I think Dan hit the nail on the head. Funny, the first tuned pipe was actually an accident. I remember reading an article where an old fashioned motor cycle had crashed. During the crash, the exhaust bent, creating a shorter pipe. The shorter pipe created more power at a higher RPM and the cyclist had more top end.

I am not an expert, but apparently the exhaust pressure wave hits the end of the pipe and travels back to the combustion chamber to "stuff" more mixture in. Unfortunately, the pipes are only tuned for ONE specific RPM. Not a bad thing for our boats I guess since we run WOT all the time.

I believe CVEC or some RC car company tried to create a pipe with a spring such that the back pressure was adjusted, but I don't know if it worked or not.