I have a 351w in my fox and my guess is, it makes between 420 and 440 at the flywheel. I've figured 2.5" exhaust is enough. It has 150 shot on it....at what point does it need 3" exhaust. I've heard at 400 it should and then I've heard at 500 its time to go bigger. It's a street car with Mac 1 3/4 long tubes, x-pipe, & spintech mufflers. I shift it at 6200 with the factory cpu. Attached is a article I found but I just cant see a 400hp car needing 3" exhaust. Am I wrong?