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Cylinder Compression
- Thread starter tta983
- Start date
I just did mine not to long ago. Measured 152-154.
The difference between cylinders is considered most important. I'd be concerned if there is greater than 5% difference between cylinders.
There are other variables to consider however. Are you doing it on a cold engine or somewhat warm engine. And how good is the measurement tool.
I did mine after a few hours of letting it cool down.
The difference between cylinders is considered most important. I'd be concerned if there is greater than 5% difference between cylinders.
There are other variables to consider however. Are you doing it on a cold engine or somewhat warm engine. And how good is the measurement tool.
I did mine after a few hours of letting it cool down.
What is considered to be normal compression when you do a compression test? In other words what range is considered normal or good.
Hi tta983,
Your cylinder compression is what the compression ratio is. The TTA comp/ratio is 8 to 1, this means that there's 8 times compression over atmospheric pressure which is 14.7 psi. This means that if you are at 152 psi per cylinder presssure you have a different piston in the motor and I don't understand how you can be making boost with that kind of pressure. The actual tolerance is 10 percent on each piston. I think you may need to check your gauge. The GN is 9 to 1 which is 132 psi which is why they run lower boost than the TTA. I know sayin this will cause a s@#* storm but this is how it is.
Just sayin!
Hi tta983,
Your cylinder compression is what the compression ratio is. The TTA comp/ratio is 8 to 1, this means that there's 8 times compression over atmospheric pressure which is 14.7 psi. This means that if you are at 152 psi per cylinder presssure you have a different piston in the motor and I don't understand how you can be making boost with that kind of pressure. The actual tolerance is 10 percent on each piston. I think you may need to check your gauge. The GN is 9 to 1 which is 132 psi which is why they run lower boost than the TTA. I know sayin this will cause a s@#* storm but this is how it is.
Just sayin!
The measured numbers are what i posted not the original poster.
My understanding there isn't a direct way to calculate compression ratio from measured compression.
Compression ratio is a number calculated based on volume of the chamber, piston stroke, and bore.
Measured compression is a way to tell the health of an engine.
My WRX runs 8:1 compressions ratio and i measured the same thing (150) when healthy.
The important thing is the difference between all of them.
I was able to diagnose my wrx the same way. Blown headgasket showed no compression on the driver side cylinders. Three years later, car seems to run fine but going through oil. Drivers side i'm measuring 60psi and 120psi. Took it to mechanic to confirm issues and they verified it.
the measured numbers are what i posted not the original poster.
My understanding there isn't a direct way to calculate compression ratio from measured compression.
Compression ratio is a number calculated based on volume of the chamber, piston stroke, and bore.
Measured compression is a way to tell the health of an engine.
My wrx runs 8:1 compressions ratio and i measured the same thing (150) when healthy.
The important thing is the difference between all of them.
I was able to diagnose my wrx the same way. Blown headgasket showed no compression on the driver side cylinders. Three years later, car seems to run fine but going through oil. Drivers side i'm measuring 60psi and 120psi. Took it to mechanic to confirm issues and they verified it.
really!
The measured numbers are what i posted not the original poster.
My understanding there isn't a direct way to calculate compression ratio from measured compression.
Compression ratio is a number calculated based on volume of the chamber, piston stroke, and bore.
Measured compression is a way to tell the health of an engine.
My WRX runs 8:1 compressions ratio and i measured the same thing (150) when healthy.
The important thing is the difference between all of them.
I was able to diagnose my wrx the same way. Blown headgasket showed no
compression on the driver side cylinders. Three years later, car seems to run fine but going through oil. Drivers side i'm measuring 60psi and 120psi. Took it to mechanic to confirm issues and they verified it.
Hi I Wantatta,
I'm not saying this to start a problem but I think you should know. The compresion ratio is determind by the cam. You can retard or advance the compresssion by moving the timing chain on the cam. Has your motor been worked on? A lot of shops build a 350, don't work. No offence intended.
No problem, i'm curious to understand it also. I tried asking the same question years ago and never got an answer.
From my research, measurements have usually been all over the map for the same car. Just different tools and users. The important thing is to have consistency over the cylinders. Going over to the Subaru forums and you can find tons of posts where people measure different things on the same engine but they are at least in the ballpark. There seem to be too many variables to get an exact answer.
My car is on the stock engine. As far as i know it has never been opened. Car has been inspected by two of the more knowledgable buick guys around (i've had non-engine work done by them).
From my research, measurements have usually been all over the map for the same car. Just different tools and users. The important thing is to have consistency over the cylinders. Going over to the Subaru forums and you can find tons of posts where people measure different things on the same engine but they are at least in the ballpark. There seem to be too many variables to get an exact answer.
My car is on the stock engine. As far as i know it has never been opened. Car has been inspected by two of the more knowledgable buick guys around (i've had non-engine work done by them).
Hi I Wantatta,
I'm not saying this to start a problem but I think you should know. The compresion ratio is determind by the cam. You can retard or advance the compresssion by moving the timing chain on the cam. Has your motor been worked on? A lot of shops build a 350, don't work. No offence intended.
I came across this which explains why you might measure more than expected
Puma Race Engines Technical Guide - Compression Tests
It explains it a lot better than i can.
A simple multiplying factor he uses instead of taking into account all the variables is multply by a factor of 17 or 20 which will give you a range 136-160 for an 8:1 ratio.
I came across this which explains why you might measure more than expected
Puma Race Engines Technical Guide - Compression Tests
It explains it a lot better than i can.
A simple multiplying factor he uses instead of taking into account all the variables is multply by a factor of 17 or 20 which will give you a range 136-160 for an 8:1 ratio.
Hi iantatta,
Well, I decided to respond. If you are actually running 160 lbs per cylinder that means you are running at a 10.3 compression ratio.
You are correct in the measurment aspect. You do need a dial indicator to mesure top dead center and bottom dead center to calculate center line of the piston, which depends on the cam and recommendation to where it should be set. I use a rollmaster timming chain so I can get as close to where I want.
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