What year is the car, and what transmission does it have?
The first thing a Mustang needs is gears. Forget the custom chip for now. Advance the timing, throw on a set of pulleys and a K&N. The cold air kits really are no better than using a panel k&N with no silencer.
Then add headers, an H-pipe, and a cat-back exhaust system.
See my car at:
http://www.mustangworks.com/cgi-bin/moi-display.cgi?179
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1988 Mustang GT
11.8 @ 123 (7/14/01)
1992 BadAzz Wrangler
1993 Explorer
See my car at:
www.mustangworks.com/cgi-bin/moi-display.cgi?179