[ntj]

Hi all,

New to the forum and BMW. Recently purchased an '18 M240i in estoril blue, and absolutely loving it thus far. I do have a question though....

The car is spec'd with an MPE, and my understanding is that the MPE does not have any valves. So I'm curious why the overrun sounds so different between comfort and sport modes? Is there a valve in the exhaust and I just don't realize it? I thought overrun was generally controlled by valves.

[Denk]

Yup, no valves, more burble due to less restriction. Down pipes, more burble, res delete more burble. Add all those with an MPE or other aftermarket exhaust and it'll be one tootin machine.

[ntj]

Thanks for the reply. So what is it that makes the overrun more pronounced in sport mode then? And, for that matter, is there a way achieve the overrun in comfort mode?

I had wondered if it was just a matter of lower RPM's in comfort mode due to gear selection, but in my non-scientific experiments, even if I rev the engine to the same RPM in comfort vs sport, the overrun seems far more subdued in comfort.

[PeterWT]

Some of the overrun you hear may be the ASD. But, there is a real sound difference between sport and comfort as well. I think it is tuning, e.g. fuel delivery continuing after throttle is closed in sport.
As I understand it (and from my testing), the exhaust valve only closes for cold start, opens shortly after and then stays open. It is not actuated by the drive mode selection.