i have a 93 rx7, with a hks DP, hks n1 catback, and apex turbo timer, when i let off the gas i get the gun shot sound, why is that?
its driving me crazy!! please help........

 

It's called a back fire. Snicker snicker. It is caused by large amounts of gas being ignited while in the exhaust pipe. I would love to drive behind you to see the 3ft flame you must be throwing. I would guess that it is mostly caused by the combination of the N1, and the DP. The main cat should prevent the back fire, but I guess you are flowing too much exaust for it to do any goo.

 

Do you still have a working Air Pump? If so you must have a poorly working Cat.. I have the same DP and I don't get large backfires. Or you are running very rich or your "Anti Afterburn" solenoid is not working, this cuts fuel on deceleration. Nice little trick we used to do with carburated 10A's and 12A's is rev the engine to 7000 in neutral and turn off the ignition - about 30sec to a minute later there was a huge backfire with flames and everything! Don't try this with a 3rd Gen.!