a faster turbo response, it also means that management of the heat from the exhaust must be considered, to prevent too much being radiated back to the cylinder head; hence the heat shields.

Thermal expansion of the exhaust system is considerable, given the high temperatures involved. The retention of the original cast manifold means that any 'give' in the downpipes between the engine and turbocharger is minimal. According to Thorby, the single rear mount on each side of the exhaust system, which retains the original exit position through the rear grille, has to cope with 15 mm of movement due to thermal expansion.

Fig. 1 - The turbochargers are packaged very close to the engine (Courtesy of McLaren Automotive)

Fig. 2 - The new McLaren has impressed in early races. It uses heat shields to protect vulnerable parts, and maintain heat in the exhaust flow (Courtesy of McLaren Automotive)

Written by Wayne Ward

In addition to high levels of vibrations and shocks from jumps and so on, rally exhausts have to cope with the possibility of damage from rocks and gravel when drivers tend to hang the wheels of the car over the edges of ditches and gulleys to get the fastest line. An additional and unusual problem is the rapid quenching effect experienced when a very hot exhaust system is plunged into cold water when a stream is crossed.

The turbocharged engines naturally run with higher exhaust gas temperatures than their naturally aspirated counterparts, thus making the materials work closer to their limits. Anti-lag systems, often used in rallying, can take a number of forms, but a lot of anti-lag engine strategies rely on a lot of unburnt fuel entering the exhaust and burning there. This keeps the turbocharger spinning at low engine speeds and throttle opening without the engine producing power, by increasing exhaust mass flow rate.

In the absence of such systems and control strategies, the transient delay in the plenum reaching full boost pressure while the turbocharger accelerates makes the car difficult and unpredictable to drive. These anti-lag systems and control strategies create higher temperatures and pressures than would be normal in a turbocharged engine installation. The pressure spikes due to this abnormal exhaust combustion create additional stress in the exhaust.

There is a requirement therefore to place particular emphasis on weld quality. Given the lower quality of stainless steel materials compared to Inconel, attention to weld quality in the lower-grade material is critical if they are to last for any reasonable length of time. High-mileage failures of exhaust systems are often due to combustion of fuel in the exhaust.

Pat Barrett of Primary Designs, who has experience of the manufacture of rally exhausts for high-level competition, told me that the most common materials used for the production of competition rally exhausts are Inconel 625 and stainless steel. Inconel is a high-temperature material, beloved of many of the higher-budget race teams in many branches of motor racing, and there is a compromise to be struck between reliability and exhaust system cost. WRC cars tend to use Inconel, but competitors who aren't as well funded are more likely to choose a stainless steel system.

Beyond the turbocharger, exhaust temperatures are lower and the possibility remains of using a material that isn't so temperature resistant. While this means stainless steel for a lot of people, titanium is a realistic option for those who see reduction of mass as a priority.

Fig. 1 - Rallying imposes unusual demands on exhaust systems, including high internal temperatures and pressures, rapid quenching and impact damage

Written by Wayne Ward

Perhaps the greatest challenge in terms of mileage are the endurance races, of which there can be no more famous example than Le Mans, the Mecca for exponents of sportscar racing for many decades. Exhaust systems are expected to last for the practice and qualifying sessions, as well as the race itself. This can easily be in excess of 4000 km, during which there is little opportunity (or hopefully zero opportunity during the race) to inspect the exhaust system. Sportscar racing has embraced the move toward the higher specification materials with the use of Inconel. Titanium has little chance in endurance race exhausts, although lightly stressed parts could conceivably be used with success.

The cars are expected to be silenced and to come within strict guidelines on noise, and over the course of a 24-hour race, silencer degradation can be serious, with loss of silencer packing material being a problem where the traditional perforated tube silencer is used. With the marshals and technical officials perhaps forcing a car to come in for repairs if an exhaust system becomes too noisy, there is a real incentive to ensure the reliability of the system from the point of view of the headers and the silencers.

Turbocharged engines have an advantage over naturally aspirated engines, as the turbine takes a lot of energy from the exhaust flow, so they generally require less silencing. However, with high mass flow for a given engine size, the exhausts used on a turbocharged engine have to cope with much more heat and therefore higher temperatures.

As anyone who has seen the diesels in action - love them or hate them, they are very fast - will attest to their very low noise emissions. Part of this is due to the requirement for exhaust after-treatment, where the exhaust gas has to pass through filters in order to burn off particulates. Again, these filters have some noise attenuation, and the exhaust flow arriving at the filter will have had some energy removed by the turbocharger.

The issue of heat at Le Mans is a serious consideration for the race team and the engineers who design the cars. The race is often hot, and the engines in the prototype cars are fully covered, with no louvres or chimneys in the engine cover to aid cooling. Radiant heat from hot exhausts and the general heating effect due to convection under the engine cover can lead to reliability problems for other components, especially electronics. Unless sufficiently protected from heat, soldered connections can fail due to the solder being melted. This is just another challenge for the exhaust designer, who needs to keep heat away from critical electrical components.

Fig. 1 - Le Mans presents a particular challenge in terms of exhaust durability, silencing and protection of adjacent components from exhaust heat

Written by Wayne Ward

Most of these have exhaust systems that follow a similar 'recipe'; the engines, having four cylinders and producing similar power at similar engine speeds, will have broadly similar requirements in terms of pipe diameters, lengths and so on. Being well supported, they are generally made of titanium, which offers a lightweight system at reasonable cost. Most exit on one side of the motorcycle, as is generally the case for a road-going motorcycle.

Fig. 1 here shows a picture of the Yamaha MotoGP machine from 2002, with an uncommonly long silencer exiting on the right-hand side of the engine. The 2011 model from the same manufacturer has a silencer in the same position. Both of these machines are inline fours.

Honda, Suzuki and Ducati all run a V4 engine in MotoGP. Honda splits its exhaust flow to two silencers; one exits at the side of the machine, the other under the seat. Presumably the exhaust under the seat takes the flow from the rear two cylinders of the vee and the side silencer exhausts the flow from the front two cylinders. Ducati and Suzuki differ in their approach from that of Honda in that they exhaust the whole flow under the rear of the seat unit. A picture of the Ducati exhaust exit can be seen in Fig. 2.

As was mentioned in the recent Race Engine Technology magazine article on exhausts (issue 52, February 2011), the position and direction of an exhaust exit can affect vehicle performance, whether that vehicle is a motorcycle, car or aeroplane. One only has to see the effort being expended in Formula One this year on exhausts to see that advantage is being taken of the use of this high-energy flow.

In terms of the motorcycle, there may well be real benefit in terms of engine performance and drag reduction in directing the exhaust flow into the low-pressure area behind the rider using an underseat exit. Where exhausts are ducted into areas of low pressure, engine performance is increased due to improved cylinder scavenging. This was discovered many decades ago in the study of aero engines. At a similar time, the study of the gains to be had in terms of thrust found that this also presented worthwhile gains in top speed. In terms of the MotoGP motorcycle, filling the turbulent area behind the rider with a significant mass flow of exhaust gas may have value in keeping the flow over the rear of the machine attached, with an attendant reduction in drag.

Fig. 1 - 2002 Yamaha MotoGP machine, with side exit silencer

Fig. 2 - 2011 Ducati MotoGP machine, with underseat exhaust exit (Courtesy of Bonnie Lane at CanBe Images)

Written by Wayne Ward

When it comes to exhaust manufacture, NASCAR uses much of the same advanced manufacturing technology and materials as Formula One. NASCAR exhaust development places a lot of demands on exhaust manufacturers, some of these report that NASCAR is the one series that stands out due to the amount of exhaust development carried out.

High-temperature materials, such as Inconel are not as popular in NASCAR as they are in some other series such as Formula One where high-temperature materials are universally used for exhaust systems. It is reckoned that approximately half of the Sprint Cup teams still use stainless steel, despite the fact that there is a slight weight penalty compared to Inconel. The strength and durability of Inconel means that it is possible to manufacture exhaust systems in impressively thin sections. In order to keep stainless exhausts competitive against these systems, stainless materials need to be pushed to their limits on wall thickness, sometimes using material as thin as 0.028 in thick, although 0.035 in is more common. Durability is a concern for NASCAR exhausts, and teams are trying to get exhaust designs that can run comfortably for 3,000-5,000 miles. The lack of availability of tube in the desired combination of diameter and thickness demanded by the various teams means that exhaust suppliers often manufacture much of their own tube, even though this requires investment in expensive equipment. Where Sprint Cup teams use Inconel and have an affiliated Nationwide team, some Inconel systems are 'passed down', but non-affiliated teams who have to purchase new systems generally specify stainless steel materials.

With long races in NASCAR Sprint Cup, exhaust durability is very important as the next opportunity to check an exhaust system might be more than 500 miles away. A lot can happen in 500 racing miles, so quality has to be 'built in' to each exhaust, with careful quality control procedures being very important.

With engineers from the NASCAR teams pushing development, exhaust manufacturers need to keep track of the latest developments in manufacturing technology. It often isn't possible to have the very latest technology owing to the costs involved, and keeping an eye on the maturity of new manufacturing technology and the costs of introduction mean that NASCAR exhausts employ new technology which represents real value.

Away from manufacture and looking to design, the advent of 3D CAD systems means that some suppliers are able to design and manufacture a NASCAR exhaust without having sight of the engine or car. If teams can supply accurate CAD data for their car, the exhaust can be designed 'remotely'.

Fig. 1 - These Inconel exhausts are destined for use in NASCAR, although not all teams have adopted this material (Courtesy of PRO-FABrication)

Written by Wayne Ward

Along with the snorkel (air horn), they are the only components able to influence engine performance that are not deemed to fall within the 'sealed engine', inside which no development or design changes are allowed, except for clearly defined reasons such as cost-cutting or reliability. Since engine homologation has been imposed in Formula One there have been a couple of opportunities to re-optimise engine performance, owing to the reduced maximum engine speed limits, first bringing the maximum rpm down to 19,000 and then reining the engine manufacturers in further with an 18,000 rpm limit.

Given the restriction on other areas of engine development and the freedom to do as the development engineers wish with regards to exhaust system design, we might therefore expect exhaust development to have been an area of frantic activity. This isn't true though in many cases, and most major changes are due to chassis influences rather than for engine performance reasons.

In more normal circumstances, as in with free development, as the engine manufacturers look to improve some aspect of engine performance - besides peak power or torque, this may equally well be fuel economy or 'driveability', even in Formula One - they would alter several aspects of the engine design. Changes in port design and cam timing might well be accompanied by changes to inlet length or exhaust geometry as we seek to influence the gas exchange processes.

With the current Formula One rules, there is no scope to use the broad range of options outlined above that are normally available to the development engineers working in other race series. Changes to exhaust systems in isolation are likely to have limited effect in terms of influencing engine output (unless large changes are made) as the other parameters we might choose to alter are fixed due to homologation.

The exemption of the exhaust system from the homologated engine regulations is designed to allow freedom for the chassis manufacturers and race teams to do as they wish with the design of the car packaging and aerodynamics. There is a considerable mass flow through a Formula One engine, and the careful direction of the exhaust gas can affect car performance and durability. In the past couple of years we have seen how influential aerodynamic advantages have been for the teams; the 'F-duct' in particular was an important development, making a real performance difference and being visible to the average punter watching the TV.

Unfortunately for the race engine business, this freedom is deemed more important than allowing similar technical latitude to the engine manufacturers. As the manufacturers are notably coy about releasing any technical information, and take special care to keep engines covered at all times, the FIA has clearly noted that there is more technical interest for the average Formula One fan in the developments they can see on the chassis - and which are detailed in the general motorsports press - than those they never see or hear about concerning engines.

There may be a lesson in there somewhere.

Fig. 1 - Formula One exhaust systems, despite being free of development restrictions, are often changed for chassis performance reasons. The large exhaust mass flows can have an important effect on car performance (Courtesy of Good Fabrications)

Written by Wayne Ward

One criterion expected of performance exhausts is their need to work at sustained elevated working temperatures, which has necessitated the use of more sophisticated and complex materials. Increasing temperature exposure has also called for greater resistance to corrosion and creep. As mentioned in previous RET-Monitor articles, this has led to the use of Inconel.

Inconel is a trade name given to a group of high-temperature nickel chromium 'super-alloys' that exhibit exceptional high-temperature corrosion resistance, resistance to creep, fatigue and, in the case of Inconel 625, excellent weldability. Inconel has proved itself in the nuclear and defence sectors and made its way into performance marine and automotive exhaust systems because of its exemplary mechanical properties.

According to Joe Ellis, managing director of BTB Exhausts in Daventry, England, it seems that there are few problems with using Inconel compared to stainless and titanium systems. Although Inconel 625, for example, shows good weldability and needs no post- or pre-heat treatment to maintain mechanical properties during the welding process, the fabrication of Inconel requires the use of pre-annealed material.

Because of the atomic structure of these high-nickel alloys, they are subject to high rates of strain hardening when cold-worked. Using pre-annealed material increases ductility and allows the material to be bent to the required radius without cold-working to failure. The use of sheet material to form tubes also allows the seam to be concealed in a position that satisfies aesthetics, as well as improving strength by positioning these seams correctly.

Ellis says, "Many people think Inconel will be lighter, but its density is higher than that of stainless or titanium." Therefore, to maintain or reduce weight over a titanium or stainless steel equivalent, the Inconel system will necessitate a reduction in wall section thickness. Although this reduction can have benefits such as aiding heat transfer, it is rare to see these super-alloys in applications where impact resistance is a prerequisite.

Inconel 625 can be welded using a number of techniques depending on the application and required properties. TIG welding - or Tungsten Inert Gas welding, so called as it uses a thoriated pure tungsten electrode - is favoured in exhaust fabrication due to the thin-wall section used in construction. In fabrication, cleanliness and fit is key, in order to minimise distortion and maintain integrity it is important to maintain an excellent tube-to-tube fit.

Eliminating distortion is not always possible, but this can be accounted for within the jig-and-fixture design. It is possible to omit a root gap because of the thin-gauge section used. The use of a pure argon back-purge helps not only to maintain the internal cross-sectional area and finish, this process also protects the weld root from reaction with oxygen during the welding and cooling process. Removing this internal argon gas protection can lead to porosity and oxidization.

Welding research by major organisations is ongoing; there is still the weldability of a number of super-alloys that require investigation, which will enable us to see ever more expensive and exotic materials being used in exhaust fabrication. With a similar commitment to filler and welding development, could exhaust metallurgy rival that of the aerospace and nuclear fields?

Fig. 1 - A BTB header fabricated using an Argon back-purge

Written by Chris Thwaites

One company, Good Fabrications, with extensive experience of many types of exhaust materials has applied its knowledge of special high-temperature materials used in other race series with great success to motorcycle racing, making the Austin Racing exhausts as used by some Aprilia and BMW entrants in British Superbike racing.

In the British Superbike championship, there is a two-tier set of regulations, with the 'Evo' regulations allowing far less deviation from the standard production specifications in terms of engine tuning. Splitlath Motorsport runs two riders in the series on 1000 cc V4 Aprilia RSV4 machines. Proven to be very competitive in World Superbike, Splitlath had a successful season in BSB Evo, finishing second in the points and its bikes are equipped with Inconel exhausts from Austin Racing, which manufactures the systems in conjunction with British specialist Good Fabrications.

The grade of Inconel is unspecified, but the general wall thickness of the system was 0.7 mm in the beginning, soon progressing to 0.5mm. This according to Austin Racing's Richard Austin means that "the weight is now comparable with the Titanium systems". A typical titanium system is around 1mm wall thickness; a length of 0.5mm Inconel tube of typical bike-racing diameter is around 12% lighter than a 1mm titanium tube of the same length.

The photo here shows an Inconel RSV4 exhaust from Austin Racing. With extensive experience of using Inconel in Formula One exhausts for many years, Good Fabrications knows how to make low-mass reliable systems in this material. Richard Austin offered his opinion on other benefits of Inconel exhausts "Inconel offers other advantages such as longevity and keeping the ambient temperature low, thus helping to keep shock absorbers and the rider's limbs cooler. Inconel fixings on link pipes etc are stronger and less likely to break".

We can be sure that, if titanium offered acceptable durability coupled with a weight saving over Inconel, the Formula One teams would all use titanium. Thin-walled Inconel tubing is expensive and difficult to source in small quantities, so it will probably remain the preserve of specialist Inconel fabricators who use it for high-end race systems.

There are other advantages to using Inconel though, other than durability and heat retention. It can be successfully welded to stainless steel, so adding bosses for lambda sensors and so on can be much more cost-effective than using titanium, where these bosses are more expensive to source. Titanium is also more difficult to bend and requires comparatively expensive tooling to do so.

Fig. 1 - This Inconel race system from Austin Racing for the Aprilia RSV4 is much more durable than a titanium system (Courtesy of Good Fabrications)

Written by Wayne Ward

1) Tubes linking certain primary pipes together; the link tubes being much smaller in diameter than the primary pipes they connect
2) Larger-volume 'bottles' being used to link adjacent primary pipes together
3) Pipes with closed ends being attached to the side of exhaust systems on single-cylinder exhausts.

We don't see the same things in photos of recent Formula One cars, however, so should we be concerned that the motorcyclists are somehow being conned?

The short answer is no. The aim of most of these devices is to change the shape of the torque curve by introducing different wave reflections into the exhaust at certain engine speeds.

In any given 'plain' exhaust system - that is, without any of the accoutrements listed above - wave reflections arrive back at the entrance to the cylinder as either a positive-pressure 'pulse' or a low-pressure pulse, at a certain time after the initial pressure pulse when the exhaust valve opens. Depending on the speed, these pressure pulses affect how well the exhaust gas is evacuated from the cylinder, and at certain speeds these reflections can be unhelpful, pushing a significant amount of burned gas back into the cylinder.

Thus the charge in the cylinder for the next cycle contains a large percentage of 'residuals'. While we can always expect a certain amount of residual burned gas to remain in the cylinder, large amounts affect the engine output, and we can find large 'dips' in the torque curve where, within a certain speed range, high volumes of residual exhaust gas prevent efficient cylinder filling and combustion.

The devices mentioned above can make a real difference to the torque curve, improving the rideability of the motorcycle. Given the small tyre contact patch a smooth, tractable engine is a bonus in most cases. A dip in a torque curve can cause the rider to open the throttle too quickly and, once out of the 'torque dip' the rapid increase in torque over a very narrow engine speed range can cause the tyre to lose traction. Although this has very little effect on the peak power, the effect on rider (or driver) confidence can mean better lap times.

The 'link tubes' referred to above were used by some teams running V8 engines in Formula One before the rules mandated the use of a V10 engine, but in the modern era it seems that only motorcycle exhausts are widely exploiting the advantage of such devices.

One particular feature of motorcycle racing is that the exhausts are generally supplied by a company whose main business is making aftermarket exhausts for road-going motorcycles. By developing race exhausts that improve the tractability of an engine, they can 'feed' these developments back to their other customers.

Fig. 1 - Link pipes on these Yoshimura motorcycle headers are typical of many current systems
Fig. 2 - This Akrapovic exhaust has a 'bottle' attached to the side of its single header pipe

Written by Wayne Ward

The company uses racing as a way to improve its products through the increased speed of development, and the teams it supplies are an active part of this process. It names CDS Racing Team and SGM Tecnic as teams with a lot of experience that, according to Nicali. "help us test our product on track and boost performance under racing pressure".

There are very strong similarities between the design of its race systems and those supplied for road use, with the main differences being in the materials used in their manufacture. As the road products are aimed squarely at the sportsbike rider, having a strong race image is important.

I asked Nicali about material choices and manufacturing methods for its exhaust and, as we might reasonably expect, the choice depends on the intended use, with stainless steel being for production motorcycles, but "titanium welded with the TIG method" for race systems. (Note: TIG stands for Tungsten Inert Gas)

The price of full titanium systems is beyond the budget of most roadbike riders, but race teams demand lightweight systems and the higher price is one that they deem to be worth paying. The silencers are available in titanium for race systems, with the ends machined from Ergal, an aluminium alloy, or formed in carbon-fibre reinforced polymer (CFRP). Fig. 2 shows a typical TIG-welded system.

I asked the company about its most common exhaust system configurations for racing, as applied to four-cylinder machinery. Although two- and three-cylinder engines are eligible for Supersport, Superstock and Superbike competition, four-cylinder engines still dominate in terms of numbers participating, and inline four-cylinder engines in particular are the most numerous.

Lesto says the most common system now is 4-2-1, and in this it agrees with a number of those teams competing in World Superbike. The advantage of these systems is improved 'rideability' of the motorcycle, with the rider being rewarded with a greater spread of torque than is available with a 4-1 system. Lesto did say though that it is working on improving performance further.

I asked Nicali what special features Lesto has in terms of exhaust system design, and he went into detail on some features which offer improved silencing. He says the 'triangular' section silencer is "very useful in terms of efficiency of the exhaust". The company also has an adjustable device in the silencer it calls the DB Killer which, as its name suggests, is aimed at noise reduction. It can be adjusted and fixed in one of five positions, and its purpose is to change the length of perforated triangular pipe within the silencer, thus affecting the silencing capability of the system.

Fig. 1 - Exhaust system fitted to a motorcycle competing in the Italian national championship (Courtesy of Lesto Racing)

Fig. 2 - Typical TIG-welded construction (Courtesy of Lesto Racing)

Written by Wayne Ward

There are various reasons why Formula One teams choose a 4-1 arrangement - the fact that the packaging can be easier, the peak performance is better and the mass of the exhaust system is lower are all important factors.

The arrangement was popular in motorcycle racing but has largely been supplanted by pipes being coupled in pairs before the two secondary pipes are brought together in the final collector. This system is known as four-into-two-into-one (4-2-1)

Eschenbacher confirmed that the titanium exhaust systems supplied to the Sterilgarda Yamaha WSBK team do not follow the same pattern as current Formula One systems, and they use a 4-2-1 arrangement; their particular arrangement has the final pipe splitting into two before the twin silencers (4-2-1-2) which exit underneath the seat unit.

It's acknowledged that this 4-2-1 style of exhaust costs a little performance in terms of absolute peak power, but Eschenbacher says the overall effect of the 4-2-1 type of system is positive for the Yamaha compared to the 4-1 arrangement, owing to a wider spread of torque and improved rideability. Even for tracks where top speed is felt to be important, the 4-2-1 system is retained.

The Yamaha R1 motorcycle used for World Superbike racing has a different firing order to many 'across-the-frame' four-cylinder machines, owing to its twin-plane crankshaft. This has been pioneered in recent years by Yamaha after successful results in MotoGP using the same configuration.

Most four-cylinder machines of this type have a single-plane crankshaft, and for 4-2-1 exhaust systems the tuning has traditionally relied on the pairs of primaries being linked in each case firing at equal intervals of 360 crank-angle degrees. So, conventionally, we would link primaries 1 and 4 together so that the wave reflections to each cylinder of the linked pair are the same in terms of magnitude and timing.

But even for conventional four-cylinder engines with a flat-plane crank, it can be advantageous to link primaries together that don't obey the traditional pairings (1 & 4, 2 & 3) to achieve good results. We should not therefore assume that the Yamaha suffers greatly in terms of exhaust tuning owing to its unique firing order, compared to conventional 4-2-1 systems on four-cylinder engines with equal firing intervals. The unusual firing order also leads to a unique exhaust note - certainly among in-line four-cylinder machinery currently racing - with the machine sounding much more reminiscent of a four-cylinder vee engine.

As with its MotoGP programme, Yamaha has had recent World Championship success with its unconventional superbike engine. The 2009 World Superbike champion Ben Spies rode this machine before moving to MotoGP this year.

Fig. 1 - Rear views of the exhaust system from Ben Spies' 2009 machine

Written by Wayne Ward

Part of the reason is that, in some countries, diesel fuel is much cheaper than petrol (gasoline). In the Netherlands, for instance, petrol is 30% more expensive than diesel. But even in countries where diesel comes at a premium compared to petrol, such as the UK, diesel vehicle sales continue to account for an increasing percentage of overall sales. That's because diesels have provided good fuel economy figures for many years and, even in the UK, the costs of running a diesel are likely to be less than for an equivalent petrol car.

So it's no surprise that the European motor manufacturers have pressed for regulations that make the diesel competitive against gasoline-powered racers. The obvious example of this is at Le Mans where, since the rules were changed, the gasoline competitors have been consigned to fighting over the minor placings in the premier LMP1 category.

The regulations are contentious, with gasoline engine suppliers complaining bitterly about disparities in the rules. With diesels enjoying a significant power and torque advantage, the gasoline manufacturers may have a point here.

One significant regulation concerning endurance racing, which is governed by the Automobile Club de l'Ouest, concerning all engines but aimed mainly at the diesels is regulation 5.5.3, which states, "The engine must not produce visible exhaust emissions under race conditions." As is obvious to anyone who has attended a truck race - the European type, where lorries are raced, rather than the NASCAR type truck races, which are something different altogether - diesel engines can produce a lot of smoke. Diesel particulate filters are therefore necessary to comply with this regulation.

These are commonly fitted to series production cars and commercial vehicles. But they're not a filter in the same sense as, say, an oil filter. The aim of an oil filter is to trap particles above a certain size permanently on the surface of a porous filter, while allowing the liquid phase of the flow to pass through relatively unimpeded. Over time, the filter becomes blocked and the pressure-drop across the filter increases, hence the need to change the oil filter periodically.

A diesel particulate filter (DPF) is different in the sense that its aim is not to trap particulate matter permanently. In appearance, the DPF is similar to an exhaust catalyst, although on closer inspection there are differences. The face of the filter looks rather like a chessboard, with half of the square passages blocked. Each passage through the filter is blocked by an impermeable wall at one end, while the axial walls of the filter are permeable. Figure 1 shows a typical filter element for a DPF.

The aim of the DPF is only to trap particulates on the surface of the filter until the temperature of the exhaust flow is high enough to oxidise them. The gas products of this oxidation can then pass freely through the filter. Filter sizing is therefore determined such that exhaust 'back pressure' is kept within reasonable bounds. A larger filter implies lower gas velocity across the porous media, so the associated pressure losses are lower. The filter only becomes 'blocked' if the temperature in the exhaust system is too low for oxidation to take place.

Particularly sedentary drivers of road vehicles may sometimes see a warning light when the DPF has become clogged; the reason being that the filter has not been hot enough to burn the trapped particulates. The owner's manual will invariably encourage them to go for a drive under high load conditions, but while this may go against their instincts for economical driving, they harm the economy of their car by having a blocked DPF. Perhaps they should give their car to a Le Mans racing driver for a while?

Fig. 1 - Diesel Particulate Filter (DPF) element
Fig. 2 - Flow through a DPF. The particles are trapped on the wall of the filter until they are oxidised.

Written by Wayne Ward

Some of the very well-funded teams and engine manufacturers take great care with exhaust design to prevent flow separation in exhaust systems, by making measurements on exhaust systems or, more commonly, undertaking computational fluid dynamics (CFD) simulations of the flow in critical areas of the system. Where taking physical measurements will determine that there is a problem, CFD can provide a visual description of the problem, showing the areas where the flow begins to separate and the volume of recirculation etc. Furthermore it is easily possible to take the same measurements as one would take physically. The early Renault V10 exhaust system may have incorporated an exhaust with excessive flow separation; now such a system would be subject to much CFD optimisation to ensure minimal separation.

So, we might ask, how tight is too tight? As with a great many questions, the answer is not a simple one. It isn't possible to specify a certain bend radius, nor a ratio of bend radius to pipe diameter. If only there existed such a magic number, our lives might become somewhat simpler. The answer to the question of what constitutes a bend that is too tight will depend on a great many variables such as flow rate, exhaust gas composition, temperature and the state of the flow upstream of the bend. Therefore the problems associated with bends which are excessively tight will perhaps only be apparent at certain engine speeds and throttle openings, although we are generally only interested in the effect at full throttle in racing engines. Certainly any performance loss at anything less than full throttle can be dealt with by opening the throttle a little more.

Without being able to specify what is too tight a bend, we can make some generalisations and the obvious one is not to use very tight bends unless absolutely necessary. Where road vehicle exhaust systems are concerned, the header pipes or primaries are generally quite similar from cylinder to cylinder, but it is often the case for single-seaters, sportscars etc that the primaries are quite different from cylinder to cylinder, and by introducing tight bends into the primary of one cylinder we can make that cylinder behave differently from the others.

Having said all of this, there are measures that can be taken and design features which can be incorporated to minimise and prevent the separation of flow in areas where bends are tight.

Fig. 1 - CFD wasn't extensively used when this exhaust system was designed

Written by Wayne Ward

There is an alternative philosophy besides insulating the exhaust and this is to insulate the components which are at risk of heat damage, or to insulate parts of the engine where excessive heat pick-up would cause performance to be impaired, or where excessive heat transfer to the cooling and lubricating fluids of the engine would render the cooling provisions of the engine insufficient.

Heat shields are a common sight on may racing cars, not least in Formula One, where these are often polymers reinforced with either carbon fibre, 'Kevlar' or a mixture of the two. The advantages of these types of materials, besides their relatively low thermal conductivity, are their ability to be formed into complex shapes which, for example, are an offset of the engine castings or other components. The heat shields are often covered, on the side closest to the heat source, with a reflective covering, and these have been the subject of extensive trials in the past to determine which proves to be the best at reducing radiant heat transfer. There are alternative fibre composite materials which may prove fundamentally better than carbon or Kevlar as insulating heat shields.

For those without access to 3D CAD geometry of their engines to produce models of such parts, or the funds to make them, a new solution is available which promises to tackle heat transfer on a number of fronts. Wayne Ward spoke recently to Dr Andy McCabe, Technical Director of Zircotec about this new development. The supplier realises that it isn't the correct solution for everyone to have their exhaust systems coated, and that for many the best solution to the problem of heat transfer from exhausts is to locally insulate vulnerable components. Their new product is within reach of everyone but the most cash-strapped of racers. It is a ceramic backed aluminium foil which, as can be seen from the accompanying picture, has the ceramic applied in a regular pattern. Whilst the incomplete ceramic coating lends the foil some compliance, there is another important reason for this feature. As the ceramic is quite thick, the gaps in between it form a thin layer of still air which, as we are probably aware is a very effective insulator in its own right. The ceramic offers a high thermal resistance owing not only to low conductivity, but also by virtue of the low contact area. The final method by which radiant heat transfer is mitigated is due to the foil itself. Whilst not highly polished, it is more reflective than most cast surfaces and will therefore absorb less radiant heat energy.

Fig. 1 - Zircoflex, a ceramic backed aluminium foil.

Written by Wayne Ward

In light of this, we discussed exhaust coatings with a British company, one of whose specialities is the application of thermal barrier coatings to exhaust systems. Zircotec, who are based in the middle of an area rich in motorsport expertise, have a number of exhaust coatings which are made available on a sub-contract processing basis. They claim to be able to reduce convective exhaust heat transfer rates to the surrounding area by up to 30% with their premium coatings and as much as 25% using their lower-cost processes. I have seen independent test data from a source which proves the voracity of these claims with some very exhaustive testing (no pun intended). The report that I have seen regarding these coatings was conducted using an engine from a high-output premium road car in controlled conditions and using varied simulated wind speeds to mimic the effect of airflow over the outer surface of the exhausts. One important point that is apparent from the results of the tests is that the increase in gas temperature due to the thermal barrier coatings is very small and of such a small magnitude on the scale of absolute temperature that I seriously doubt that any retuning would be necessary in terms of exhaust dimensions. There is no data contained in the report concerning heat transfer by radiation, which would be a significant effect in a racing engine, especially if it is supercharged or turbocharged.

The coatings offered by this company are not paints, but are the result of a plasma-spraying process. Plasma is produced by the dissociation and ionisation of a gas, generally by means of an electric field. A plasma gun injects gas into a strong electric field and the result is that a plasma flame is emitted from the front of the gun. It is into this plasma flame that the powdered media is injected. The temperatures inside the plasma flame are sufficiently high to melt even ceramics. Owing to these high temperatures, there are very few materials which cannot be spray coated, providing of course that it can be supplied in a suitable form, this being a very finely divided powder. Whilst the temperature may be several thousand degrees, the mass of the sprayed particles relative to the mass of the comparatively much cooler substrate means that there is little heating of the substrate and little chance of damage being caused. Whilst we are unlikely to find an aluminium exhaust system, plasma coatings can be applied to low-melting point materials without physical damage and I have seen ceramic coatings applied by similar methods to aluminium parts for use in race applications. Certainly, when we talk of exhaust system materials such as steel, stainless steels, titanium or super alloys, there will be no damage caused by thermal spray coatings. The accompanying picture shows a coating being applied using the plasma spraying technique as described.

Fig. 1 - Plasma application process.

Written by Wayne Ward

The simple answer is that we are definitely not finished. The advent of ‘direct metal laser sintering’ and other ‘additive’ manufacturing technologies can offer us the possibility to make shapes and sections of exhausts which would either be extremely difficult or impossible to make by hand. These are essentially ‘traditional’ rapid prototyping technologies applied to metallic parts to produce functional parts. The quality of the process, the improvements in surface finish and the strength and density of parts is vastly superior to the state of this technology just five years ago, and it is rapidly becoming widely accessible, albeit at a cost.

It gives us the chance to put proper fillets in places where welding is difficult and so it may offer us the chance to finally cure that collector that keeps cracking. Where a number of pipes meet at a shallow angle, it is inevitable that there is a severe stress concentration where the pipes meet, because it is difficult or impossible to weld between each pipe. Examining such geometry on a small scale, we have manufactured a ‘crack’ at each junction. The fabricator may through his skill be able to reach in to melt the sharp edge sufficiently to mitigate this problem, but in DMLS and other technologies we have the chance to define the exact geometry that we want and in the process eliminate the stress concentration.

It is obvious that this technology offers us the chance to also make other exhaust parts which are either impossible to make by other means, or very risky owing to the difficulty in manufacture and weld access. In a recent Formula One exhaust project that I was involved with, we experimented with different internal configurations in a collector with the same basic dimensions in terms of tube size, angles, positions etc. There is certainly a clear effect on the engine from these changes, but there were many configurations where the collector would have been too risky to run even at a test session; race use was definitely out of the question owing to problems with durability. Some of the parts were just too difficult to make using conventional techniques. The ‘rapid prototype’ approach would go some way toward solving these problems, especially as high-temperature materials are among those available.

Written by Wayne Ward.

There is another reason why it might not be desirable to insulate the exhaust system. If we retain more heat within the exhaust, we must therefore have a higher gas temperature at any point along the length of the exhaust system. The speed of sound in any gas increases with temperature, and it is for this reason that insulation has a direct effect on the design of the exhaust system. If we retain more heat in the exhaust, the time taken for a pressure wave to reach a point of reflection (an open end, or a sudden expansion or contraction are examples) is reduced, and consequently the time taken for the reflected wave to return to the engine is affected in a similar manner.

If we take a very simple example of having a uniform exhaust temperature of 723 degrees C (this is 1000 K in absolute temperature) for an uninsulated exhaust, and this is then increased to 1200K by means of thermal insulation, we would find that the increase in the speed of sound of the exhaust gas increases by the ratio of the square root of the absolute temperatures, i.e. for the 10% increase in gas temperature, we would see a 4.9% increase in sonic speed. In theory, if we wish to maintain the same exhaust tuning behaviour, the exhaust would need to be 4.9% longer. So, in addition to the extra mass of the coating, bagging or wrapping, we might need to increase the length and therefore mass of the exhaust system by at least 4.9% as in this example. Many people would not go so far as to re-optimise the exhaust tuning behaviour when insulating the exhaust, but in critical applications where the exhaust has been optimised and the decision has been taken to insulate, this re-optimisation might be seen as a necessary step. Not only does this involve additional mass, but also time, design and testing resource at no little expense.

Noting the above, you should therefore consider carefully the options for exhaust system insulation before committing to any choice. As ever, there are compromises to be struck here, and in solving one problem, we may inadvertently create another, especially if the exhaust system has been carefully optimised beforehand.

Written by Wayne Ward.

The management of this heat under the engine cover or bonnet (hood) is a concern in many applications, most especially where turbocharging is employed. Whilst no longer used in Formula One (where power outputs approaching 1000 hp per litre were suggested in qualifying trim), turbocharging is still popular in many branches of motor sport such as rally and sports cars. Before we concern ourselves with heat management, we should consider what the problems are with having a red-hot exhaust.

A problem which can often be seen where the exhaust system is too close to the bodywork, we can see blistered paintwork and sometimes holes burnt into the bodywork. In a famous case in 2008, Kimi Raikkonen’s Ferrari exhaust failed during a race, resulting in the contents of the airbox being revealed to the television-viewing public around the world; I’m sure that these pictures would have been looked at in detail by their rival engine suppliers. Whilst due to exhaust gases rather than radiation from hot metal, this shows the damage that can be done by excess heat. Radiated heat from exhausts can lead to increased coolant and lubricant temperatures which would lead to the requirement for coolers of increased dimensions and, where cars are aerodynamically optimised, increased drag. The lower viscosity due to the oil being hotter is generally held to give an increase in performance due to lower frictional losses, but this is often more than offset by the aerodynamic loss.

Excessive temperatures under the engine cover due to exhaust heat often leads to failure of electronic components, or mechanical failure of adjacent components.

If aluminium components become overheated their strength can be decreased with an attendant loss of fatigue life, thus requiring early replacement or re-engineering.

Where the inlet system is subjected to this radiated heat, it can lead to increased charge temperatures and consequent loss of performance, and this can lead to very serious losses of performance in turbocharged engines.

So, where we have to deal with a hot exhaust, what can be done to eliminate or mitigate the effects of heat? There is the option of providing cooling to the affected components or areas of the car, but this can be complicated if there is more than one component to provide cooling for and can also have an aerodynamic penalty. The second option is to try to contain the heat within the exhaust so that it cannot radiate as effectively. There are a few options for this, including wrapping the individual exhaust headers in an insulating cloth which is available from many motor sport suppliers, creating a ‘bag’ or a ‘blanket’ from special insulating materials, or having the pipes coated to lower the thermal conductivity. All of these options have the aim of lower surface temperatures. As temperature (absolute temperature) has a fourth-order effect on radiative heat transfer, any lowering of surface temperature has a very significant effect. A 10% lowering of absolute temperature (approximately 100 degrees C for an initial temperature of 750 C) gives a 46% reduction in radiating power output. The remaining strategy for dealing with the heat is to shield the parts concerned so that radiated heat is absorbed by an intermediate part. Such heat shields are very effective but need to be at such a distance from the exhaust so as not to touch in service.

Written by Wayne Ward.

In this article we shall look at a technology which is relatively new to motor-racing, although it has been used successfully for decades in the manufacturing industry, and to a limited extent for exhausts. Hydroforming is a process which stretches a pre-form (this can be as simple as a piece of straight pipe) to a given shape.

Whilst this process was, until fairly recently, the preserve of those making small runs of prototype exhausts, it is now finding favour for some limited production aftermarket exhaust systems, especially for the motorcycle market. Again, Corvette are among the avant-garde here (let’s ignore those shouting ‘leaf-springs’), using hydroformed headers on the LS7 engines in the C6R production car.

The first application of hydroforming (certainly that I’m aware of, and I await the barrage of comments telling me that I’m wrong) was used to manufacture two-stroke exhaust systems. Leon Moss at Ledar in England was certainly using this technique around 20 years ago to manufacture expansion-chamber exhausts. In this process, two identically-shaped pieces of sheet metal are welded together along their edges and then ‘blown’ to shape by filling the gap between the sheets with oil, or water is also commonly used, under high pressure. The resultant expanded shapes are basically bent tubes of varying diameter which are then cut into sections and re-welded to form the finished article. No mould tools were involved, and by today’s standards the process was quite basic. There was little stretching of the material, which is one of the fundamental differences to many of today’s processes.

In more modern applications, the hollow exhaust pre-form is inserted into specially designed mould tools, and expanded to fill the cavity in the mould, which is no longer limited to a basic circular cross-section as per the ‘early method’. It is common for the perimeter of the mould section to be greater than the external perimeter of the corresponding pre-form section, and the material is then stretched. Where ‘excessive’ stretching might be required between the pre-form and the finished part, there might be an intermediate stress-relief anneal stage and two sets of tooling used.

The possibilities for the hydroforming part of the exhaust manufacturing process are many – one type of hydroforming uses a sheet pre-form and the work-piece is expanded into the mould by filling a cavity above (or below) the work-piece with the working fluid. This is an alternative method for making parts which might normally be produced by pressing. It can offer an improvement by eliminating stretch marks often seen on deep-drawn parts. As is evident from looking at the Corvette LS7 headers, multiple branched pipes can be made from a single pre-form, and therefore requiring only one (albeit very long) weld. On production vehicles, flexible EGR pipes are often made with hydroformed flexible ‘bellows’ sections. Aside from the manufacturing advantages, people have used this technique to fit exhausts into spaces where packaging is very tight. It has been used in Formula One for this reason.

Written by Wayne Ward.

In the article dated June 14, we mentioned pressed mends as a means of being able to produce very tight radii where space constraints require this. Of course, there are other applications of pressed parts in exhausts, especially where the space envelope is tight. As we know, the aero departments have a great influence on the design of the car and the tight spaces left after they have draped their desired bodywork shapes over the chassis, engine and gearbox mean that our pipes are often, by necessity, quite contorted. In this case, pressed and welded exhaust sections can offer us the chance to use different cross-sectional shapes. By flattening the exhaust pipes locally we are able to fit within spaces where a number of round-section pipes might not. In view of non-circular pipes, we need to be careful not to flatten the profile too much without consideration of the results. If we don’t have the luxury of CFD analysis, then here we need to consider hydraulic diameter. Many people are not familiar with the concept of hydraulic diameter. It takes into consideration the flows in non-circular ducts and equates this to the behaviour of flows in a circular duct. This circular duct has a smaller cross-sectional area than the non-circular pipe and therefore has higher pressure losses. The calculations are based on the diameter of this analogous duct – the hydraulic diameter.

If we take a circular pipe and compress it in one direction between two flat plates, it distorts to form a shape that is often called a ‘racetrack’ (more like Indianapolis than Spa!), similar to the profile of the pressed sections we have discussed. Whilst we would all accept that the internal cross-sectional area of the pipe remains pretty constant, the effective flow area does not behave as such. For regular polygons (equal sides and angles), the hydraulic diameter is taken to be the diameter of a circle inscribed inside the wetted perimeter of the section. For other irregular sections there is a (thankfully simple) formula to allow us to calculate this:

Dh = 4A/P

where:
Dh = hydraulic diameter
A = measured cross-sectional area of the duct in question
P = wetted perimeter of the duct

Let us work through a trivial example of a circular tube of 50 mm inside diameter and compare this to rectangular section duct with an aspect ratio of 2:1 of the same area. The cross-sectional area in both cases is 1963 mm2. We can therefore calculate that the sides of the rectangle are 31.33 mm x 62.66 mm. The perimeter of the rectangular section is 2 x (31.33 + 62.66 mm) = 188 mm.

The hydraulic diameter is then 41.76 mm, which is only 83.5% of the diameter of a circular pipe of the same cross-sectional area. We can make an estimate of flow-losses from textbooks on fluid flow due to this decreased equivalent diameter. It should be noted that this effect is not always negative, and irregular cross-sections have been used to control and mitigate the effects of other flow phenomena….

The June 14 article also implied that the section of the pipes would be round immediately after the cast flange which joins the primaries to the header pipes. This again isn’t necessarily the case, and a number of teams use quite a length of fabricated transition on each primary before finally coming to the round section. Too quick a change of section can lead to separation of the flow from the walls, and hence to higher pressure losses in the system. The result here is the same as having a section of pipe of smaller cross-sectional area.

Written by Wayne Ward

Whilst there are a lot of exhaust systems still being made in steel, it isn’t as widely favoured as it once was for a number of reasons. The obvious one is that it corrodes very easily. Welding steel to stainless is a good way

to make the metal corrode even more swiftly than plain mild steel, and one motorcycle manufacturer discovered this very costly mistake on production motorcycles. If you are going to be welding different materials together, you should be sure of materials compatibility and corrosion beforehand.

Possibly the most common material choice for racing exhausts, although not in F1, is stainless steel. Austenitic stainless such as 304 is a popular choice for all manner of exhaust systems for everything from tuned road cars and motorcycles through club racing to Le Mans and beyond. This material has good corrosion resistance compared to mild steel, but is not so easy to work, owing to the rate of strain hardening. If there is very severe bending required, it may be necessary to stress-relieve the material by annealing part of the way through the forming process. There are other stainless materials used commercially such as 441 which is a ferritic grade which has a good degree of corrosion resistance coupled with greater formability than the austenitic grades. Austenitic grades of steel also suffer from galling, which is a micro-welding process where joints between similar or identical materials with sliding movement become cold-welded to each other with very little force applied, and this phenomenon is not limited to exhausts, or to stainless steel.

Titanium is very widely used for exhausts on motorcycles, although Formula One has largely shunned this material, and there is a good reason for this. In general, motorcycles support their exhausts with more than one support. Their pipes are obviously fixed to the engine at the cylinder head, but they also benefit from further supports either underneath the engine, or close to the end of the exhaust system, or possibly both. This largely mitigates the bending stresses which would be present if the extra supports weren’t used. The main advantage of titanium is its low density. With around 40% lower density than stainless steel, the advantages are obvious, especially for motorcycles. These exhausts are made from standard titanium materials, although some with increased high-temperature capabilities are available and have been tried in Formula One, although without success. In the same way that sportscar teams support their exhausts (especially those using turbocharged engines) to reduce the bending stress at the head, this technique could be employed to make titanium a realistic possibility for Formula One. Let’s not forget that the turbo Formula One engines used exhaust supports so there is a precedent for this measure. There seems to be little reason why a titanium tailpipe couldn’t be used, and perhaps some Formula One teams already do this.

Now we turn to Formula One, and the materials of choice here are the high-temperature ‘super-alloys’ containing lots of what we often hear called ‘strategic elements’. These elements which are used by the military and coincidentally are the ones that attract the ever-unpopular alloy surcharge, namely Nickel and Chromium. Inconel 625 is the popular choice, although there are some good alternatives available with potentially better performance, and certainly some teams already use these alternatives, with others looking at newer superalloy materials.

There is some evidence that non-metallic materials have been tried and tested for some exhaust components, although this seems to have been limited to small-scale testing in Formula One. There is no doubt that the material in question is impressive, and it seems to have a natural niche in aerospace and defence markets.

Written by Wayne Ward.

In Formula One however, there are much tighter constraints on the exhaust, particularly in view of the dominant position of aero in the overall design of the car. After all of the other necessary components are in place, namely the radiators and coolers, there is little space left in which to cram the exhaust system. This is the reason why we see the beautifully crafted but tortuously curved systems on the ‘spy-shot’ photographs we see on engines at the track. (It is quite common to equip show engines with dummy parts, including exhausts, in order to mislead the competition and, having been in the fortunate position of working on more than one manufacturer Formula One engine programme, I have seen that this is indeed the case).

The Formula One exhaust system is incrementally forced into smaller and smaller spaces each year, with pressure on the exhaust manufacturers to make the systems ever lighter and more durable. Leaving materials choices aside for the moment, we will look at the traditional manufacturing methods used for a typical racing exhaust system. In general the exhaust ‘stubs’ which are used to mount the exhaust system to the engine will be investment cast or, in some circumstances, fabricated by hand using a mixture of tube, sheet and machined components. The casting route is more precise and allows the exact form of the transition from the shape formed by the port at the cylinder head exit to the diameter of the first round section to be managed precisely. This ensures that the area schedule decided by 1D simulation can be adhered to closely.

Once the cast transition has reached the round section to which the main primary pipes are welded, we are then into the realm of the fabricator’s artwork. Each time I look at a set of primaries I have to admit to being very impressed, and this is true of those systems still made by the teams themselves, as well as those made by the few excellent external suppliers that supply the Formula One teams. Until relatively recently, the remainder of the exhaust system was made of a collection of bent tubes both in Formula One and other formulae. However, as space restrictions have become tighter, particularly in Formula One, the fabrication methods have become more specialised. Individual bends are sometimes made as pressed parts to allow a much tighter bend radius than can be achieved by tube-bending equipment. The process involves pressing individual halves of bends from flat sheet and welding them together to form a tight radius. It is possible in this way to form a centre-line bend radius which can be as low as 70 percent of the tube diameter, i.e. a 42.5 mm bend radius on a 60 mm diameter tube. I certainly wouldn’t suggest that such tight bends are commonly used, but they are possible. It is common for an Formula One exhaust to have one or more steps in the exhaust and these are formed either by fabrication or by having sections cast. The size of this step and its severity change from team to team, but they seem to be universal at the moment.

In general, the collector will be fabricated from a mixture of tube and sheet, with various degrees of internal complication. The collector is generally made complete with the tailpipe, and these can be of various designs ranging from a simple tube to a section which changes constantly to form an elliptical exit.

In future articles we shall examine some of the other design features, more modern manufacturing methods including those not yet in widespread use in Formula One and materials choices.

Written by Wayne Ward.

As a result of this development and in reaction to Peugeot’s more adventurous 908 design, Audi began work in autumn 2007 on their R10 replacement, the R15, which debuted at the Sebring 12 Hour earlier this year.New this year is the R15’s powerful V10 Tdi engine which is both smaller and lighter than the 12-cylinder engine it replaces. This is the basis of an improved car package, which takes a more radical approach to LM P1 aerodynamics. Fundamental differences can be seen both front and rear and whereas the two exhaust pipes of the R10 exited from the back of the car, the R15 exhausts exit through the upper rear bodywork, echoing the current trend in Formula One.Ulrich Baretzky, Head of Engine Development at Audi Sport said, “The exhaust [shape] is determined by the total package and the aerodynamics.” It seems that due to reduced space at the rear of the car (in spite of the more compact engine), rerouting the exhausts became expedient and in any case Audi also claims that by rerouting them in this way, the gases exiting the pipes are channelled through the rear wing set-up, which in effect has an additional aerodynamic function. The gases do speed up the air flow above the bodywork as Wolfgang Appel, Head of Vehicle technology, Audi Sport confirms: “Because of the higher flow velocity (energy) the rear-deck-gurney is impinged in a better way and so [it is] more efficient.”

This comprehensive replumbing has enabled the Audi engineers to substantially reduce the weight of the exhaust system by shortening the outlet pipes, which are now made from titanium, instead of stainless steel. Although the engine is the same capacity, that is 5.5-litres, it has two less cylinders and therefore two less outlet pipes from the engine. Asked if the exhaust system from the engine to the turbochargers is the same as the R10, Baretzky commented, “Lets say in principle, yes.”Although Audi is remaining tight-lipped as regards the exhaust outlet diameter, Baretzky explained that this was determined by two main factors namely, backpressure versus mass flow and, the geometry of the diesel particulate filter.However, some of the biggest changes to the R15’s exhaust system are to be found in the particulate filters themselves. Over the past three years Audi has worked closely with Dow Automotive to develop their advanced lightweight, compact ceramic technology particulate filters, with proven results. Baretzky again: “This was the only company that could offer us very advanced ceramic technology with lightweight, high efficiency [particulate filters] and a very low backpressure. It is difficult to find all these factors combined in one product, and Dow wanted to prove their new technology in the hardest environment they could imagine, and that is Le Mans.”The Audi/Dow particulate filter works in such a way that the gas enters on the engine side, while the opposite side is closed, which means this is not a so-called ‘open’ system. Some manufacturers provide an open particulate filter, but this does not offer a true total purification of the gas, up to 100 percent, as there will always be particulates that escape through the open system. In the Dow system, 100 percent of the particulates are trapped in the filter as the exhaust gas has to pass through the extremely thin ceramic walls within the particulate filter. “The porosity in these walls of the particulate filter defines on the one hand the backpressure, and on the other hand the percentage of purification which in the end, is 100 percent. And this is the technology that is the secret of this particulate filter,” Baretzky explained.

The key technology that Dow has developed in the last few years has made this system not only stable but very lightweight and very effective in terms of cleaning the exhaust gases. “You can come and wipe inside our exhaust after the race at Le Mans once it has cooled down, to see how clean it is. We do this regularly every year, its very impressive,” Baretzky added.