Compression help

So i'm doing some testing on my 81 XJ650 Maxim and when i did a compression test i'm getting between 120 and 130psi, but the manual has it listed as 9.2:1 and i'm not sure how to compare the two. Can anyone help me understand this?

Thanks

 

The Haynes manual says (page 37 Routine maintenance 31):

Standard 156 psi
Minimum 128 psi
Maximum 171 psi
Max Difference 14 psi

It does "say" for UK models only...but I think that holds true for all of them.

Compression ratio cannot be directly compared to the measured compression, because there is overlap between the intake and exhaust valves. Assuming adiabatic compression, P*V^gamma is constant http://en.wikipedia.org/wiki/Adiabatic_process . Compression Ratio is a measure of the volume of the combustion chamber from bottom dead center to top dead center. Using that formula (gamma = 1.4, and atm pressure of 14.7 psi), the expected measured compression would be 190 psi :!: 8O .

That all being said, the cylinders are close (good) but low (could be an instrument issue). Did you hold the throttle wide open?

 

Thank you. I did not hold the throttle open. i'll have to look at my valves and possibly the cylinder rings.

 

No No....if you didn't hold the throttle open...do it again with the throttle wide open. If you restrict the air, then less air (mass) means less measured compression. To be extra sure, I'd measure again with the throttle held open. Don't forget to disconnect your TCI (you don't want to fry it) and fuel.

But you will probably find that your numbers go up and are within spec. Make sure your battery is fully charged for all four cylinders (a slower turning piston can result in a lower measurement).

IF you get one low cylinder, the excepted wisdom is to put a teaspoon or less of oil in each cylinder and test again. If you see the pressure between the cylinders line up w/ oil - it's likely rings (oil coats the rings making them seal better). If the low one stays low relative to his buddies, then it's likely valves (oil in the cylinder doesn't help the valve seal).

 

Throttles Wide Open.
BATTERY Fully charged.