Compression Ratio question

I have a question converting compression ratio to psi when checking engine compression.The tractor engine I'm checking is suppose to be 6 to 1 compression ration. On my compression gauge it checks 130 psi. That caculated to 21.66 psi per ratio. Is the psi per ration the same on all gas engines or does it change when checking low compression engine versus high compression engines...Thank you...Ben

compression ratio and cylinder pressure while somewhat inter connected do not have a direct relationship. Camshaft overlap will take a huge compression ratio ( like 14 to 1) and turn it into 125 lbs of cranking pressure. like wise a tractor engine with 5 to 1 might have the same pressure.Paul

It changes. Compression ratio is dependent on stroke, piston diamiter , and piston design. Long stroke compresses more volume of air, bigger pistons compress more volume (air) and if the piston top is shaped to fill the entire combustion chamber the compression ratio increases. If you are trying to resolve engine issues, compare compression PSI in each cylinder. If each cylinder varys it is due to piston ring leakage, or valve leakage or a leaky head gasket.

I would say that it must be more than 6 to 1, more like 8.8 to 1, based on 14.7 psi being atmospheric pressure.
It is possible that there is a buildup of carbon in the cylinder, but probably not that much...
What kind of engine are you testing?

I would say that it must be more than 6 to 1, more like 8.8 to 1, based on 14.7 psi being atmospheric pressure.
It is possible that there is a buildup of carbon in the cylinder, but probably not that much...
What kind of engine are you testing?

Its a 1952 cockshutt 30, 153 ci buda gas engine.

Well, at least Skip has figured out the the camshaft profile enters into the compression pressure equation!

"Well, at least Skip has figured out the the camshaft profile enters into the compression pressure equation!" The toyota and Ford hybrids have used the effect of cam timing to adjust cylinder pressure to good effect.

The Prius hybrid, which has an Atkinson cycle engine, has a mechanical 13/1 compression ratio built in, which makes great part throttle mpg, but would detonate like heck on pump gas at wide open throttle.
To fix that, they use variable intake cam timing to hold the intake valve open as the piston rises on compression. Some of the air in the cylinder is allowed to blow back into the intake manifold through the still open intake valve, thus reducing peak compression pressure.
If the pressure is high enough to detonate, the knock sensor tells the intake cam to retard a bit more and bleed off more cylinder pressure.
This keeps cylinder pressure just below detonation at part throttle for more burn efficiency and top mpg.
They consider it to be 5 cycles, 1 intake 2/3 compression / blowback, 4 power, 5 exhaust.

The down side to an Atkinson 5 cycle engine is that it must blow off so much air at wide open throttle that it can't make as much full throttle power.
Thats why you only see them in a hybrid where the battery / electric motor can make up for the gas engines lack of WOT power.

The system seems to work amazingly well, my 2010 Prius does 0-60 in 10 seconds, 115 mph flat out and has AVERAGED better than 50 mpg at the speed limit (65)for the 8,000 miles I have owned the thing.
I wish someone would put that type driveline in a light duty 1/2 ton pickup for the city cowboy and farmer "gofer" pickup market. I suspect the thing would make 30 mpg or better.