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Compression Ratio?

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I understand compression ratios, but Im looking at the new Yam YZ catalogue and the four strokes have a single compression ratio.

The two strokes are listed as follows:YZ 125 8.6 to 1 - 11.0 to 1

YZ 250 9.1 to 1 - 10.6 to 1

Does anyone know why this is?

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I think that when a two stroke get "On the pipe" the reflective wave of pressure increases the amount of fuel and air in the cylinder. Some of the unburned fuel and air that goes through from the intake port to exhaust port gets pushed back into the cylinder. Not a tuner, so there may be a better explanation.

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The 2-strokes have a power valve which changes the compression ratio by somehow changing the exhaust port based upon RPM.

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2-strokes bring the air/fuel mix in thru the crankcase as the piston rises, the piston coming back down compresses and pushes the mixture up thru the transfer ports to the combustion chamber, where it is compressed a final time. So the 1st ratio is the crankcase ratio, the 2nd ratio is the piston/combustion chamber....... 😢

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I think that when a two stroke get "On the pipe" the reflective wave of pressure increases the amount of fuel and air in the cylinder. Some of the unburned fuel and air that goes through from the intake port to exhaust port gets pushed back into the cylinder. Not a tuner, so there may be a better explanation.

😢😢🤣😢😢😢

This atricle explains San Diego's post and sounds like the best explanation. as your engine approaches the tune of your pipe, you'll get the most compression and power.

The power valve adjusts the back pressure, not the engine compression, based on engine RPM.

i dont think that the piston moves down enough to compress the fuel charge in the crankcase before opening up the intake ports on the cylinder to yield an 8.6:1 (125) or 9.1 (250) compression ratio...thats pretty huge.

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Again, I think that the powervalve changes the exhaust port timing. The Guillotine slides down and changes the point at which the exhaust port is open. This changes the pressure wave and the RPM at which it bounces back into the cylinder. Variable timing should allow better pressurization of the fuel air mixture in the cylinder at a larger rev range. ie. wider powerband.

Tuners change not only shape and length of the exhaust cone, they change the height and shape of the ports.

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Again, I think that the powervalve changes the exhaust port timing. The Guillotine slides down and changes the point at which the exhaust port is open. This changes the pressure wave and the RPM at which it bounces back into the cylinder. Variable timing should allow better pressurization of the fuel air mixture in the cylinder at a larger rev range. ie. wider powerband.

Tuners change not only shape and length of the exhaust cone, they change the height and shape of the ports.

*surfer realizes that he is hard headed and unwilling to concede this issue*

after some thought, yes, the power valve does change the timing....BUT

zounds to me liek the power valve is only optimizing the already existing and returning pressure wave. the ALL of the compression increase comes from the pipe. the power valve, however, attempts to spread this power over more RPM's (by timing) but can do absolutely nothing for acheiving the maximum specified compression ratio, that max compression figure is ALL pipe (when engine is running at the pipe's resonant frequency).

it also seems to me that if a tuner were change pipes, then he would also need to match the power valve timing to really get max power. i have never even seen or heard of such a product...hmmm never mind

*surfer goes to drawing board*

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