what's it's function?

 

The powervalve opens at a certain RPM (around 5500). When it opens, it affects the exhaust flow in a certain way to increase high end power. I believe it closes off the expantion chamber, and allows the exhaust to flow directly out.

-Matt

 

Fast1: The exhaust valve on the Zill is different than the majority of other exhaust valves. This valve changes the header pipe volume by opening a little chamber at a given RPM (~4,500).
This changes the dynamics of the pipe.
The theory is to have good bottom end along with decent top end.

Other exhaust valves simply alter the height of the exhaust port which allows more agressive porting for top end without sacraficing any low end power.