Compression: It's Good, But What Does That Mean?

As a general rule of thumb, compression pressure around 90psi is the minimum required for combustion to occur in a gasoline engine. That is, if the compression test reveals less than 90psi, that particular cylinder can be declared "dead." On the other end of the spectrum, ~200psi is about the most compression pressure you will see in a non-racing gasoline engine. Where "normal" is for any particular engine depends on the compression ratio, which varies widely from one engine design to the next. Also, compression figures can vary considerably depending on how the test was performed and under what conditions. For example, was the engine warm or cold? Was the battery strong enough to spin the engine at the same speed throughout the test procedure? Was the throttle kept open or closed? Was one cylinder tested at a time with the other spark plugs in or out? There are many ways for the test to be done, which results in a variety of different pressure readings. Bottom line: A compression test is not really about measuring the pressure itself to determine the health of the engine; rather, it is more about measuring the pressure differences between cylinders under the same test conditions—typically to help diagnose a problem.

This is because modern piston engines maintain practically unchanging compression for the life of the engine. In fact, piston/cylinder/ring design technology has come so far that this part of the engine just doesn't wear out anymore—as long as the engine has not been starved of oil or overheated. These days, you can tear down a modern engine that has logged 300,000 miles of normal use and find that the cylinders still have the machining marks on the walls—they often look virtually brand-new, which is truly amazing. Thus, a compression test is not a very good measure of the overall health of the engine (or a measure of how much "life" is left). That is a common misunderstanding, plain and simple. Good compression test results indicate that the engine does not have a compression-related problem at the moment—and that's about all.

The thing is this: Any mechanic worth his weight in salt should already know that something is wrong with the engine prior to doing a compression test. The engine will not be running smoothly—especially at idle—and the check engine light will be on, if so equipped.

Testing compression is really nothing more than a way to figure out which cylinder has a problem (because you already know there is a problem!). Once you determine which cylinder has a problem, then a leak down test is run to further pinpoint the issue. The three possibilities for low compression in a particular cylinder are:

1. Past the rings: damaged piston and/or cylinder and/or rings
2. Past the valve: burned valve
3. Past the head gasket: blown head gasket

Equipped with this information, the mechanic can figure out how next to proceed.

The next time somebody tells you, "It has good compression," you can answer, "So what?" If the vehicle was manufactured after 1990, and has a working check engine light, don't waste your time on a compression test. Just take it to a mechanic who knows his ass from a hot rock—that is a much better bet.