compression test out of car, motor cold, how much does it affect compression readings

[Indolent]

before i slap in this motor, i'm going to compression test it, i read that it's best when the engine is fully warmed up, well i can't really do that but 'how' does it affect compression readings? like how off would it be or something if it were warmed up? basically i just want a better understanding why it is so.

[RightWheelDrive]

I did a cold compression test just last week. It is better to do a compression test when the engine is warmed up to operating temperature because the metal expands when it gets hot, which makes the gaps in the engine smaller, which in turn gives you better compression. You should still be getting good compression numbers when the engine is cold since the metal expansion will not make up for worn rings and such. I hope that helps, Good Luck.

[turbozxi]

You will get a lower reading, but aslong as the readings are similar between the cylinders you should be good.

[mikesim]

Originally Posted by turbozxi

You will get a lower reading, but aslong as the readings are similar between the cylinders you should be good.

i did a test once when cold, it was almost 30psi lower across all four cylinders.

[formeRiceR]

I read somewhere that what you can do is do a compression test cold and then you do one hot. The difference between the two figures tells you how worn your rings are ... or something like that....