Compression ratio ?

[Brian VT]

Anyone know the approximate compression ratio of an EC300 pushing 165 psi ?

[hannesd]

a 300 pushing about 165psi is somewhat on the low side...
did you fully open the throttle when measuring?
is the meter reliable?
the CR can not be calculated by the amount of psi its pushing.
you need to measure the amount of cc in the combustion chamber and then calculate that in reference to the amount of cc in the stroke.

[Brian VT]

Originally Posted by hannesd

you need to measure the amount of cc in the combustion chamber and then calculate that in reference to the amount of cc in the stroke.

I kinda figured you'd need more info. than I have.
No biggie. Thanks.

[iancp5]

Originally Posted by hannesd

a 300 pushing about 165psi is somewhat on the low side...
did you fully open the throttle when measuring?
is the meter reliable?
the CR can not be calculated by the amount of psi its pushing.
you need to measure the amount of cc in the combustion chamber and then calculate that in reference to the amount of cc in the stroke.

Are you sure you can't work it? Surely the pressure has a fixed relationship to the compression ratio PV/T = constant? So if pressure has gone from 15 psi to 165 psi that's approx 11:1 effective? Now that is obviously quite different to what you would calculate on the simple swept volume change but might be accounted for by the resonance effects and reed valves etc? I.e. it could be a more realistic measure.

Just wondering.

[lankydoug]

Originally Posted by iancp5

Are you sure you can't work it? Surely the pressure has a fixed relationship to the compression ratio PV/T = constant? So if pressure has gone from 15 psi to 165 psi that's approx 11:1 effective? Now that is obviously quite different to what you would calculate on the simple swept volume change but might be accounted for by the resonance effects and reed valves etc? I.e. it could be a more realistic measure.

Just wondering.

Static compression ratio is a straight forward mechanical math equation. All the above mentioned including things like leakage past the rings affect dynamic compression which is what really matters when you are talking about how the engine actually runs but is impossible to put a number on.

[iancp5]

Originally Posted by lankydoug

Static compression ratio is a straight forward mechanical math equation. All the above mentioned including things like leakage past the rings affect dynamic compression which is what really matters when you are talking about how the engine actually runs but is impossible to put a number on.

But if you measure 165psi you HAVE just put a number on it! If you are spinning the engine at a fixed speed/rate then you have a very good measure of real compression albeit still not the reality but maybe more representative than a simple swept volume after port closure to combustion chamber volume.

[lankydoug]

Originally Posted by iancp5

But if you measure 165psi you HAVE just put a number on it! If you are spinning the engine at a fixed speed/rate then you have a very good measure of real compression albeit still not the reality but maybe more representative than a simple swept volume after port closure to combustion chamber volume.

That's right, cranking compression is an attempt to get an idea of dynamic compression but it's used more as a tool to diagnose problems such as broken or stuck rings rather than to figure compression ratio.

[Brian VT]

I didn't mean to start a fuss. I'm just curious how my bike might run on the (ethanol-free) Sunoco 110 I bought. I can't ride it because the snow is 2'+ deep.

[GMP]

It will run fine on anything from pump 93 up. Octane is no issue at 165 PSI just check your jetting in the cold. What have you been running in the past? Is ethanol in the gas new to VT?

[Brian VT]

Originally Posted by GMP

Is ethanol in the gas new to VT?

Yup. About 3 months ago.