Brett Jenkins

While I have the crossmember off to pull the oil pan and check the bottom end of my engine, I'm thinking about installing the headers that came with the car. Obviously, they are longer than the stock manifolds, so I can't use them with the stock cats. If I buy a couple hi flow cats, and take them and the headers to an exhaust shop, along with the stock manifolds and cats for reference, they should be able to build me something, right? What size cats should I buy as far as ID/OD, etc? S4 engine/S4 exhaust.