Thanks for throwing this out there, SBC. I've been wondering the same if there is an actual formula to figure the ideal sizing. My mechanic suggested the 3" for my 402 but I always wondered if that hurt the velocity/scavenging. Switching from 2.5" with glass-pack side pipes to 3" with flowmaster 40's took 0.3-0.4 seconds off my 1/4mi ET but how much from the mufflers and how much from the plumbing? I don't know.