It all depends on your motor setup. if I were to put a 2.5" muffler on my lawnmower it obviously wont make optimal power, same goes for my mustang. I wouldn't stick a 1" muffler for a modded sbf. but that is just the obvious stuff. Every setup has its ideal part. Like what was said before, it all depends on how its built. but it is known that the lightly modified 5.0L mustang loves some back pressure. to give an idea to how much it likes it is when you run just mufflers and dumps, the car will feel different with just tail pipes. there are many variables and each car is different. But all in all some back pressure helps. It is just how to find the fine line that separates you from getting all your torque to robbing power.