There has to be more to it than that. There is no way BMW would use a more restrictive exhaust on the 328 and M3 than on the 323. Virtually everyone (except Hawken) has said that the 328 and M3 exhausts flow much better than the 323 exhaust. Perhaps overall diameter isn't the whole story, and that by having a larger overall diameter in the 323 exhaust, scavenging and velocity is hampered, thereby reducing power. Everyone says that a 328 or M3 exhaust on a 323 is the single best performance modification in terms of gaining power.
If the 323 exhaust was superior, we'd all be putting 323 exhausts on our cars... Besides, if the 323 exhaust is 2.5 from the cat back, and the 328 has two 1.5" pipes, that's a half-inch more total from the cat back compared to the 323. Am I having problems with my maths? The 323 most definitely does not have a nearly 5" single pipe from the cat to the muffler.
Last edited by ZeGerman; 02-15-2013 at 01:30 PM.