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Compression Test Hot or Cold

Chippster1

Active Member
I hear all different opinions on whether compression tests are more meaningful hot or cold
Would love to open a discussion

Ted Chipps
RV10 498EC
 
Warm/Hot

Your engine never is cold in operation except @ start. False reading IMHO, cold. A bad hot reading is most likely no better cold.
 
Yeah, another popcorn 🍿 thread :D

Do them cold, if not good enough repeat hot ;)

PS
I usually go for a quickie (flight…), and then do the hot items, compressions and the oily stuff…
 
I prefer to do them hot, because you’ll get better numbers to put in the logbooks….

That said, if something is broken bad enough to give you less than 60/80 (or the engine is just so tired that you are getting less than 60/80 on multiple jugs), then hot or cold - you know the answer…you have a problem. So diagnosis wise, it rarely makes a difference. But there are those logbooks that stick with the engine forever…..

Paul
 
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Hot is best practice. However, cold will generally produce lower results. Therefore, if you are just looking for problems and you get acceptable results cold, you can call things good. If looking for long term trends, tests should always be done at similar temps and never cold.
 
Don't think it makes much difference. Primary interest is valve sealing. Even the manufacturers don't agree on acceptable ring leakage.
 
cold

Agreed, doesn't make much difference.
Just use the same procedure each time you do it, hot or cold.
Cold is much less trouble to deal with. When doing my annual, I have the cowl off, along with various other parts and heating up the engine during this process is just very inconvenient.

Lycoming says a lot of things :
Lycoming says “Hot” .. see step 1…

They also say running one of their engines with ethanol doesn't work.
See my tag line.
 
Cold!

Cold test is much faster: you don't need to warm it up. If it's bad then run it and retest. If the number are not perfect don't put them into the logbook if that's a concern!
 
I hear all different opinions on whether compression tests are more meaningful hot or cold

20 years ago a cold compression check in the 60s on my first RV scared me. I added some heat, suddenly it was at 78/80. A significant emotional event, which showed me the value of doing compression checks with a hot engine.

Today, I want meaningful, consistent results that I can check against Lycoming's published criteria (https://www.lycoming.com/sites/default/files/Cylinder Compression.pdf). I achieve that by doing compression checks under the conditions Lycoming specifies (hot engine).

I normally do a compression check at annual, when I'm also doing an oil change, which also requires a hot engine for easiest oil draining.
 
Is a hot engine cylinder compression number ALWAYS higher than cold? If not, what would cause a cold number to be higher than a hot number (besides not properly performing the test)?
 
Is a hot engine cylinder compression number ALWAYS higher than cold? If not, what would cause a cold number to be higher than a hot number (besides not properly performing the test)?

No.

The valves will generally seal the same, or at least very close, hot or cold. The rings, however, are sitting in a piston land without a lot of clearance. The clearance can fill up with carbon (coaked oil) and this, when cold, can squeeze on the ring and can prevent it from seating well on the wall (ring exerts pressure on the wall via tension). Once hot with fresh oil moving around, things loosen up and the ring can better exert force against the cyl wall, and provide better readings. Same concept as morning sickness with exh valves. Heat expands things, so when cold, various things can be tighter and not do their job as well as when hot.

Larry
 
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