How does power valve help low-end power?

The fresh fuel-air mixture in a two-stroke is prevented from flowing out into the pipe only by the rising piston closing off the exhaust port. At low rpm's the piston takes longer to close the port, so to retain enough mixture for good power you need a low port height. But, a low port chokes flow at high rpm. A power valve has a moveable component that provides a low port height at low rpms and then moves upwards out of the port to provide a larger port for high rpm flow. This is simplified, as experts could write pages on the fine details.

I always thought it was hype...then I disconnected mine for fun. The bike felt like a slug. You could adjust it for good high rpm power or low, but functioning it had both.

I thought that the powervalve help control the timing of the return wave in the expansion chamber of the exaugst. At lower RPMS, you need the wave to take longer to bounce the pushed out fresh mix back in, and at higher rpms, the wave needs to come back quicker. Check on the explanation on the justkdx website. Its very good, and will deffinatly help you to understand how these things work. Deffinalty not hype. :aj:

there are plenty of different ways it affects power. some alter port timing some alter port volume,some alter the return shockwave from the expansion chamber. some try and alter any combination of the above.

on most two stroke engines the exhaust port timing < when the piston exposes it open> the exhaust port volume and shape and the return wave from the expansion chamber have dramatic effects on the type and how much power the engine will produce. Basicly by having a port design that favors high rpm power and thru the use of rotating drums, guilotines,butterfly valves and flappers<depending on what color and year the bike is > you effectively lower port hieght, close off part of the volume and block some of the return waves of the pipe to aid in low rpm power.