[roadkillrob]

Thank you for your email to BMW Customer Information.

We can advise that the exhaust flaps adjust at differing engine speeds in order to make the most effective use of the exhaust silencer. When the vehicle is running at idle or at low RPM, the flap ensures that the exhaust gases use as much of the silencer's volume as possible, keeping exhaust noise down, e.g. when you're in slow traffic or driving in high gear.

Whereas when the vehicle is running at high RPM, the exhaust flow characteristics are adjusted by the flap to give the exhaust gases a more direct route through the silencer, thus using less of its volume. This also has the effect of allowing the engine to rev more freely as there is less restriction on the exhaust flow.

The primary reason for the flap is to make the most effective use of the exhaust silencer volume at differing engine speeds.

We trust this information is of some assistance to you but if we can help you further at any time, please do contact a member of the BMW Customer Information Team.

Yours sincerely

BMW Group