[eddman303]

Can anyone shed any light on what that valve body at the bottom of the down pipes does and is it important? Any help would great. Cheers

 

[chaosExpanding]

It's kind of an EXUP system. It partially closes at low rpm to provide more back pressure giving you more torque. It opens up at high rpm to increase exhaust flow to increase peak horse power.

Useful on a streetbike. Not useful on a track bike.