New 3" Exhaust / Car Now Backfires

[84SVOGARY]

JUST HAD A CUSTOM 3" SINGLE EXHAUST INSTALLED WITH DOWN PIPE AND NOT CAT. IT HAS A 3" DYNOMAX MUFFLER. SO FAR SO GOOD. NOW FOR THE BAD NEWS. THE CAR BACKFIRES IN BETWEEN SHIFTS AND WHEN DOWNSHIFTING. IT EVEN BACKFIRES ( RAPIDLY ) IN IDLE. THE CAR IS A MOSTLY STOCK 84 AND THE 2" EXHAUST THAT WAS ON IT BEFORE IDLED SMOOTH AND HARDLY EVER BACKFIRED. ANY HELP APPRECIATED.

[AT6svo]

Backfire

Welcome to the world of no exhaust backpressure. Fairly sure this is a somewhat normal phenomena. My 84 backfires fairly regularly upon down shifting and occasionaly at ilde, esp when its cold (i.e running rich). I am running a ported E6, stock dp, stock cat with no muffler and about 1 meter (3 feet) of pipe off the cat.

There was a thread a month or two ago about this very same subject as well (backfiring).