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Hi

I own a 1995 328i Convertible (twin tailpipe) and I've noticed that the tailpipe on the right handside has a butterfly valve which is open when the engine is switched off but closes when the engine is running.

Can some when shed some light on what this is for?

Ta

:jaw-dropping
 

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i can :)
its vacuum controlled and is shut at low revs...but it opens at around 3k.
its so the exhaust gasses can better fill the silencers and is there as a noise reduction measure.
over 3k exhaust egress is more important than silencing so the butterfly opens to facilitate this.
hth
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