2006 R52 S, 6-speed, I replaced my down pipe/cat/assembly with a header pipe, without a catalytic-converter, disconnected my O2 sensors, and have it all free flowing until the mufflers. I have a K&N intake in it and the thing has all kinds of new HP. Problem: it is running rich, of course, is generating lots of back-firing, like a F1 car, and lost about 2 miles/gallon. Any tips on re-tuning? Thank you.

 

Put the O2 sensors back in.

 

Post a sound video..