Exhaust system selection is more crucial than header choice because performance and noise levels are of utmost consideration. Cabin noise levels are more important than performance because excessive cabin noise will damage your hearing. Exhaust pipes must be the right size. Mild street Mustangs need 2 to 2 ¼-inch pipes. They do not need 2 ½- or 3-inch sewer pipes. When you start courting 2 ½- and 3-inch pipes, fit becomes a huge challenge. Keep it simple and modestly sized.