Originally Posted by ArthurJGuy
Not only does removing the cats not change the commanded AFR of the engine (that is commanded in the tune), but with a target based ECM like the M has it will have no effect because the ECM will do anything within the physical limits of the car to hit that target AFR.
They are illegal to remove, I'll give you that one. That is why most all catless pipes say off road use only, but we all know no one takes an M off road.
right, as I said it'll prob do nothing, but why give BMW the ammo to deny your warranty. Believe me that if it went to litigation BMW can say what they want (no judge is gonna dyno the car for comparison sake) and you'll lose. Simple as that. Either swap x pipes back and forth or leave it alone.