Difference in comp. between hot and cold

[brookskb]

Checking the compression on a c25mlht I am getting about 15 psi less than in the past. In the past the check was with engine at temp. This time engine cold. Is this a reasonable difference due to temp?

Thanks

[TownsendsFJR1300]

IDK how much the difference would be between hot and cold but as you noted, it'd be more accurate to operating temp.

Rings (and other parts) are going to expand, seat better when warm (and more efficient) when warm/hot vs cold. Reading briefly on-line, (and of course everything on the net is TRUE!) 15 PSI is not unusual and can vary a lot between different engines.

Simply testing cold, then warm it up and see the different #'s would answer your question and would be interesting to know(hint, post back what you find)

On certain engines, its a also best to have the throttle WIDE OPEN when cranking/testing.

**Note, a leak down test would give you the BEST indication of the health of your engine.


I have a 15 year old 7 HP Briggs and Stratton mower engine, OHV, with a 98% efficiency...

[zenoahphobic]

Sometimes cold can read higher because the cold oil is more viscous (seals better), or if oil / fuel is left behind from previous unsuccessful starting attempts, displacing some of the space.
Sometimes there is a ritual you need to consistently follow to get accurate comparisons between readings. You need to crank several revolutions WOT and watch how the pressure progressively increases.
A lower reading after successive (excessive) cranking can occur because lubricating oil can diminish because in a 4st not fast enough to raise piston pickup or high enough oil pump pressure. In 2st simply can disappear out of exhaust port and is not replenished with a fresh charge fuel/oil (even with oil injection into intake- but with no petrol to disburse it).

[brookskb]

Got it running well enough to check at temp

OK. It is running well enough to warm up. The warm readings with the new gauge are up to 120 and 125 psi. The temp. change brought the pressure up 15 psi over cold.

Thanks for the help!

[zenoahphobic]

We take it that there was nothing wrong with your engine, that you were only concerned about that actual reading?

[99yam40]

thanks for sharing what you found.

I believe it is not just the lubrication,
it has to do with the pistons, rings, and cylinder walls all getting up to temperature & expanding so they fit as they should at running temp.

But I am sure they all play a part in it

[TownsendsFJR1300]

OK. It is running well enough to warm up. The warm readings with the new gauge are up to 120 and 125 psi. The temp. change brought the pressure up 15 psi over cold.

1^^ thanks for the update.

And, in your case, obviously, with a warm engine, parts are better seated/to temp, the engine is sealing tighter..