Took my TD's X pipe off
 

and got me the GMMG. It sounds great but now I have an exhaust backfire when I'm decelerating or at idle during a stop. I know the backfire is due to the unspent gas in the exhaust burning off. But is the backfire occurring because the GMMG system is more restrictive?

Y pipe now right?

If so, thats prob why.

the y-pipe is not as free flowing as the x-pipe obviously, and the direction of the exhaust running into each other and in a sense "slamming" into each other and not being able to flow out fast enough.

Your X pipe does not play well with others.

make sure u go over every exhaust connection. I had an exhuast backfire forever until last week when I fixed an exhaust leak I had. Every since then, the backfiring went away.

hahaha true john.

yo vin does your cut out leak at the valve at all?

I don't think it does..I've actually never put my hand in front of it with it closed. You can hear the motor close the valve pretty tightly so I really don't think it leaks any.

mine does not leak.

hmmm i gotta check mine out then

Here's the thing. I use to run the stock Y with cats and no problem. The weirder thing is, I drove the car today and it hasn't backfired yet.

My car with the stock tune had a little bit of pop to it once in a while decelerating. GMMG is definitely not one of the better flowing cat-backs(chambered pipes do not flow very well) but it sounds good and is stainless which is nice. Maybe your computer is making adjustments after you've driven it around a bit. A more restrictive exhaust would not pull raw fuel into the exhaust stream like a freer flowing exhaust will so that wouldn't cause the popping.