Marc, what you are talking about will work for piston engines, rotaries just like low back
pressure. When a piston engine with a small diameter head pipe is at max rpm or max power, you can
often use the paint trick to locate the standing wave in the pipe. For most piston engines under 2
liters with around 10K max rpm this length ends up around 18 inches. Most of the time it will only
work with straight pipes. without a collector. many times it doesn't work at all. Most of the
designs we use to route the exhaust within the cowl don't have long enough head pipes anyway.
Rotaries really like a wide open exhaust. The optimum rotary exhaust would probably be a
completely straight megaphone sticking out of the port perpendicular to the engine. The length
would set the optimum rpm/powerband. Such a exhaust system on a rotary would be so loud that it
would hurt people used to listening to top fuel dragsters without ear plugs. Your best bet is
probably some variant of the square pipe system Paul and others have been talking about. Low back
pressure while muffling is the key. Good Luck. Bill Jepson