Compression vs. Leakdown

I am sure a "search" would give you many more thoughts... But:

Laymans terms:

Leak down test can show you the amount of pressure that is "leaking" out of the cylinder and or valves/head. You pump a set PSI into the spark plug hole when the piston is at TDC for that cylinder, and then read a gauge on a leak down tester showing the % that is either leaking by the rings, or the valves.

Compression test is when you turn over the engine (allowing the piston to sweep through the cylinder) with a compression gauge plugged into the spark plug hole. This gauge will give you the pressure that the piston makes in the cylinder when it goes through the compression stroke.

A compression test gives you a figure for each cylinder for you to compare. If all of the cylinders are close to the same (say within 10%), then you can say you have a fairly good (or equally worn) engine.

The leak down test can also tell you if the engine is worn in each cylinder by reading the % that leaks out of them. However, the draw back of the leak down is that it is only testing that TDC section of the cylinder wall. On the other hand a compression test will test the whole stroke of the piston. If you have a cylinder bore with an awful taper, the leak down test can miss that.

If I feel the car needs to be tested - I use a compression test 1st, to see if there is an issue to begin with, and if there is, then I proceed with a leak down test to pin point the issue (rings or valves etc.).

Consult a competent shop.

 

It all depends on who you're asking. A Urologist would give you one answer, a neurosurgeon another, and a mechanic, yet another.