I know what the guy's trying to say, but he might want to word it a little differently. Exhaust scavenging plays a large part in the shape of the power curve. While back pressure is never a *good* thing, exhaust system diameters that are larger than neccessary for an engine's particular state of tune can create exhaust gas velocity so low that they affect scavenging, and cylinder filling. Obviously, engines tuned to develop their power in the upper part of the band can utilize larger diameter tubes without severely reducing gas velocity. Milder engine with broader torque bands probably would do better wiith smaller diameters. Hopefully, I've said it a little better than the Clifford Performance guy.

Bob