Originally Posted by evilstib
Replaced stock midpipes with (previously owned) CP-e catless midpipes. The Downpipes & muffler are stock.
I love the tone, but I get a lot of backfiring now. I didn't think removing 2 of 4 cats and the resonator would cause this.
Anyone else see this before?
It's probably burbling/crackling or "popcorn" as I've heard it referred to before. I doubt that BMW programs it into the tune, but some other vehicles have tunes that specifically inject fuel on overrun/vacuum to give the crackle on decel. If you think this thing crackles, you should hear my stage 3 SRT4 with a 3" Side exit exhaust. I can set off car alarms with the crackle, so I tuned most of it out. I think that some of the crackle exists only because you're always going to have a small amount of overlap on the cam, and that's what you're hearing.
If it's a true lean backfire, and it's only on decel, then no big deal (BMW probably just got agressive and turned off the fuel injectors completely on decel), but if it's a lean backfire at WOT, then that would be bad.