Compression Testing, Hot and Cold

[Deleted User 62]

I know a compression test should be done with the engine at operating temperature. If it was done on a cold engine, how would it effect the results, higher or lower? This question came up on another forum. Having never done back to back tests, I'm really not sure.

[Major Softie]

Lower and more inconsistent.

A cold engine can sometimes give significantly different numbers from one cylinder to the next, but test fine at operational temp.

You can do it on a cold engine, but it only can give you positive information: that is, if it tests good cold, it's probably good at all temps, but if it tests bad cold, you don't know much at all.

I don't know how much it varies among different designs, but, knowing metal expansion rates, I would guess the difference would be most problematic with iron cylinders, somewhat less so with iron lined cylinders, and least problematic with Nikasil cylinders. That's all guess-work, but (I would think) likely to turn out to be true.

[Deleted User 62]

Got it, makes sense. Thanks, Major!

[CVA-42]

Good stuff. Duane has a lot of compression test tips on his website.

[montmil]

It is my understanding that an engine going through a compression test requires unrestricted air flow into the cylinder/s.

With the CV Bing carburetors, it's necessary to remove the carbs from the cylinder head mounting stubs to achieve full air flow and the best compression numbers. The lowered CV slides heavily restrict air flow into the cylinder and may present the tester with inaccurate figures.

Just my $0.02 worth.

[Major Softie]

Yep, that's really important too.

"Standard" slide carbs only need the throttle to be held open (i.e. Dellorttos), and remove the air-cleaner. CV carbs need the throttle AND the slide held open, or, as most people do, just take the carbs off.

[Duane Ausherman]

What MS said.