I actually found the stock one to work amazingly. I had a problem during the winter with the car lagging. I had an inncident in the snow and I thought I cooked the cat. It just didn't feel right. So I threw it up on a lift at work, and I did a back pressure test in the bung that's before the cat. I forget what the stock backpressure is supposed to be, but it was so free flowing that the meter barely moved off of zero.