125 rider

Sponsoring Member
Oct 1, 2000
408
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)
 
Last edited:

Buckholz

Member
Mar 15, 2000
396
0
Calculated, assuming the power valve would seal tight. Higher compression at low speeds, lower at high engine rpms, where the power valve is open.
 

spanky250

Mod Ban
Dec 10, 2000
1,490
1
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.
 
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