I'll have to agree with HiFlow50, it really isn't a good idea to gut the cats, but i've never heard any specific dyno numbers on whether you lose power from gutting them or not, some people say you gain power, some say you don't, i think that magazines are reluctant to dyno testing on that because they'd get alot of sh/t for doing it from the EPA or other emissions enforcers (environmental groups or what have you) It's a better idea to get an offroad h-pipe like he said because with the better mandrel bends and consistent pipe volume it has been PROVEN in dyno tests to increase power, plus they're pretty inexpensive (usually between $110 to $160). All i know is gutting your cats will make it louder, i was just guessing on how much you COULD possibly gain. I've heard dyno tests on removing the smog pump that you do increase power because of a fairly high parasitic drag at higher rpms and because a smog pump does move ALOT of air, just look at the inside of one, which would explain the drag, it's also a reason why people, even people who drive on the street alot, convert from power steering to manual because there is a slight weight reduction and also because the engine doesn't have to turn the pump, this is also a principal idea of pulleys, they reduce the amount that the accesories spin and thereby reduce the amount of drag they put on the engine. But staying emissions legal will keep you from getting in trouble. If you want to keep your car relatively stock and want an easy install, 1 1/2" shorty headers work better than stock and i've heard increase torque as well as horepower all accross the power range on a stock engine.
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previous name: 84stangLX
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT