Stock exaust size, I'm pretty sure, is 2.5in. After coming down from the headers and after the cats, it hits a flex pipe. Then going to the "3rd Muffler, which is 2.5 in and 2.5 out goes to a Y-pipe. Going back to 2.5 inlet and 2.5 outlet on both sides of the Y. Then you get to see the ugly non tipped pipes.