Originally Posted by Andy

No idea, but here was my take on exhuast piping size. Think of a river, where it's big and wide, the water run slowly, where it tightens up, the river flows faster. Too big a pipe, exhaust slows down, too small a pipe, exhaust gets chocked and you don't get as much exhuast out as you could. Right sized pipe, exhaust flows quickly and efficently (at least for certain rpm's as more exhaust cycles means more exhaust gas). No idea if it's true or not, but just some dumb thing I thought up one day.