Compression Test: The Basics and What to Expect


If you own a Subaru or any car for that matter you have heard of or experienced the recommendation for getting a compression test.  Sounds like some sort of contortionist nightmare for your engine and well I guess to a point it is, but in such a good way.  Simply, compression is the amount of air sucked into the cylinder and compressed while the piston travels to top dead center (both valves closed).  The act of filling the cylinder charge is vital to engine performance, smoothness and longevity.  Also a compression test is essentially a gateway test to narrowing down further potential problems.

In the performance world, we like to use a compression test to gauge a motors health prior to modification.  This ensures to us that your motor is a candidate for increased performance and the chances of engine failure are significantly lower (but still possible).  We use a compression tester on built engines once the engine break-in process is complete as baseline compression numbers and to ensure even cylinder sealing.  Then every time spark plugs are changed we prefer to redo the compression test and store those numbers.  They then can be compared against previous results and any large deviations can be addressed and corrected before any catastrophic failure occurs.

The steps to a compression test are simple: all spark plugs are removed and a single cylinder is fitted with the tester, the engine is cranked over (fuel system disabled) with the throttle plate open till peak cylinder pressure is achieved – rinse and repeat for all cylinders.  The spark plugs are also read in terms of gap, color etc. and give insight to any variance in cylinders before testing even begins.

What we are looking for mainly is even compression among all cylinders above a useable level.  For example, three cylinders with 135psi and one with 60psi indicate an internal motor issue that needs to be investigated further.  Generally speaking, we prefer to see the spread of your highest and lowest reading cylinders within 7-10% with anything 15%+ raising concern for further investigation.  Anything in the 25%+ range usually has visible symptoms that may have led you to this very moment.  They absolutely can read the same psi for every cylinder and is always a cause for celebration when this happens.  If all is well, we always install a fresh set of NGK / DENSO spark plugs and send you on your way with piece of mind.

With that said, you may be asking yourself what the test number should be!?  We typically see 2.5L Turbo cars in the 130-155psi range, 2.0L USDM Turbo cars in the 150psi+ range and we haven’t done enough JDM 2.0L Turbo cars to know the range but estimated in the 120-135psi range for healthy motors.   PLOT TWIST!  These numbers DO NOT mean a thing as every compression tester is different and will have gauge variance etc. so take them with a grain of salt.

If your results weren’t so hot, this is where I spoke above about the compression test being a gateway test.  If we suspect something is bouncing around in your cylinder (maybe damage to the tip of the spark plug) we can use an endoscope to visibly see the inside of the cylinder and inspect for damage.  If all goes well with the camera probing, we lean towards a cylinder leakage test which specifically can pinpoint if compression is being lost past worn rings (ring land issues), valves that aren’t seating properly or burnt, head gaskets that are failing etc.  That test will be discussed in depth at another time.

So long story short, you absolutely should have a compression test scheduled at least annually or every 30,000 miles when replacing spark plugs (sooner for cars with increased performance over factory).  The compression test runs less than two hours’ time and wont set you back much more than the cost of removing and replacing your spark plugs.

Chris Keefe Racing LLC.
149 Production Rd.
Walpole MA 02081


2 Comments on “Compression Test: The Basics and What to Expect

  1. Good write up, I’d like to add a few things.
    1. The compression test should be done on a warm engine.
    2. If the readings are low in a cylinder, squirt some motor oil into the cylinder through the spark plug hole and recheck the cylinder. If the readings come up it usually means the rings are worn out.
    3. If the readings on two ajacent cylinders are low it usually means there could be a problem with the head gasket or the head itself is warped.
    4. A compression test generally only indicates the condition of the top piston ring and will not indicate the condition of the oil control ring.

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