The answer may surprise you, concerning OEM mufflers and cats, the mufflers are the bigger restriction! This can partly be true to the giant leaps in catalytic converter efficiency. Back in ‘74 when they were first mandated for every vehicle they were horrible, bad design and did nothing but robbed horsepower (you could delete the cats back then and expect atleast 10hp if not much more). Today the screens in the cats (and even better aftermarket high flow cats) offer such ease in flow that replacing them with a straight pipe would only give a slight increase in power. Whereas mufflers today are still designed in nearly the same way. To quiet the roar of the engine for grandma. Couple a glasspack with a good cat and good pipe and you’ve got a great free flowing exhaust.n