a smaller diameter pipe will add low end power., headers for street driving use small diamiter primary tubes and a longer collector, my mpg's did go up when i installed them, and the race headers use bigger primary tubes and shorter collectors and hurt low rpm but add highend, like frank said in and of itself it might not work out that well, as your engine from intake runner length/diameter,to cam lift/overlap/duration/advance,to compression ratio,valve size,and exaust all need to be matched (with-in reason), exaust pulses at certain rpm's are a certain size and velocity ,and when they all follow each other out without "bunching up" (to big of a pipe), or running into each other (too small of a pipe) it is more efficent. ,engine designers have to design the engine to operate over a broad range of rpm, if we never go above 2000-3000 rpm ,i'll bet we would see cars with much smaller pipes, ,,,,,But i could be completly wrong about all of this!
