An X or H pipe. The reason I'm asking is I had Mac l/t's with a prochamber and Mac cat back. It sounded good and really raspy, not to loud at cruising considering having l/ts. I removed the l/t's becasue they were interfering with the 7 quart oil pan, so I installed FMS shortys and a Mac off road h-pipe. All I can say is its just to damn loud, and I don't like it, it just doesn't sound like a 5.0 mustang. I was thinking about installing some convertors or just ordering a x-pipe with cats. Does anybody but know if the b-303 sounds better with an x pipe?