I had a BTM welded on by a shop. I thought it sounded retarded right from the start BUT read on...I got so sick of the awful whistling/hissing noise out of it I bought a new exhaust. When I went to change out the stock exhaust with BTM to an aftermarket 4", I noticed the idiot that welded it on put the inlet/outlet on the wrong ends. So don't do that! I can tell you it sounds horrible.