It's been a while since I cleaned the valves on a Yami but I'm pretty sure they have a stopper on them to keep the valve from falling into the combustion chamber. That's Yamaha engineering for ya. Something Polaris obviously had no interest in doing.
Quote originally posted by Luke68Polaris
Call me stupid, but dont the valves on sleds open and close the ports, so even if it "floated" like you say, it would just be full open (or closed) and it would just run like crap?
A valve can "float" on a 4 stroke, because it will open and be whacked by the piston, but the valves on sleds are at the same x axis all the time, the just go up and down (y axis)
If that makes sence to anyone.
The valves sit diagonally(z axis). They cover part(not all of) the exhaust port at low RPM in order to imitate a motor with lower exhaust ports(creates more torque and better MPG). That's because it allows all of the power from the fuel/air mixture to be used completely after it's burned. The lower the opening of the exhaust port is, the longer the burned mixture will stay in the combustion chamber.
At high RPM the valve opens and does not block off the exhaust port at all, the motor runs exactly how it would if the exhaust valves were not there. Higher exhaust ports are required to exhaust the burned fuel/air mixture because of the high RPM(less time for the mixture to leave the cylinder). This fast exit of the burned mixture also helps somewhat to "pull" the incoming fresh charge of fuel and air into the combustion chamber for the next power stroke.
If the valves are stuck closed the power will be less than optimum at high RPM, if the valves are stuck open the power and MPG will be less than optimum at low and middle RPM's.
Yamaha and Arctic Cat powervalves are servo motor controlled and have only an open and closed state, nothing in between.
Polaris powervalves are controlled by cylinder pressure(500-800 VES), the CFI sleds and race sleds are solenoid controlled though I believe.
Ski-Doo's powervalves are also controlled by cylinder pressure. An advantage that they have is that there is two different valves within the housing which creates three different stages for the valves(for low, mid and high RPM operation). Just another reason why Ski-Doo's have good fuel economy.
2009 Ski-Doo Summit XP 800 154" X package - Quick Clickers, Between the Lines Designs Ice Scratchers, Tunnel & Rail Stiffeners
