To understand how powervalves work you need to have some basic knowledge on porting. When you port
a motor, it is referring to modifying the 3 main cylinder ports with a small grinding tool, usually air powered hand grinders, the 3 main ports that get "ported" are the intake ports
, the transfer ports
and the exhaust ports
. A good port job will involve porting all 3 of these ports, but a cheaper portjob will only have the intake and exhaust ports worked on. The exhaust port is where the powervalves are located, so I wont even discuss the intake and transfer ports. When an engine builder ports the exhaust port he widens and re-shapes the port, but more importantly he rasies the height of this exhaust port. A typical exhaust port height is raised\cut just a hair... usually no more than 1.5 mm, raising the exhaust port height gives ya mucho power, the problem tho is you can never "un-port" it, once the material is ground away, you can never put it back. By raising\porting the exhaust port you will unfortunatly lose power down low in the powercurve. For example if you have your exhaust ports cut for a "drag racing" port job, the port heights are raised pretty fuggin good, this makes alot of peak HP but is not suited for a trail machine, it would be such a handfull to ride in the trails you would tire out from the peaky abrupt powerband and loss of low end HP. Now.... lets get onto the powervalves.
One powervalve per cylinder, the powervalves are found right at the exhaust ports. They raise and lower exhaust port height in conjuction with the engine's RPM's. When the powervalves are fully closed the port height is lower and you have the benifits of a stock exhaust port height that makes great low end power, when the powervalves are raised, they raise the exhaust port height and you get the benifits of a ported exhaust port..... without the loss in low end power typical with a port job.
There are 2 styles of exhaust powervalves, electronic powervalves
and exhaust pressure acutated powervalves
. The Yamaha YPVS(Yamaha Power Valve System) found on the RZ 350 and 500 streetbikes use the electronic style powervalves. The T-Rex and FMF monoblock powervalve cylinders use the exhaust pressure to sense when to raise and lower the valves.
I know a little more about the electric powered RZ powervalves, so I'll just write about these, someone else here can help explain the exhaust pressure acutated powervalves.
Ok... when the RPM's in the motor start to build, the spinning flywheel sends RPM information to the CDI box, the CDI then processes that info and controls a small single servo motor that is mounted on the frame of the quad near the cylinders to acuate the powervalves. This servo motor has cables attached to a "wheel" that are located on the side of the left-hand cylinder, the servo motor moves the cables, which turns the wheel on the side of the clyinder, the wheel is attached to the powervalves, the wheel turns the powervalves and effectly raise and lower the exhaust port height. At lower RPM's the exhaust port height is lowered so you have ALL of your low end power, and as you give it some throttle and RPM's increase the YPVS gradually raises the exhaust port height until the valves are fully open, I think
when the exhaust ports are fully open, it simulates a pretty wild port job, about a 1.5 mm raise in port height. Since the powervalves are always raising and lowering it really really really
smooths out the powerband, from a strong full powered low end hit, smoothly transitioning too the mid range and then right into a powerful fully ported top end powerband kick, GONE is the lightswitch on\off peaky hard hitting Banshee powerband and instead you get a more "electric" "250r" motorcross style powerband. Much easier to ride, less tiring on the trails and track with all the low end power, smooth powerband and ported topend in one neat package. Its truely the best of both worlds. Powervalves are not recommended in drag motors, because you dont want any low end or smooth powerband when your dragging. A stock non powervalved Banshee motor will put out on average 34 HP, while a stock powervalved RZ motor(which is nearly identical to the banshee's cylinders except for the YPVS) puts out 62 HP.... 62 Hp with low end a smooth powercurve and all the benifits of full porting.
Here are RZ cylinders....
Here are the RZ powervalves.
Here's my exhaust port on my 404 motor, they are ported for MX.