Can someone review the compression test vs. leakdown vs. Italian tuneup debate?
 

I recall a debate a while back when someone got a bad compression reading and people said that does not mean anything definitive. Like it could just mean the valve might just have some deposits that may get burnt off after a spirited drive. Am I remembering correctly? Is this a possible outcome to a bad compression test?

Is a leakdown test more conclusive than a compression test? Or could the same fault/remedy potentially apply to a bad leakdown? I can't remember if one was more definitive than the other.

I am asking since another poster here has received a bad compression test on a cylinder. On his behalf, and for forum documentation, I am just wondering if that 100% means top end job.

Is Italian tune-up sometimes a remedy for bad compression test?
Is Italian tune-up sometimes a remedy for bad leakdown test?
What situation calls for an Italian tune up as a possible fix?

Compression test gives you cylinder pressure readings & tells you if you have leakage somewhere.

Leakdown test tells you if you have leakage somewhere & can help you identify what's not sealing properly (intake valve, exhaust valve, rings).

Italian tune-up can help blow out some carbon, so if there's a small bit on a valve seat that's keeping the valve from seating fully it might get heated up & blown out. IMO, Italian tune-up probably won't do much for worn rings but might help reduce some oil leakage by heat cycling gaskets, etc.

In the 911 Engine a bad compression can also be valve lash to tight if done on a cold Engine. The valve lash grows wider by 3X when the motor is hot. A 0.1mm lash will grow to near 0.3mm fully warm motor. And the lash gets tighter as the Engine ages with milage, so it's possible to have a valve slightly hung open on a cold engine that has not had a proper valve adjustment.

Steve Wong tuned my 1988 on a chassis dyno a few years ago. We did 10 pulls. During one of the early pulls a fair amount of "stuff" exited the exhaust tip (car has a pre-muffler and Dansk sport exhaust), it wasn't anywhere near rolling coal, but it was noticeable. Both Steve and the dyno operator commented regarding "blowing out the carbon."

I don't know that you can duplicate the loads associated with dyno pulls, but it's worth a shot.

As an aside, first pull showed 190hp, last pull 210hp (car had 55k miles, the aforementioned exhaust, open airbox and Web 20/21 cams). May have been one of the happiest days of my life. 🤣

https://amoroso.smugmug.com/1988-Porsche-911-M491-M470-Poi/1988-911-Dyno-tune-at-Game

Lots of wife’s tales on differential compression testing. Unless it’s done HOT it’s rolling the dice with accuracy.
Done a couple thousand of these:
https://www.lycoming.com/sites/default/files/Cylinder%20Compression.pdf
http://continental.aero/xPublications/xService%20Bulletins/ECi%20Archives/10-4/

Without the ability to rock the piston back and forth it’s almost impossible to get a perfect ring seal consistently. For a car, oil consumption and spark plug reading should be enough. The simple act of removing a spark plug can dislodge carbon into the cylinder and possibly under a valve. The best use of the test is as a diagnostic before there is a problem.