do-it-yourself undertail exhaust

[mouse]

I've been kicking around the idea of building my own undertail exhaust after reading about such an installation on a speed triple (kuhlka) over on the S3 site. Has anyone else given this any thought? There are several inexpensive used Honda CBR1000RR mufflers available, but there appears to be some sort of exhaust valve in the pipe. What is it?

looking for input,
TM

[TBSstunta]

You could prob do it if you use the headers aft of the EXUP. Basically its a butterfly valve that chages the amount of backpressure based on the RPM. Its one of the reasons that the fours have such healthy midrange compared to years back.