RE: What pipe to use?
the diameter affects the strength of the back pulse, the length (to the high spot) affects the intended RPM band. One is trying to creat a tuned exhaust restriction that creates back pulses to jam more mixture into the cylinder just before it closes the exhaust port. so, if you have a giant pipe diameter on a small engine but with the right length the "pulse" of the engine will "get lost" in that giant ole pipe and have little effect. using too small of a pipe will have a hyper effective pulse coming back and more back pressure (again think of the exhause pulses like a slinkee toy laying on the floor with one end being "pumped, no giggling!). I think the smaller pipe will work, but will have a bit of restriction. think of pipe sizes as a function of the area of a circle and how that relates to displacement and the sizes will make more sence. I'd use the 15cc pipe and cut it to the length you want, if it's ultimate rpm you want look at the noisy pipes or the marine ones.