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After many years of racing, countless hours on the dyno and many exhaust
systems in the scrap heap, I picked up on something interesting that was
later confirmed through both empirical measurement and observation of other
successful systems. That "something" is that an efficient exhaust system
will have two attributes. It's internal bore will be sized to the horsepower
of the cylinder it is connected to (28 Hp per square inch) and it will be a
constant bore for 28"-30" before ending or transitioning. The remarkable
thing is that the length rule is true regardless of engine RPM. I suspect
that this is an effect of the sonic flow past the exhaust valve upon
opening.

Most aircraft exhaust systems are garbage from a flow standpoint. Any
fiddling with the shape or angle of the collector discharge will have little
effect on flow. It is akin to trying to cure diarrhoea by reshaping the
toilet. While it is possible that in some installations increasing the
backpressure at the collector discharge will improve the overall flow
coefficient at the exhaust valve, these improvements would be eclipsed by a
proper tuned exhaust. I have seen this on the dyno when we were testing
rotary engines. Our test exhaust system had water injection into the
collector for noise abatement. Horsepower increased by 3% when the water was
on. This was not the case when the tuned racing exhaust system was fitted
(we tried it).

Drag is another story. Streamlining of the exhaust discharges did cause a
small but noticeable effect in both speed and cooling on my airplane. Seeing
those long exhausts, at the Lancair tent, sticking into the airstream caused
many head scratches. How many knots are you willing to give up for a clean
belly.

On the topic of thrust, there isn't enough available energy in the exhaust
to contribute significantly to thrust. Fred Moreno and I ran the
calculations once and determined that a perfectly designed exhaust would add
about 2 pounds of thrust to the approximately 300 pounds generated by the
prop at altitude. Good for less than a knot, if you could measure it.

When noise regulations first came out in sport racing, we were campaigning
one of the faster and quieter cars. At one race we attached oddly shaped
aluminum baffles to the end of the exhaust pipe. Theories in the pits were
rampant and the next weekend several competitors sported similar, useless,
appendages. We then, of course, removed the originals after qualifying and
ended winning the race. Our competitors spent precious race prep time
considering and fabricating the "do nothing" parts while we spent our time
preparing the car.

Unless you are prepared to do controlled, back to back tests, don't bother
chasing rumored speed. Spend your time maintaining and inspecting your
airplane. Do not be lured by Zerbachian claims and rumors.

The nice thing about racing is that it is an absolute. Either you win or you
don't. Every looser has an excuse. "So and so would have won if it hadn't
been for bla bla bla....". When I lost to what I thought was a slower
airplane in the '97 Cross Country Race it was because he had a better plan
and was the better pilot that day. I lost and he won. Next time may be
different.

Regards
Brent Regan

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