You're referring to what's called "tuned exhaust".... what you do.. is... you take shoe polish, and you wipe a fkn line every 8 inches from the collector on the extension.... or about 3 line...the last line of shoe polish that burns off.... is where you would attach the muffler....imo that has NOTHING to do with the diameter required...smaller would probably move that line further down the pipe even..'more heat'. Need to know what you're doing and not let some Ck biter steer you queer.
