I asked a related question about PowerFlow Exhaust systems, which produced another question for me: How do they go about finding the resonance frequency so they know how long to make the exhaust pipe?
I guess I'm interested in the technical aspects of it. What formula do they use? Do they have special equipment?
I think it's interesting that they now offer a "short stack" for some mods:
“Classic” or Short Stack? The Classic is our original design from 1999. It is the “ideal” tuned length for this engine. Everybody loves the performance, only a mother (or an engineer) could love the looks. So, we came up with a compromise: The Short Stack is virtually indistinguishable from the stock Cessna exhaust and will give you about 85% - 90% of the performance gain of the Classic in a much more attractive installation.
Is this one of those instances of the 80-20 rule (approximately)? It seems interesting that a similar length gives you 85-90% of the gains. I'm guess then, that it's not just about the length of the new exhaust, but also the width?