Compression Results - What does this tell me?

[Marccus]

'89 w/465K

Here are some compression results done about a year ago:

cylinder # psi(w/o oil) psi(w/oil)
1 185 -
2 185 210
3 190 -
4 200 -
5 205 -
6 205 -

What does this tell me?

 

[vortex2450]

Those numbers are healthy, you've definitely got some build up on the pistons if that mileage is on an original engine and the oil probably helped raise #2 because your rings are simply getting worn. At least that's my $ .02

 

[Marccus]

What about the vast difference between cylinders (210 vs 185) and the fact that three cylinders are at 210 and the other three at 185? Does the wearing of the rings have something to do with the firing order of the cylinders?

Compression ratio is 9.8:1 for this engine. Yet I am getting much higher values. Is that because of buildup on the cylinder walls? Shouldn't I start to get knocking as the actual compression ratio goes up and/or short life and burnt tipped spark plugs?

 

[Off Road SHO]

What three cylinders are at 210? And 205 to 185 aint bad of a spread. And Carbon only builds up on the top of the piston, not the cylinder walls. Just de-carbonize with some water and it'll be fine.

Tom

 

[vortex2450]

What about the vast difference between cylinders (210 vs 185) and the fact that three cylinders are at 210 and the other three at 185? Does the wearing of the rings have something to do with the firing order of the cylinders?

Compression ratio is 9.8:1 for this engine. Yet I am getting much higher values. Is that because of buildup on the cylinder walls? Shouldn't I start to get knocking as the actual compression ratio goes up and/or short life and burnt tipped spark plugs?

Like Tom said, a spread of 185-205 is fine, that's within 15% range of each other, closer to 10% if I'm not mistaken. Is that a stock engine with that kind of mileage?? If so good for you man!

Your problems start when you start getting results ranging from say 165psi - 195psi. Your fine, personally if i wanted to clean any possible carbon build-up I would pour a small amount of Marvel into each cylinder and let it sit for about 30 minutes then run it out. People will debate about snake oils but the way I see it is that it burns up rather than evaporating like water would.. To each his own.

 

[Showgun]

What about the vast difference between cylinders (210 vs 185) and the fact that three cylinders are at 210 and the other three at 185? Does the wearing of the rings have something to do with the firing order of the cylinders?

Compression ratio is 9.8:1 for this engine. Yet I am getting much higher values. Is that because of buildup on the cylinder walls? Shouldn't I start to get knocking as the actual compression ratio goes up and/or short life and burnt tipped spark plugs?

Not vast enough to worry about. From memory, the highest to lowest should be within 10%. That engine is right there.
I"ll bet that engine is smooth as silk.

Wow, that's amazing service -- 465k miles! You have really looked after it (or somebody has)

 

[Lorenr]

Leak Test the Motor

I've never been good at getting realistic results from a straight compression test. Gauges differ, testers differ and getting back to back results, well, good luck. Adding a couple of drops of oil and your really messed up.

Find someone with a leak down tester, buy one on E Bay, or find a shop that will do it. You do need a compressed air source also.

On a motor with that many miles anything less than 10% is good.
A new motor with good ring seal will be less than 5%.
A really good motor might be 1-2%.

Leak the motor at near operating temperature.

Cylinders on your motor should not very more than 1-2% loss.

Does really low leakage mean power? I don't know. Does 10% leakage mean loss of power? I don't know. I've seen motors with 13% leakage make serious power, but they really did have combustion getting past the rings. I think.

If there is a significant variation in leakage, then you probably need rings and/or a valve job. Check the cylinders for the original cross hatch. I would think long and hard before boring one of these motors.

:swing: