Peculiar exhaust on z3 2.8

[swisss]

On just one of the twin exits, there is a butterfly valve that does not move/open/close with varying revs.

Is this some kind of noise adjuster? Shoudl it move? What is it?

Behind the rear box, but only on one of the exits, not both.

Weird, any ideas?

[roop]

I have noticed this before. It also appears on a number of installations of the 2.8 - have a look at a previous generation 3er and it's there too. I always thought that it was fixed and immovable - perhaps the engine runs better with lots of backpressure but BMW didn't want to put a peashooter single exit tailpipe on their top-but-one model so just blocked off or faked one exit...?

[swisss]

cheers roop


someone has just emailed me this..... you're right, it's only on the 2.8s, but it seems it should move...




That is called the Active Exhaust System Actuator (AESA). The AESA consists of the following:

Exhaust flap with vacuum actuator
Solenoid actuator valve
Vacuum reservoir with check valve
MS 41.1 control module
The electrical function of the exhaust flap operation is controlled by the MS 41.1 module.

At idle speeds, up to 2500 RPM: the exhaust flap is closed with vacuum applied to the actuator. This causes the exhaust gas to pass through the long resonator pipe, reducing the low frequency noise that is generated.

Above 2500 RPM: the exhaust flap opens. This allows the exhaust to pass through the short pipe which maximizes the back pressure effect to provide the best performance.

In the event of electrical or vacuum failure the flap will rest in the open position providing maximum exhaust throughput.

[roop]

Well there you go. You learn something every day. Pretty clever stuff by all accounts. I'd rather have the low resonating sound though

[stub]

It's there for emissions control on warm up.

[dannylt]

Nah - you see it closed when hot too. Seems to be a common thing; my 535d has one also. A turbo-diesel should never like back pressure, so presuamably a noise thing.