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Exhaust materials and their effect on design

Exhaust system design is a complex affair in some racing categories, especially where packaging around the engine is tight; motorcycles and single-seat racecars can be particularly challenging in this respect. If tubes are too small, have the wrong cross-sectional shape, are bent too tightly or are bent through too great an angle, engine performance can suffer significantly. On the dyno, many engine suppliers test with very durable dyno exhaust systems that are fairly straight and easy to work on, but where the exhausts from the car or motorcycle are used, there can be noticeable differences in performance.

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

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