On the 330i, if you look while the car is stationary, it will seem like it is always open. When you drive, do you get much more soot on one tailpipe versus another? No doubt, you do. The cleaner one is the one with the butterfly valve. It closes from time to time while moving, presumably to alleviate exhaust noise while cruising.
330i | Black Sapphire | Beige + Burl | Sport | Premium | Cold | Nav | CA | Sat prep