Compression and Leak Down test

[XtremeOverKill]

Ok I'm going to admit my ignorance here for a bit....

I know how to do a compression test
but what should one be looking for, or look at when doing the test?
I've read on this list of people having 120 Psi.... and then I've seen Mercedes that have up to 175 psi. What's the difference...? To say What does a higher compression engine yield over a "low" compression engine.

And then I've heard something about Cranking Compression and Static Compression? What's the Difference?

And then there's a Leak Down test. I understand how you set it up, and procede with the test... but what do the percentages tell you, and its a percentage of what exactly?

Humbly
Mike

[pyagid]

I higher compression motor will generally be a more powerful motor, but you also have to run a higher octane gas.


A leak down tester uses air from a compressor and measures the rate at which it leaks through the engine. You can find the source of the leakage by listening for the escaped air. Internal leakge is found by air bubbles in the cooling system, or you could hear air coming out the exhaust (exhaust valve bad) , or carb (bad intake valve)

if air is pumped into the cylinder at 100 PSI and the gauge reads 97 PSI, then the leak down percentage is 3%

[Goose]

Well the most important thing I look for when doing a compression test is even compression across all cylinders. and at least as high a reading as stock (in the book) but I dont want to see a low reading on one hole.. that means work
One thing I havent quite figured out about the AMC yet is how or why they seem to "share" compression between two holes .When I tore this one down you could see that it had been.

[XtremeOverKill]

Share Compression?????

I'm baffled................... I thought if this happened you were due for another headgasket?