Compression readings...what do they really mean?

GlennAB1 said:
Finally somebody named it right, thank you, sir.
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Not really, It will also show you a cracked cylinder.

Like I said in post #2 and now some one finally got it right. yeah right!
 
How many here know why 80 is used as the baseline, as opposed to 90 or 120 or some other pressure?
 
bluerooster said:
How many here know why 80 is used as the baseline, as opposed to 90 or 120 or some other pressure?
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They didn't want to see A&P's flying across the hangar. :D Get over 80 PSI and the prop on a geared engine can do some nasty things.
 
So close that it hurts, and another reason; it was started back in the days when round engines were the norm, and 80psi wasn't enough to significantly move the piston at TDC.
 
bluerooster said:
So close that it hurts, and another reason; it was started back in the days when round engines were the norm, and 80psi wasn't enough to significantly move the piston at TDC.
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Old round engine mechanic here, we did not use the static compression type of compression check. We used a gauge in each cylinder and cranked the engine. just like they do car engines today
 
and when you got a low reading then what did you do?
 
Runup to max rpm and if you get blue tailpipe smoke its rings, then and pull the power back quickly and if you get black smoke tailpipe its valves?
 
I've seen shiny cylinder walls that had excessive wear (no longer had surface texture pattern to retain oil) on them that could pass a differential compression test with flying colors, but really were on their last legs.

V/r,[/QUOTE]

That theory has been disproven so many times, I can't believe any one would still post it.

Oil is controlled by the oil control ring, which has nothing to do with compression. other than adding oil to the rings that blocks air escaping thru the rings.
A worn out oil control will in fact increase compression, that is why an engine that was tested cold will fail, but a hot engine with plenty of oil on the cylinder walls and rings will pass.

All scratches placed on the cylinder during honing will fill with carbon by the burning process and polished by the rings. when this happens the break in process will be complete. simply noted by oil usage stabilization.
 
I owned a jeep pickup truck that had a 6V battery and when it was HOT, it wouldnt start because of TOO HIGH A COMPRESSION caused by oil in the cylinders increasing the compression. It ran ok. We fixed it by putting in a 12V battery permanently mounted in the truck and a knifeblade switch connecting it just for hot starts. 6V sucks.
 
I get ticked if I get one below 70.




All day :cool:

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coloradobluesky said:
I owned a jeep pickup truck that had a 6V battery and when it was HOT, it wouldnt start because of TOO HIGH A COMPRESSION caused by oil in the cylinders increasing the compression. It ran ok. We fixed it by putting in a 12V battery permanently mounted in the truck and a knifeblade switch connecting it just for hot starts. 6V sucks.
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Your Jeep probably had a weak starter.
 
Jumping it off a 12V was enough to start it. Only happened when the engine was hot.
 
There is another a way to tell with an automotive compression test if its rings. If you get low compression, squirt some oil in the cylinder, if the compression goes up its rings, if not its valves.
 
You win this time, we'll see next annual ;)
 
80 pounds of pressure is used as higher could and would cause engine damage .so they say
 
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