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soccer8823
02-20-2009, 07:35 PM
What should compression be on a stock octane?
octane972
02-21-2009, 10:11 AM
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.
soccer8823
02-21-2009, 11:46 AM
are you talking specifically for the octane or just in general for around 140-150?
octane972
02-24-2009, 10:43 AM
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.
soccer8823
02-24-2009, 12:50 PM
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
02-24-2009, 12:53 PM
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
02-24-2009, 12:59 PM
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.
RMBC Freeride
02-24-2009, 01:11 PM
Exactly, thanks Matt. Isn't there a good thread about this somewhere? I searched for it, but to no avail...
Matt_E
02-24-2009, 01:13 PM
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.
soccer8823
02-24-2009, 01:54 PM
thanks for the help im off to read a bit about compression ratios