Hot vs. Cold Compression Test (1 Viewer)

[Gimme a 60]

Hi, I have done a lot of searching on Hot vs. Cold compression test- and I have had limited results.

I mean to ask, what allows for more compression? Some people on here have said that hot compression will allow there to be lower compression numbers because of thermal expansion, and when it’s cold, everything is super tight allowing there to be higher numbers.

I’m just confused since i got my compression numbers back from my BJ60 - 400 psi across the board cold.

Thanks.

 

[ToyotaMatt]

Hi, I have done a lot of searching on Hot vs. Cold compression test- and I have had limited results.

I mean to ask, what allows for more compression? Some people on here have said that hot compression will allow there to be lower compression numbers because of thermal expansion, and when it’s cold, everything is super tight allowing there to be higher numbers.

I’m just confused since i got my compression numbers back from my BJ60 - 400 psi across the board cold.

Thanks.

i have not heard of a hot and cold compression test ?


i HAVE heard of a WET and DRY .....

 

[Gimme a 60]

i have not heard of a hot and cold compression test ?


i HAVE heard of a WET and DRY .....

Idk the guys in the diesel section were talking about it.


But there was some confusion…. I wanted to clarify but I guess nobody here knows either.

 

[ToyotaMatt]

Idk the guys in the diesel section were talking about it.


But there was some confusion…. I wanted to clarify but I guess nobody here knows either.

what does your trusty OEM toyota paper print media FSM tell you ?

 

[Gimme a 60]

what does your trusty OEM toyota paper print media FSM tell you ?

It says compression test should be done at operating temperatures. I didn’t , so I’m wondering if I should be expecting better compression when the engine is hot.

 

[ToyotaMatt]

It says compression test should be done at operating temperatures. I didn’t , so I’m wondering if I should be expecting better compression when the engine is hot.

Excellent ! .


- the original paper print media OEM FSM never lies

.

 

[2mbb]

I always thought that parts fit tighter when when they were hot/warm, not cold. If the cylinder wall "expands" when hot won't the ID get smaller? If the piston "expands" when hot won't it's OD get larger? Doesn't this mean an overall decrease in the gap between the two, which means they fit tighter when hot?

I also think with compression tests there is a lot to be said about consistency. Each cylinders compression is compared against another, so you want to test all cylinders in the same way. Also, lubrication is a factor in sealing. Having a procedure to bring the engine to operating temperature, I think, will help with consistency. Providing no instructions for preparation or stating "test on a cold" engine is very vague. A cold engine could be one that ran last yesterday, or ten years ago. I would not immediately run a compression test on an engine that ran 10 years ago.

Another point, is that most of the time when the engine is running, it will be at operating temperature. This is the more valuable compression number. You won't get far running a cold engine just because it has better compression.

I'm probably overthinking this. Just do what the manual says.