If you're running duals, there's no point in going bigger than 1 3/4" or so. Each cylinder pulse goes through a different pipe, so there's no way to "stack" two exhaust pulses down the same pipe. Going too big slows the velocity down and reduces scavenging on a 2-pipe system.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!