Compression Test VS Leakdown Test, Why Do Both?

[Thomas M]

Why bother with a compression test, unless you don't have a leak down tester???

If there is say, 10% leakage, wouldn't the compression be right up there???

Is it possible to have low leakage and still have bad compression numbers???

Doesn't the leak down test tell it all??? Low leakage and, if not, where the leak is originating???

[TylerDurden]

This is what I googled:

Cylinder leakdown test-

While a compression test is a dynamic test (engine moving), a cylinder leakdown test (C.L.T.) is a static test (engine at rest). The compression test measures how much pressure the engine can produce while cranking; in contrast to the C.L.T., which measures how much pressure is lost in the engine. In a C.L.T. the engine is placed on TDC of the cylinder in question and using a similar type of connector as the compression test, we fill the cylinder with pressure. The tester then measures the volume of air needed to maintain a predetermined pressure in the cylinder. This reading is expressed in a percentage. Good cylinder leakdown readings should be below 5-8%.

The great thing about C.L.T. is that it deals with how well the cylinder is sealing and nothing else. The readings are not affected by carbon deposits, cam timing, or even engine cranking speed.

Another great feature of the C.L.T. is the fact that you can hear where the air is leaking out of the cylinder. When a cylinder has high percentage of leakage, first check the oil filler cap. Do you hear a hissing sound? If so, you may have pressure leaking by the rings. Is there air escaping out the exhaust? Is it escaping out the intake system? Then a burned valve may be the problem. If two adjoining cylinders have similar low readings and you hear leakage out the other cylinder, then a failed head gasket may be the problem.

Being able to pinpoint the exact source of the compression loss will tell you where the problem is; and not just that you have one. This knowledge will greatly assist you in the next step… the repair.

[Thomas M]

Thanks, Tyler Durden !!

I don't see a good reason for ME to do a compression test. Your write-up stresses the value of the leak-down test for sure.

Don't get me wrong, when the time comes I will do both tests. However, I just don't see any value in the compression test except curiosity as to what number comes up. A tuner or racer might have a different point of view.

Also, as long as one has a good compression gauge, an runs the test at "normal" operating temperature and/or cold, it would seem the compression test results are a bit more objective. There are a lot of variables that effect the leak-down test as I have read on Wikipedia.

All very interesting. I'll just keep puttin' along with my 54 horsepower !!!!