from the seadoo manual,
RAVE SYSTEM OPERATION
For a 2-stroke-cycle engine to have high power capacity at high crankshaft speeds, a high
volumetric or breathing efficiency is required and the fresh charge losses must be minimized. The
result is achieved by opening the exhaust port early and utilizing the resonant effects of the tuned
exhaust system to control fresh charge losses.
When an engine of this design is run at a high speed, efficiency falls off quickly. The relatively high
exhaust port effectively shortens the useful power stroke and because the exhaust system is tuned
for maximum power, there is a large increase of fresh charge losses at lower speeds. As a result,
the torque decreases along with a dramatic increase of the specific fuel consumption. Higher
torque along with lower fuel consumption can be obtained at lower engine speeds if the time the
exhaust port is open is shortened.
Bombardier-Rotax has patented a remarkably simple system to automatically change the exhaust
port height based on pressure in the exhaust system.
Located above the exhaust port is a guillotine-type slide valve. This rectangular valve is connected
by a shaft to a diaphragm which is working against a return spring. Two small passages in the
cylinder just outside the exhaust port allow exhaust gas pressure to reach the diaphragm.
To the outside of the return spring is a red plastic adjustment knob. Turning the adjustment in or
out changes the preload on the return spring which, in turn, will change the RPM at which the
RAVE valve opens and closes.