[scottj]

DaveR,

The 3" core I had was louvered like Blake discribed and while the pipe diameter was 3", it only measures 2-3/8 or so when you consider the protruding louvers. My pipes are a true 3-1/2 straight through.

The difference I noticed was a slight carb stumble and fewer revs than with the 3-1/2's.

If your going to do it and want some muffling, go with 5" pipe. 4" pipe was a whole lot of work for next to no muffling at all.