EngineBuilding
Can we finally settle the debate. Compression test Cold or Hot?
I have 2/8 cylinders that when absolute cold read 76psi/ 40psi. Warm both jump to 160-165. Spec is 180. Manufacturer gives a 140-180psi working range.
*COLD*
| 40psi | 168psi |
|---|---|
| 73psi | 117psi |
| 178psi | 178psi |
| 130psi | 115psi |
*WARM*
The 3 strong cylinders stay strong, all others jump to 165psi +/- 5psi
Everyone seems to have their own opinion on the matter. The way I look at it, the engine spends 99% of the time in a hot state, and that's the numbers that matter the most. I only look at the cold numbers for extra insight, it tells me that something is clearly wrong on those two cylinders but the way I see it is, I have two workers that sleep a little longer but they eventually wake up.
Discussion
The answer depends solely on why you are hooking up a compression gauge in the first place.
The engine?
Tailpipe
Tailpipe ≠ engine
All assembly specs and clearances are at room temp or cold. A cold motor is cold. you can't guarantee consistent temps on a resting hot motor
Except valve clearance is nearly always given HOT
If it's low cold, eventually it'll be low hot. The ring land is a little tight when cold. It can stress the ring to crack.
You'll want to tear it down and inspect those pistons....and inspect cold.
Mine did this. There was a hole in the side of my piston all the way down to the middle ring and it was eating a hole in my block. Was also burning oil like crazy and anemic in power until warm. But I’d agree given my limited knowledge that something isn’t right
How cold?
As cold as your mother's bed....jk
Shouldn't matter that much and any difference should be minimal. You havd a problem thats bein fixed by piston expansion.
My main concern is that the numbers are close to each other across the cylinders. Hot or cold it shouldn't matter if there's a big difference, something's fucky. You could grab two brand new HF gauges off the shelf and they'll probably give you different readings, so unless you've sprung for an expensive gauge I wouldn't trust the numbers completely anyway.
Maybe sticky, dirty ring lands that free up when hot. Try spraying some oil in there when cold to coat the rings, and run the test cold. Perhaps some carb cleaner soaking in there to clean the rings. Run the cylinder to the bottom and spray a ton in there, and let it soak. Then spin the motor to push the excess out the plug hole, and squirt some oil in there to coat the cylinder walls before starting it again. Might want to change the oil, too, before running it.
Spraying oil into a cylinder will seal up the ring gaps and give you a better reading.
Perhaps I didn't explain that well. If it does, then you know it's the rings, and not the valves. There are probably off the shelf products designed specifically to clean up the rings.
If a compression test is required this might mean the engine won’t run so can’t get warm you have to work with what you can but heat causes expansion which can lead to differing results I think I’d be inclined to look at the suspect cylinders
In my experience, I came to the conclusion that Hyd lifter engine when cold and sat all night may lead to misreading due to bleed down, don't know if true, but I didn't really warm it up much, just started it up till oil pressure came up, maybe 20sec run time, all the numbers seemed more reasonable, except for the one cyl that was in question. Burnt valve
This may be part of the problem. I know my lifters are not great, if left to sit overnight they would clack a little, but they will go normal after driving a block or so. I generally just hold throttle at 2000rpm for a few seconds and it goes away. Maybe those lifters that keep bleeding oil are not allowing for proper valve actuation and causing the bad readings.
That's what I had found. Don't know if I'm right, but that was my theory
As far as I know , the proper way is Hot.
That being said - i check when cold. I want to know 1st 'pump' , I want to know that they are even at whatever point - sometimes its a consistent 1, 2, 3... sometimes i let it crank til it stops climbing. Depends on what I'm looking for/trying to resolve.
I work on old cars, and typically not expecting them to be perfect. If it uses oil - add oil. If you don't like the answer , rebuild your motor.
Is it ok and is it tuneable - thats all I need.
On airplanes we check hot. But 80 year old air cooled tractor engines are a totally different beast with differently ground cylinders… you can see the rings when borescoping through the spark plug hole.
Leak down test
My service manual says to do it hot, thats all i can tell you. It makes sense tho, thats when the piston rings seal the best
I have a question that may be relevant to the answer:
Do you spend more time running then engine when it is warm, or when it is cold?
If your cold compression is so bad the thing is hard to start, it’s a dud anyway.
Not an expert by any means, but I can't imagine a cold test being MORE relevant than a hot test. I think your assessment is fine. In the absence of any other symptoms, the engine seems to have good mechanical integrity and good compression.
Cries in Wankel
Lmfao gotta burn those carbon deposits off those apex seals.
The community swears don’t go off cold numbers… but also most don’t run enough to get warm.
In my eyes, the engine is designed to run at a set range of temperatures. That's where you should take all your measurements unless otherwise stated by the manufacturer. I work in BIG engine items to the point where RH and dew point are needed to be taken in to account.
Ahhh yeah forgot about y'all