Backfiring
 

I have an 87 with a brand new 300 crate motor. Everything runs great but
i can't seem to figure out why it backfires when your crusing at 55-65 and just let off the gas all at once. I Just replaced the exhaust system from the cat back I put a 3" pipe with just a glasspack. I was wondering if could be not putting enough back pressure on the engine. Any help would be greatly appreciated.

Sounds like you are running a bit to lean. This commonly happens when a free flowing exhaust is installed. If you are running a 2 or 4 bbl carb, step up the jets a couple of numbers. You can also advance the timing a degree or two, not much though.

Also check that you exhaust is tight from the engine back to the muffler, for if you are sucking in air thru a gap, it will cause it to backfire from the fresh air being introduced into the exhaust mixture. 8D

Its fuel injected I might try changing the timing to see if that would help but I know the manafolds have few cracks in them but I was giong to go with a set of headers as soon as I could afford to thanks for the input

Check fuel pressure. Could be a bit low causing lean condition...