I have run different ID's over the years and stuck with a 3" ID and a 4" OD on the muffler. I used glasspack mufflers and welded up the sidepipes with chrome turnouts. It might not be the best looking pipes but they sound awesome and perform great. The bigger the ID the lower the exhaust tone, and the the smaller ID will give a higher note.On your size motor I would'nt go smaller than 3"ID
