NASCAR exhausts

Thursday, March 27, 2014

Tags :  exhausts

The last time we looked at NASCAR exhausts was a year ago; at the time, some of the focus from the individual teams and the exhaust manufacturers would have been on reducing the mass of the exhaust system. A big, heavy stainless system would be a real handicap for teams trying to reach the reduced minimum weight limit. In the film ‘Snatch’, the line “Heavy is good, heavy is reliable” refers to guns, not NASCAR exhausts, and while some people might apply the same to exhausts, we might add “Heavy is slow” in the context of the current regulations.

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

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