Hi guys,
What exactly is engine compression, and how is it measured? My friend got her engine tested the other day and it was 150 - but what units are these, and what is the top or
bottom of the scale?
Is this good or bad?
justin

Engine compression is when the piston pushes the air/fuel up into the combustion chamber. The A12 has 293 cc's in each cylinder. The head has a combustion chamber of 27.5 cc's or 29
CC's. Divide the head cc's into the piston cc's (293 divided by 27.5 = 10.65) and you have the compression ratio, which should be about 10:1 after considering the head gasket and leakage
and all occurs and it is rated in PSI (pounds per square inch). Below is a chart of PSI to compression ratios.

260 PSI is 14:1
240 PSI is 13:1
220 PSI is 12:1
200 PSI is 11:1
180 PSI is 10:1
160 PSI is 9:1
140 PSI is 8:1
120 PSI is 7:1

150 PSI is 8.5:1 compression ratio. I hope this helps you understand compression ratios better. There may be others that can give a clearer description for you.

I'd say that's in PSI (pounds / square inch)
Yep, plenty more pressure than found in your tires.
Most engines should have more than 150PSI (maybe above 200psi for high
compression)
My CA18DET ranged from 120-180PSI (thus the current rebuild)

Also to note that it's generally measured under start cranking
condition, not while running!

Chris

So if the engine compression varies, that's bad aswell?
Are you getting anything special done in the rebuild? I hope I don't have to rebuild
mine for a while!
justin

so chris ya rebuilding the engine anything specail going in??
My engine needs some head rebuilding as it has been leaking oil into the exhuast port smokey on startup ect ect still goes like a cut snake with 20psi up
its guts
cheers Andrew

Yeah, going large as possible forged pistons, rods, headwork and few other things. No new cams, not sure about valves yet. BTW my T28 is toasted. It appears to have almost siezed, and
the intake wheel has been rubbing against the housing. Looks like bearings. So I might also be up for another turbo. But first comes the new car rebuild, the engine is happening on the
side. I'm letting another fella do the engine, I'm too busy with normal work and the bodywork for the new car.
I hope my rebuild will be a show car and will still do 12's. After hearing about the success of a 180SX rear end in a 1600 wagon, I'm thinking of doing the same in my 1200, something
else I'm looking into.
Just got to get on with it.

Chris.