Topic: Why warm engine to perform a compression test?

[Boomologist]

I have read in almost every manual that you should warm up the engine before doing a compression test. Aside from making the head hot so you burn your fingers, my question is why?
Do you get a different reading between cold or hot engines?

I realize that the metals will expand but does this raise or lower compression? If so, is it a noticeable difference? Or does running it place oil on the cylinder walls to raise compression? 
Thanks in advance,
     Ron

[ProTeal55]

You want the motor to be at operating temp before you check for compression, or anything else related to power/performance. A cold motor, no matter it be in a car or on a bike, will run differently (usually worse ) than when at the given operating temp...

[Lumbee]

...I always do my comp. tests cold.  No way I'm burning my hand trying to get a comp. tester in/out of cylinders 2/3 when the motor is hot.     I assume a cold test is going to read lower than a hot test, so as long as I get 120 or so cold, I know I'm good to go.

[TwoTired]

Do you get a different reading between cold or hot engines?

I realize that the metals will expand but does this raise or lower compression? If so, is it a noticeable difference? Or does running it place oil on the cylinder walls to raise compression? 

Steel sleeves and aluminum pistons expand at a different rate with the piston expanding more than the sleeve.  Also the rings expand to fit onto the cylinder wall with minimum end gap.  Operating temp is where everything fits the best as that is where operation has occurred the longest for the parts to "marry" together.  This is also the predominant run condition. So, taking readings with a warm engine is the same condition as the engine encounters on the highway, rather than parked in your garage.

Cold compression numbers have always been lower in my experience.  I'll do these on an unknown engine to determine if it warrants further attention to get it running.  But, if it runs oddly at normal temps, I'll do the comp test with the engine hot to verify the mechanicals in an "as running" configuration.

Cheers,