Compression test result diagnosis

[guzzla]

How much variation in compression points do you need to determine that something is a-miss and what would be subtle noises/performance changes that would confirm this ? I know if it had completely dropped a piston it would have a noticable miss and or busted HG but this doesnt seen to be the case.

I know the familiar causes are worn piston rings, fatigued/broken valve springs and blocked oil galleries and worn cam lobes. I guess the best way to determine which it is is with a leak down test but short of pulling it apart you got any other ideas ?

Thanks in advance

[-Scott-]

So you've done a compression test & have some numbers? Do you have "wet" and "dry" numbers? If compression comes up with the "wet" test that's a good indication that rings are the problem. If not, it's probably a valve issue (which could be a seat issue - not necessarily a spring/lobe problem.)

Also, throttle position will affect the result - an open throttle allows more air in than a closed throttle. Depending on your vehicle and the efficiency of the intake manifold I wouldn't be surprised if closed throttle readings could vary significantly from say end cylinder to middle cylinder - but I would expect less difference in an open throttle test.

Other than that, 10 psi difference I wouldn't worry about. 20psi I would think the motor is getting "tired" and anything beyond that I would start investigating/planning to do something.

[want33s]

As a general rule anything more than 10% (of the highest reading) difference and there is a problem.
As said ^^ do wet and dry tests with oil in cylinders.

[MightyMouse]

Many professional engine diagnostic analyzers can do realtime starter current plotting. These can very quickly show up cylinder to cylinder variations.

They can show variations but obviously not absolute pressures. - but its very fast.