Compression testing gives you a cylinder to cylinder comparison of absolute pressure numbers.

Leakdown testing tells you where the pressure is leaking out to... Out the valves, through the piston rings or out the headgasket.

Quote Originally Posted by nickriingen View Post
Gasoline and diesel engines have different types of compression gauge. For gasoline engines, the pressure range is from 0- 250 psi and for diesel engines the range is from 0- 800 psi. In testing gasoline engines, you need to remove all the spark plugs and disable the ignition system and insert the test hose to the spark plug hole of each cylinder. For diesel engines,its a different story. You must have a glow plug adaptor or nozzle adaptor to fit on each cylinder for compression testing. Most of independent service shops don't have this instrument unlike the casas.
Granted, diesel is different... but it's the gas tester I'm talking about. Very few shops have a universal tester for gas engines, despite it being a very simple machine.