Compression tests-hot vs cold ?

[Aviaman]

I had what seems a counter intuitive result. I got compression readings that were slightly higher measured cold, than when measured hot. It wasn?t a big difference, something like 72,73,73,73 hot vs. 76,77,77,77 cold. It seems when hot, the clearances and fit would be tighter, causing compressions to be higher. Any ideas?

[PilotjohnS]

I thought it always had to be done hot.
If cold compression is higher, curious if the rings are causing the compression loss. Thick oil trapped in cylinders will tend to help with better compression numbers due to less blow by.
Did you hear any hissing from breather during hot testing?

[plehrke]

Does it matter, hot vs cold, as long as you are consistent from one time to the next? I always thought compression checks are looking for change since absolute numbers don?t really mean a lot as long as they are pretty good.

[jrtens]

Originally Posted by Aviaman

I had what seems a counter intuitive result. I got compression readings that were slightly higher measured cold, than when measured hot. It wasn?t a big difference, something like 72,73,73,73 hot vs. 76,77,77,77 cold. It seems when hot, the clearances and fit would be tighter, causing compressions to be higher. Any ideas?

I assume you are positive that you had exactly 80 psi coming in for both tests?

[Aviaman]

Yes, I was careful to dial in 80psi.

I had a thought that the air connections are themselves sources of leaking. Since there is an air connection at the output of the measuring device, and also at the adaptor plug at the cylinder, any leaking of those 2 connections would lower the reading. Those are harbor freight air connectors. Has anyone had issues with Them leaking? It might not matter for many applications, but for a leak down measurement, it may very well.

[Scott Hersha]

One thing I learned when getting a prebuy on my RV4 is that it is called a leak down compression test, not a compression test. I used to rock the prop looking for the highest pressure, and then only held it there for a couple seconds to record the almost highest pressure. My mechanic looked for the peak pressure at or near TDC and then just held it there until it quit leaking down and recorded that. If you peak at 78, you don?t record 78, unless it stays there while keeping the 80 psi input pressure constant. If you start at 78 and then it slowly leaks down to 50 in 30 seconds, you may want to investigate further.

That being said, I don?t think a leak down check tells you a lot, unless it leaks down to a relatively low pressure, like 50?s or lower/80.

[Capt]

Correct, it is a leak down test, the ability for a cylinder to hold pressure against what's being forced into it, IE: PSI.

[AlexPeterson]

Originally Posted by Aviaman

I had what seems a counter intuitive result. I got compression readings that were slightly higher measured cold, than when measured hot. It wasn?t a big difference, something like 72,73,73,73 hot vs. 76,77,77,77 cold. It seems when hot, the clearances and fit would be tighter, causing compressions to be higher. Any ideas?

What is creating the seal are steel rings inside a steel barrel. Both have the same thermal expansion coefficient, so those two items probably don't care about what temperature you measure compression at. However, the oil is much thicker when cold, so tiny menisci between the rings/cylinder/pistons will take a higher pressure to "blow open" at cold temperatures.

Or something else...?

[Caveman]

Originally Posted by Scott Hersha

One thing I learned when getting a prebuy on my RV4 is that it is called a leak down compression test, not a compression test. I used to rock the prop looking for the highest pressure, and then only held it there for a couple seconds to record the almost highest pressure. My mechanic looked for the peak pressure at or near TDC and then just held it there until it quit leaking down and recorded that. If you peak at 78, you don’t record 78, unless it stays there while keeping the 80 psi input pressure constant. If you start at 78 and then it slowly leaks down to 50 in 30 seconds, you may want to investigate further.

That being said, I don’t think a leak down check tells you a lot, unless it leaks down to a relatively low pressure, like 50’s or lower/80.

Take a look at Lycoming SI1191A. Download it here: https://www.lycoming.com/content/ser...ction-no-1191a
"To assure that the piston rings are seated, the propeller is moved slightly back and forth with a rocking motion while air pressure is applied; thus providing a more accurate reading"

I was taught to always check compressions when the engine has been warmed up, as the SI states. Just for kicks I have checked my RV7's cold twice then followed up with a hot check. Both times the cold compressions were a few psi lower. YMMV.

BTW as far as not telling us a lot, item #3 addresses that. A compression test is a very good tool used for decades for good reason. When combined with borescope inspections it tells us much about the engine's top end health.

[DanH]

Given our primary interest is valve seating, I don't think hot or cold makes a lot of difference. Just do it the same (hot or cold) every time.