Compression ratio depends on several factors or are you after cranking pressure from a gauge which is still only somewhat relevant. If you want to check engine condition you can get a good look at the bore through the exhaust port. It's important not to have massive ring gaps but you have to pull the barrel off to check them.
I agree with Tom, Cranking pressure has lots of variables, (base gasket thickness, porting work etc, etc) I think this is why manufacturers don,t readily give this info in their manuals, it could just complicate things and lead to a wrong diagnosis of the issue.
If your gonna use a compression tester, check the reading when the top ends just been rebuilt with new parts.
Whatever reading you get, "save it" & use it as your datum point in the future.
For instance, if after a rebuild your pushing 140psi on the gauge & a month later you check again & its down to 120psi, this is a good reflection on top end wear.
Without the original reading to go off its nearly impossible to say whether you have worn components or not.
Again as Tom has mentioned, the only way to determine wear without the original reading is to strip the top end and physically measure the wear & compare your measurments to the tolerances that are listed in the manual.
factory service manuals almost ALWAYS have compression ratio's in the specs.
For instance, 89 kdx200;
7.7:1 high speed, 9.2:1 low speed
This is the difference between power valves open/closed. I would not expect a big bore kit to come with such specs, because they do not know what else you may have done or will do to your bike.
If you're talking about cranking compression like Tom said, it's only useful as a guide to ring performance. Check it when you put it together, and when it starts to go down from that number, it's time for rings.
You are going to have much higher compression with the bigger piston. I think you gain 1 point for every 2mm overbore. Also you can't go by the Japanese rated compression ratios from the manuel.
When I had a big bore kit done on my '84, I know they machined the head.
Not sure if they enlarged/reshaped the combustion chamber or removed some material from the bottom of the head. Either way, it changed the compression ratio.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
