ok i got in an argument with my buddy and he says that if he were to do the compression test on a hot motor they would be lower numbers than on a cold motor.I think its the opposite so what do you guys think?i mean metal expands when hot so it would seal better and hold more compression right?I would try it now but my adavnce auto parts compression gauge took a poo-poo on me.

ILBB

 

Cold oil has higher viscosity so it would seal better. That would cause higher compression.

Cold oil provides more resistance, so the starter motor would spin the engine at a lower rpm. Cold cylinder walls cool the air, as it's being compressed. These would cause lower compression.

These things work against each other; some engines go one way, some the other way.

 

I would think and this is just an opinion based upon the physics of the matter that the compression numbers would be lower on a hot engine because air molecules are moving at a faster rate when the engine is hot, rather than when it's cold and they are condensed......When the engine is colder and ambient air temperatures are cooler more air is being drawn in that when the molecules are spread apart it is for this reason that when compressed I would believe a cold engine to generate larger numbers than a hot one......

 

No. The compression is still controlled by the compression ratio, whatever that is (10:1?). The number of molecules in the cylinder is different hot vs. cold, but the compression process is the same.

Adiabatic compression heats the air quite a bit, same as with any other type of compressor. But if the walls are cold, it'll cool off the compressed aira little.