Hi all,

 I did a search before asking, what's it do? How's it work? Does an LT motor with one make more power? Is our motor the only one that came w one?

Thanks.
Dave

If read the link to any other two stroke powervale you going to be mislead. This is the only "power valve of its type.
ITS ACTUALL NOT A POWERVALVE ITS AN EXHAUST VALVE.
In 1987 Suzuki went away from this style valve in favor of a variable timing ex port guillotine set up. Those will make more HP and retain low end also smoogbing transition. There main function is to raise the exhaust port timing at a certain rpm. How they make more power is that at lower rpms the are closed and build more compression due to a longer effective stroke of the piston sweeping the ex port closed. At the specified rpm the valve snaps open allowing the return wave of the pipe to stuff the cylinder later and creating more volume at the port opening to be stuffed.
Kawasaki had a different arrangement where they had triple port exhausts and open and closed them to effectivally do the same thing. But there benefit being the tripple ports scaveng the top from of the cylinder more effectivlly. In later years Kawasaki developed a guillotine in the main ex port also opening the two ports at different times and rates.

Is our motor the only one that came w one?

The only other Factory performance 2 stroke atv with a PV/EV were the Kawasaki Tecate 3 and 4 with the Kips valve. 250R's and banshee's were never built with them from the factory. Most of the new high power aftermarket banshee and 250R jugs use a PV/EV.   

YA those style valves in the sleds are the Skidoo RAVE style valve, they change ex timing based on cylinder pressure.  Yamaha for sure and CAT I believe had the only rpm controlled valves they ran on a servo motor. All the others use a spring and pressure port. 

The following are some of the basics of my understanding and experience with exhaust power valves and Helmholtz resonator type valves on two strokes.

Suzuki used the Helmholtz resonator type system on the LT 250 and Lt 500s.  The Helmholtz type system makes the engine think it has two different exhaust systems.  One system is tuned for low RPM, the other is tuned for higher RPM.  A valve in the bottom of the exhaust port must be closed at the RPM where the graphs of the two power curves intersect each other.  One graph would be created with the valve fully open and the other would be created with the valve fully closed.  Where the two graphs intersect each other depends upon the porting and exhaust system used.  When using our HPR 19 exhaust system the valve needs to close in the 5500 to 5800 RPM range.

The majority of the motocross, road race, and snowmobile engines use a different type of exhaust valve.  These valves alter the port timing and the blow-down area of the exhaust port by using a guillotine, a flat-sided spool or a hinged draw bridged device in the exhaust port.  The exhaust valve is fitted close to the piston.  This type of valve affects the power curve due to multiple effects.  Changing the port timing changes the effective tuned length of the exhaust pipe.  Changing the blow-down area changes the RPM where the cylinder ports are tuned to breathe optimally.  The effective power stroke is theoretically increased, but this effect is minimal because the valves are not fit close enough to the piston to seal the combustion pressure very effectively.  These type of valves are far superior to the Helmholtz type system. 

The exhaust valve like the majority of the motocross, road racer and snowmobile engines use are activated by a fly-ball type governor, a computer controlled servo motor or the static pressure in the exhaust system.

Servo motor controlled valves are a superior method of valve control.  The servo controlled valve is computer controlled and can be programmed to open or closed at any desired RPM and the valve can also be closed gradually over any desired RPM range.

Most motocross engine are typically controlled by the fly-ball type of mechanism that is driven off the end of the crankshaft.  This type of mechanism is difficult to change the RPM where the valve begins to close and the range of RPM over which the valve is closing.

A pneumatic activated valve is the least desired method of exhaust valve control. They are highly sensitive to anything that may affect the static pressure in the exhaust system like carbon in the stinger and muffler, weld buggers in the stinger and muffler, barometric pressure, RPM and throttle position.


Kawasaki uses a combination of the draw bridge, spool valves and Helmholtz resonator in many of their motocross engines.  They use a draw bridge in the main exhaust port and spools in the auxiliary exhaust ports.  When the spools close the auxiliary ports, a passage way in the casting to a Helmholtz resonator tank is opened.  The tank or volume is hidden within the casting of the cylinder.  Most engine builders are not aware of this tank and is though to be part of the water jacket in the cylinder. 

Thanks MotorGeek, I see how you got your name, I enjoyed reading your very detailed explanation. Just like the modern day diesel, mechanical to computer controlled. I'm very happy w my Helmholtz system. I never had a 2-stroke racequad before, I'm quite impressed.

Thanks again for better helping me understand the diff power valves guys!

Dave