problems with backfiring

just recently me and one of my friends just traded mufflers.
i had an pilot muffler that sounded really sweet he had an apc that looked pretty cool. so we traded but now my car is having all types of crazy problems. the first is every once in a while when i shift i hear my car backfiring also when i rev my engine white smoke comes out. someone told me it is water getting into my engine. also last night and this morning after about fifteen minutes of driving my car started sputtering every time i pushed on the gas, and the check engine light came on. the only other things that i have done to my car is and air intake and ported out my cat.

check your oil and see if water is in it. when you drain the oil let it sit for a couple of minutes and see if any water comes to the top. if there is you might have a leaky head gasket or even a cracked cylinder head.

what code are u getting thrown ? changing mufflers wont cause this to happen on your car. u say white smoke comes out, this is after the car is warmed up right ? white smoke does signify that water is being combusted, so it could be a blown head gasket, which could be why your car is sputtering. does it idle funny too ?

no it idles just fine. and i was playing around with it today and drove it around but it still backfires when i shift into 3rd that is the only gear it does it in.and how would i tell what code it was?

does it still sputter ? well it could sound like its backfiring because of the muffler, u said its a cheapo apc muffler right, yeah those mufflers suck, they are loud too. the backfiring is probably because of you lack of a cat and that cheap muffler

idk how to check the code, sorry. u know what, u say its backfiring, is it loud like a gunshot or is it just more like a softer pop sound that u can hear when u let off gas and shifting

it is a soft pop. what type of muffler would you recommend i like them loud but i dont like them when the sound all high and spread out i like them more with a low deep growl any thoughts?