[Iscadean]

Hi all,

New to this forum but I have been reading about the electronically controlled exhaust ( muffler) flap. Can anyone explain how this works and if it needs maintenance or adjustment?

 

[New2rt]

BMS-K controlled, cable operated using a remote actuator.
The GS911 can be used to calibrate it, end points.
Some say it's there for sound, I believe it's much more then that.

 

[Iscadean]

Thanks. I looked up the BMS-K and the Can-bus info and it does seem complicated. The flap must be controlled however by some data - any idea what that is and how it determines the opening and closing? Is it based on revs? Or something more complicated?

 

[Skidmarc]

I think it's there to control back pressure, and thus the torque curve. But I could be wrong...

 

[TWheels]

I removed the muffler as part of a recent tire change and had a look while I had the chance. The butterfly valve is about 1/4 closed until approx 4k rpm and then opens almost completely. It seems to have only the two positions and not "stepped" as I assumed.

 

[New2rt]

Thanks. I looked up the BMS-K and the Can-bus info and it does seem complicated. The flap must be controlled however by some data - any idea what that is and how it determines the opening and closing? Is it based on revs? Or something more complicated?

I don't have a good answer for you. It can be just RPM's or RPM plus load (like the fuel system, alpha n)
I really have no idea.

 

[Gaby]

I think it's there to control back pressure, and thus the torque curve. But I could be wrong...

I think you are right.
If I remember well, the principle of adapting the exhaust volume (and thereby its resistance in order to have the vacuum wave arrive at the
exhaust port just in time to help evacuate the burned vapors) was first introduced by Yamaha (EXUP) on their FZR series (in the nineties?).
Since the correct timing changes with RPM's of the engine, these systems are (at least) rev governed.

 

[lkchris]

Here's what was writtne in BMW Motorcycle Magazine about the similar flap on the HP2 Sport (where this first appeared):

This constantly adjusts the internal diameter of the exhaust system to suit engine revs and ensures good low to mid-range performance for a linear power output all the way to the rev limiter.

 

[Iscadean]

Thanks everyone. It was a silly trivial question I know, but you seem to have answered it. I'll listen intently now when I breach 4k on the rev counter.

 

[heavynikonf3]

Thanks everyone. It was a silly trivial question I know, but you seem to have answered it. I'll listen intently now when I breach 4k on the rev counter.

You are not alone......I always wondered myself.