Compression test cold and warm egine: differences

DimitrisIX · Jan 12, 2009, 06:48 AM

I just did a compression test on my relatively new build. It came out 100 psi across all cylinders but the engine was cold.
Has anyone tried it on a cold engine, then warm it up and check again. How much difference can the engine temp make?

thanks

SloRice · Jan 12, 2009, 06:50 AM

or just warm the motor up.....

cold compression tests are useless.

DimitrisIX · Jan 12, 2009, 06:53 AM

I just installed my BR COP i cant bother to remove it :-) i know that the engine should be ok because they are all 100 i just wanted to know how much difference can engine temp make. I searched and found posts reporting 140 psi warm on a IX. So by warming up the engine you can get 40 psi more? it seems weird to me but I am not a mechanic.

SloRice · Jan 12, 2009, 07:09 AM

There is no formula that says it will be xxx psi when cold and yyy psi when warm.

As for the 40psi increase you said from the motor being warm. When metal gets warm, it expands, thus tightening all the clearances in the motor. That's how it creates more pressure. Simple physics.

JakeTheVIII · Jan 12, 2009, 07:39 AM

Like he said, your test was useless. If you ever re-do it with a warm motor, that will give you a real measurement.

CNano8 · Feb 5, 2009, 12:01 PM

Dont mean to bring back a thread from the dead but how would you go about compression testing a motor thats out of the car already? Like if you were to buy a second longblock off someone.

IMPdemon · Feb 6, 2009, 04:16 PM

FWIW I got close to 180 across all cylinders when I was doing a pre-buy last spring on my 8, and that was ice cold.

For testing a block out of a car, doesn't the starter motor stay w/the block? I would hook that up to 12 V and crank away.