Originally Posted by max427
sounds better than what i said. although i do dissagree that everything after a collector is meaningless, especially on a car with more than one cylinder bank. H and X pipes have been dyno proven to increase power.
And tubing diameter is important on n/a cars to keep the exhaust gas velocity as high as possible without being restrictive... also, changing tubing diameter can hurt, sometimes a lot. changes in velocity, from slow to fast or fast to slow will hurt performance and the more changes in velocity (changes in tubing diameter) the more it will hurt performance. I dont know any formula to determine optimum diameter but i im sure there is one. the 2.5L subaru n/a motors are best around 2.25" ID under 200HP. The guys @ TWE said if aggressive cams are used or other mods that will make power over 200hp, a larger diameter tubing will be beneficial. seems like a rough rule of thumb but probably a good one since they are just about the best when it comes to subaru exhaust.