Well, when I added headers to my car. I'm assuming I used a size too big because my car seems slower. I don't know, it's probly just the 143000 miles on the 305. But anyway, When I put them in, i took it to a shop to have them make me a custom Y-Pipe. And by the looks of it they used small pipe. It appears to me to be about 2.5in. I'm not sure. I havn't measured it. They did however get rid of the T intersection that came with the stock pipe. So I would guess that it pretty much evens out after that. One other thing is...how do you know if your cat is even working? I have hit it to make it shake and it makes some rattling noises. When they put it back on, they welded the air line bad and that broke, so the cat isn't getting any air. I need to get it fixed. So, would that size of Y-Pipe effect much? O forgot to ask about pipe size when I went in. STUPID ME!!!!