Octane Octane compression

If my memory is correct the minimmum by the book is 115, but I have seen 1200s (3 cyl version of same motor) run with less. I have seen compression as high as one 150psi, but they're usually around 130-140psi.
 
I have only checked one octane (my own). I have checked many 1200cc polaris engines which is the same as the octane motor, but with one more cylinder. My octane compression I think was 135 & 140 with the stock domes. I installed "pump gas" domes from Watcon.com which raised it around 150psi. When I mention 150psi earlier I was saying that was the highest compression I have seen on a stock 1200cc polaris. That number is an kind of an exception---It's usually about 140 or a little less.
 
What is the reason that high compression domes on the polaris yield around 150 while on a yami or kawi it you get compressions around 180 or 190?
 

RMBC Freeride

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Pueblo, CO
Because the cranking pressure numbers mean nothing! The Polaris engine has totally different porting, larger bore and stroke than a 701 Yamaha. Because of this it needs lower (indicated) compression than a 701 or you will melt it down.
 

Matt_E

steals hub caps from cars
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Because the cranking pressure numbers mean nothing! The Polaris engine has totally different porting, larger bore and stroke than a 701 Yamaha. Because of this it needs lower (indicated) compression than a 701 or you will melt it down.


For this reason, compression ratios are more meaningful than cranking compression pressure numbers, which can vary wildly from gauge to gauge, vary with altitude and weather, and from engine to engine.
 

Matt_E

steals hub caps from cars
Site Supporter
Location
at peace
I don't think so.

Compression ratios are not very convenient to use for anyone but a builder. They are not easily checked or verified.
Sticking a pressure gauge on a hose on the engine is so much more convenient. :biggthumpup:
You just have to realize the limitations of such measurements.
 
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