Outer Limits 4x4 Board

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

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.

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.

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.