E90 330 exhaust butterfly valve??

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
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Your engine is not this one, but the explanation should be the same ...
On M52 engines, two resonator pipes (short and long) are incorporated into the rear muffler assembly. An exhaust flap is integrated into the outlet side of the short pipe. The flap is operated by a vacuum actuator via the engine control module. The flap is closed at speeds below 2500 RPM. This allows exhaust gasses to flow through the long resonator and reduce noise. Above 2500 RPM, the exhaust flap is open and allows exhaust gasses to flow through the short pipe for maximum performance.
You should not view this as a cheap shot. BMW wants your driving experience to be quite and restrained at low engine speeds, but lively and responsive at high speeds. The valve is a way to accomplish precisely what you set about to buy. Drive happy ...

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Jeff, excellent description, thanks much!
Interesting bit of acoustical engineering.
Mike

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For sound, many people mod this with a golf tee... unplug the vaccum line and plug it with the tee, then tie wrap it up out of the way. The flap will stay open, more sound comes out, but you won't go any faster!

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