"differential pressure tester" that is one of those tools that you really only get good at by using and there just are not a lot of opportunities to do so. I borrowed one for years from the guy next to me and he never really ever used it, just watched me use it. They are kind of ambigous to use, rarely to you get a glaring, obvious fault. More often than not they tend to leak a little bit from everywhere. They were standard practice at BMW when we were replacing short blocks like they were going out of style. During their "engine test" there was a starter amp draw test where the computer could determine the compression of the engine based on the current draw from the starter (nothing new about that either), it would then identify the weak cylinders and you had to do a leak down on them and if they were greater than 25% then it was motor time. My Mac tool guy is throwing in the towel so I swooped up the one he'd been carting around town for the last year, got it for $50.00.

Mark