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Compression test...hot or cold?


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Quite the opposite Simon.

 

It's better done cold to highlight any issues when tolerances are at the far end of the scale, after all, any machining work is carried out cold or at best room temperature, someone will come along with a more comprehensive reason & explanation.

 

You're of course aware of a "wet compression test" to prove piston compression rings???

 

Richard.

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I was always told to do it with the engine warmed up as that's how it is when the engine is running.

 

In may ways it doesn't matter greatly in that what you are looking for is disparity between the cylinders that would indicate a localised fault. 10% difference is considered significant.

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Quite the opposite Simon.

 

It's better done cold to highlight any issues when tolerances are at the far end of the scale, after all, any machining work is carried out cold or at best room temperature, someone will come along with a more comprehensive reason & explanation.

 

You're of course aware of a "wet compression test" to prove piston compression rings???

 

Richard.

+1 and it means wide open throttle

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+1 and it means wide open throttle

 

Not sure. Correct operating conditions includes oil at operating temperature. If the oil is cold,hence thicker, it will affect all tolerances and give a false reading. But worth doing the exercise both hot and cold. Ideally the reading should show lower compression when hot

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Not sure. Correct operating conditions includes oil at operating temperature. If the oil is cold,hence thicker, it will affect all tolerances and give a false reading. But worth doing the exercise both hot and cold. Ideally the reading should show lower compression when hot

I am.

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I've had a wee search on the Interweb and it would seem I stand corrected. Any page I've found on engine compression testing suggests it's carried out at normal operating temperature. None give any great detail on why???

 

In my trade as an Industrial/Marine Diesel Engineer I always carry the test out on a cold engine, probably because I don't want burnt digits and, on many slow-medium speed diesels by the time you've removed the injectors (half a shift later) the engine will have already cooled to ambient temps.

 

Anyway, every day is a school day!

 

Richard.

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Many modern pistons are machined slightly oval when cold. And become circular when at running temperature.

http://www.partinfo.co.uk/files/SB2169%20-%20PISTON%20SKIRT%20CLEARANCE.pdf

So hot test seems correct to me.

 

Dont know about TR pistons, but I doubt that precision of machining was used back then ??

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I took a compression test on one cylinder this morning stone cold and then dead hot after 1/2 hours running to verify the assertions. My assumption was that the reading would be more accurate under operating conditions; that taking the compression cold would give false indications and that the compression would be lower with engine hot.

Result:

COLD: 161 PSI

HOT: 151 PSI

Bingo...

 

 

 

 

post-11469-0-51017000-1414721628_thumb.jpg

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Ah... measurements, great yes I agree.

But science would ask why is the hot reading less?

I suggest the hot air in the inlet manifold was less dense than in the cold engine- so gave less psi when compressed.**

And in support of that contention:

http://resources.yesican-science.ca/SS_version1/ss/g08/fl/density.gif

So the ratio between 20C air and 50C air in hot manifold is 1.3 to 1.15 or about 0.88

The ratio between the hot and cold compression readings was 0.94

So air density not mechanical influences can expalin the hot vs cold compression readings.

That's science!

 

 

And like all science always up for debate.....

 

 

Peter

 

 

**

Gas Law

P.V=m.R.T

So P = ( m/V) . R . T

Hence P is proportional to density m/V

Edited by Peter Cobbold
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Both cold and hot readings can be relevant on occasion - depends on the symptoms and the nature of the enquiry.

 

Cheers

 

Alec

Indeed so,you must find maximum when cold as that is when the engine is getting most strain on it.

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