I have a Flowmaster 80series catback exhaust on my car. It was made for the z28 and I modified it. It is 2.5in out of the cat, then we welded a cupplet which was 2.5in inlet, to a 3in outlet, welded the 3in exhaust pipes to it, then the muffler has 3in inlet, 2.5in dual outlets. Sounds NASTY now. I have a vid of when I first put it on. Once its broken in, it sounds amazing!