Banshee, you beat me to the Jim Hand articles on Pontiac Street Performance, but the content is generally what is presented in Jim's book, "How to Build Max Performance Pontiac V8's".

Seems to me that CDJr and 8point hit on the heart of the issue. Depending on the application, whether street, drag racing, or Nascar, the exhaust requirements differ greatly, but the key is the negative pressure or vacuum that is created in the pipe. It's the vacuum created from the hot exhaust gases expanding in the pipe that creates the vacuum necessary for efficient scavenging. (I believe that is why having an x-pipe enables you to take advantage of both vacuum pulses instead of just a single pulse from true dual exhaust.) If the available expansion volume (the pipe diameter x length of pipe) is too great, then the vacuum generated would be weak and the reduced performance would be seen. Reducing the pipe diameter in Banshee's applications apparently strengthened the vacuum pulses and generated more power. I think the required expansion volume would be related to the engine displacement and operating rpm - the rate and volume of air you are putting through the 'air pump' that TOHCan mentioned. I think this explains why the high revving, big displacement engines of Nascar and many drag racing applications would warrant bigger exhaust pipe diameters.

IMHO-

Todd