This should be interesting ... I don't think you'll see much in the way of an HP increase, I don't the cats are that restrictive. Might be a slight TQ decrease. As a rule of thumb, Harley's like a straight pipe with a 1 7/8" ID with a length of 42-44", for the street. You need a little restriction or TQ fall off the map. Actually, given piston diameter and stroke, a (single) Harley cylinder is very similar to a (single) SB chevy cylinder. The S&S has a bit more displacement and the M3W pipes are definitely longer ... did you check what the ID of the pipes is ? From the pictures, the pipes look 'chambered'. S$S designed the exhaust, the mufflers are basically straight through 'glasspacks'. It's designed for max performance, as is. I think S&S R&D rewrote the book ... unless they share some of their secrets, it's going to be trial and error. Got a dyno handy ?
