The cat-back system replaces everything from the catalytic converters back, hence the name cat-back system, it consists of flowtubes (from the H pipe to the mufflers), the mufflers, and the rest of the pipe out and new tail pipes. An H or X pipe is where the cats are, the offroad versions have no cats, and are not emmisions legal. Some H and X pipes have only 2 cats and are 50 state legal.
Running just a cat-back system with the stock H pipe will be quieter than a system with both an aftermarket H or X pipe and a cat-back exhaust. A cat-back system by its self should provide anywhere from 5-15 HP depending on the other mods you have on your
Stang.
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Jesse Berger
90 LX, Previous 4 cyl. (Decommissioned, 351W on the way)
H/C/I+Bolt Ons
Best ET: 12.54@106 mph
2001 V6 5 Speed Daily Driver
Windstar Intake, Dual Exhuast, CAI, Headers
Best ET: 15.6@88 mph (stock trim)