Quote:
Originally Posted by Figjam74
IMO, backpressure IS always bad. Backpressure increases your pumping losses. The trick however, is minimize backpressure without sacrificing exhaust velocity. It become trickier yet when you realize that the amount of exhaust gas you're tuning for varies with engine speed.
Absolutely correct.
At full noise (19 000 RPM) a current F1 engine (2.4 litres) puts out about 11 400 litres/minute of exhaust gas most of which is put to use aerodynamically to fine tune the rear wing and other bits and pieces the wind tunnels tell the designers can be used for downforce such as when exiting a slower corner.
I would have assumed that F1 cars were using short exhaust pipes due to the wide range of RPM's that they need to run at, while the Nascar teams can tune for specific RPM ranges that the cars typically run at. I'd also imagine that weight is a bigger factor in F1, and that NASCAR is supposed to be 'Stock' car racing, and rules would require a full exhaust system.
Also correct on both points.
NASCAR is slightly different since the engines run at fairly much constant speeds over a narrow rev band and virtually all of the action takes places at the top end.
As far as using the exhaust as an aerodynamic aid, my guess would be placing the exhaust somewhere in the middle (taking a hint from Lamborghini here)
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The Lambo is using a set of venturi ducts either side of the engine and inboard of the rear wheels so the exhaust pipe is in the middle as a result.
I suggest the volume of air travelling through the venturi ducts is the dominant factor and it may even help the exhaust by creating a relative vacuum either side of the pipe.
Pete.