It's called "regeneration"...... Passive regeneration occurs normally while driving if exhaust temps are high enough.....active regeneration occurs when temps are low, and as the back pressure in the catalyst rises, the system begins to regenerate.....and this is what you might smell when you pull in the garage .......I haven't sensed any performance issues, and I don't know if BMW is able to control either turbo to raise temps during regen which might be the performance issues you've sensed....

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HP gets you to speed, Torque keeps you there !

1928 Model A Coupe
1957 MGA SOLD
1994 MBZ SL320
2007 Acura RL
2010 X5d 11/13 PCD