Mufflers don't add horsepower. I don't care what the companies say, the only ones that give any gain is magnaflow's because they're a straight through, and were talking maybe 5 hp.
Go with what you like the sound of and go to
www.mustangexhaust.com to hear them. You have to sign up, but there are a ton of different combos on there.
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1997 Mustang GT "The Freak" - 13.80 @ 101.70, 2.07 60'
1995 Honda VFR750 - not much @ really fast (actual data pending.)
1964.5 Mustang 289
Rice Haters Club Member #13