compression?

125 rider

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Oct 1, 2000
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#1
why?

Can someone explain why on some bikes the compession ratio changes for low and high speed. For instance my bike is: 8.4:1 (low-speed), 10.5:1 (high-speed)
 
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Mar 15, 2000
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#2
Calculated, assuming the power valve would seal tight. Higher compression at low speeds, lower at high engine rpms, where the power valve is open.
 
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#3
The power valve changes the height of the top of the exhaust port. When the valve is closed at low rpms, the top of the port is lower, so the piston effectively has a longer compression stroke than it does when the valve is open, raising the top of the port. Remember, a two-stroke doesn't start compressing the mixture until the piston has risen far enough to close all of the ports, so the lower the ports, the more stroke is still left for compression after all the ports are closed.