x-pipe

Maybe 3hp @ the wheels, put some longtube headers xpipe and cat back on there, that's good for around 40hp (depending on what longtubes you go with. Oh and have it tuned

For a stock (or near stock) normally aspirated engine, especially an NPI (pre 99), the stock H-pipe is simply not that much of a restriction even peppered as it is with cats. Like fast97 says, the real gains in exhaust systems are long-tube headers and even then it's not so much because they are free-flowing but rather because their length and primary tube-size tunes the exhaust and promotes better cylinder scavenging.

Change to an X or H midpipe if you want to change the sound character of the car, not with an eye toward big HP gains. If you have emissions inspections in your area either go with a catted pipe or retain your factory H to re-install if you have emissions hassles. (I'd personally recommend holding on to the stock H-pipe no matter what, resisting the temptation to sell it on eBay or something...)

Sharkys97Stang said:

Really! Thanx u just saved me some time and money. Would it pass smog if i went with headers x-pipe with catback?

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Depends on where you're located, what sort of testing they perform and the quality of parts you choose. LTs can be pretty finicky because:

a) Being long, thin-walled tubing, they tend to bleed heat off and the cats don't tend to run as hot and thus not as effectively.

b) LTs naturally move the cats further away from the engine exacerbating the heat-loss issue.

c) Catted X-pipes intended for use with LTs are often "high flow" types that have less surface area -- larger cell sizes, shorter lengths etc -- and are simply not as effective as factory parts at scrubbing the exhaust.

In some places removing the factory H-pipe is considered tampering with the car's emissions control equipment (and it kinda is...) and the car may fail the inspection part of the test, before it even hits the rollers (if you have tailpipe sniffer testing...)

Check local speed shops and local Mustang guys (club?) for their experiences.