What is a power valve and how does it work?

 

Before you talk about a powervalve, you need to first understand a little about two-stroke pipe design. On a two-stroke, the size (volume) of the expansion chamber and length of the stinger have a great effect on the power delivery of the engine. You notice that you can get a torque(bottem end)pipe, mid-range, or rev pipe for almost any application. That is because the pipe designer can target a specific range for peak efficiency and the other areas suffer performance wise. If you have ever done a before and after ride on a two-stroke with a good pipe, you can feel the difference.
We also need to understand a little about porting and port layout. This is my weak area because porting is half science and half magic/art. What is important to know is that the two-stroke relies on the ports to transfer the air/fuel mixture and scavenge the exhaust. It relies on the speed of the incoming mixture and the pipe to keep the fresh air/exhaust moving in the right direction. This is why two-strokes typically operate at higher rpm levels than four-strokes do. The faster the air/fuel travels, the more efficient the engine becomes (generally).
What the power-valve does is open up the exhaust port to the pipe at higher RPM's to make the exhaust gases move out faster, thusly pulling in a fresh fuel air mixture. So when the engine is at lower RPM settings you have the power/torque you need to power out of a turn or make it up a hill, or whatever. As the RPM's rise, you get the ability to make power in that range, and the ability to have a nice over-rev.
Now, please, this is just from a lay-man, so it may not be exactly correct, but it is close. If anybody can correct me, please do so. There are many different designs out there.

 

Thanks ridzhard for the laymans description. I was wondering where this valve goes on the engine. I imagine it takes place of the reeds somehow like the rotary valve 2 strokes of the 70's although I don't think it works that way. I also imagine it as some sort of a blower that force feeds the cylinder. Judging by what I think you're saying it regulates the exhaust so that power can be adjusted via the power valve.
I probably have this all messed up so if someone could provide a website or diagrams that would be great. I think it would be neat as a long term project to get a basket case banshee and RZ350 it.
Till then I'm happy with the "tame" banshee I've got now.
ps thanks for the note about the banshee 400 kit from FMF.

 

You can see a picture of a 250 power valve cylinder here:

www.sanbdbros.com/mike330r/gordons.htm

scroll down, it's the only picture of a cylinder on that page.