Does your x-pipe have cats? If not you could be hearing one of two things:
A) Not enough backpressure, so you get small lean pops.
B) You have a small exhaust leak somewhere, and cold air is rushing into your exhaust pipes and causing a backfire sound.
If the nut on my EGR valve is ont all the way tight, my car will backfire when I let off the throttle. If I get on it up to 6k and shift, it sounds like an M-80 went off in my back seat.
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1994 Mustang Cobra
308ci, Vortech S-trim 11#, ported GT-40 heads, E-303 cam, FMS 1 5/8 headers, BBK Offroad H-pipe, 2-chamber flows, and several other goodies.
Rebel Racing. Home of Bay Area Mustangs