[QUOTE=CRUZER;897982]I have yet to find a good description of EXACTLY what that flapper valve does and when it is closed or open. I believe I can hear it open just after the engine starts but can't be sure. Can anyone offer a succinct summary?[/QUOTE]
Here are the words from [i]BMW Motorcycle Magazine[/i] issue 2/2008 when they tested the newly arrived HP2 Sport (the model that first had overhead cams and first had the flapper):
[quote]An exhaust flap in front of the muffler is actuated by an electronically controlled servo-motor via a cable. 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 output all the way to the rev limiter.[/quote]
Think about it: the HP2 Sport.
It's not a milquetoast system.
This issue of this magazine, BTW, is from when it was a German publication, i.e. before takeover by our wonderful friend in Tucson.
The K1300S has it, too.