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.