Have about 1000 mi on the new E90, 330xi.

When viewed from the rear, I notice the left of the two exhaust pipes looks
more used that the right one.  As if the right one is never (or hardly)
used, but the left one is used all the time.

They do this with some Pontiac's and Cadillac's with "dual" pipes but only
one is really used (the giveaway in the winter is that one has exhaust
coming out, the other is just iced over).

Would shudder to think that BMW would also pull the same stunt - giving you
the "two tailpipe" look when only one is operative.  Ugh.

SO, I stuck a twig up the right one to see if it is a dead end.  And noticed
an obstacle.

A butterfly valve in the right exhaust outlet!  And a real one at that -
there's a shaft with a vacuum actuator on it.

I can only assume the left pipe (no valve) is used all the time, and the
right pipe is "opened" by the butterfly valve to relieve pressure at hi revs
(or closed?).  Perhaps all this fancy valving has to do with backpressure in
the exhaust system?  At rest with engine off, the valve is wide open.  Maybe
when engine starts the valve closes.  I have not checked on it yet.

Thanks for any ideas on how all that works.

Mike