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


Subaru EJ2x Timing Belt Service

Subaru EJ25 DOHC WRX STI Built For 500+HP With Timing Covers Removed

Your Subaru Timing Belt Service is definitely not maintenance that you want to sleep on. We have all read and seen the horror stories on what happens if its not properly addressed. If your new to Subaru, congratulations on your purchase and here’s some information to chew on until your time comes (every pun intended)!

Your timing belt is just that, it is simply a belt that is used to keep your engines internal components mechanically synchronized. Your timing belt is guided by an assortment of sprockets, pulleys and a hydraulic belt tensioner as seen below. Over time these components begin to wear (some faster than others) and need replacing. Subaru places this service strategically in the 105k mile service interval in your Subaru Owners Manual. So bear in mind this shouldn’t be news to you but if it is then now you’re part of the informed.

Gone are the days of a simple all inclusive Gates Timing Belt Component Kit with Water Pump. This has been the standard for many years in our industry but as of lately non-original equipment manufactured components have made their way into the kits. We see more and more Belt Tensioner, Idler Pulley and Water Pump Pulley failures then I could live with while trying to sleep at night offering that to our customers (your friends and family). We now only offer OES (Original Equipment Supplier aka Subaru) Genuine Replacement Parts with an OEM (Original Equipment Manufacturer) Timing Belt.

“What does a timing belt service consist of” your probably asking by now right!? When your Timing Belt Service is done at CKR it is standard to replace the entirety of your Timing Components (Belt, Tensioner & Idler Pulleys). When your Timing Belt is replaced, it is mandatory to replace your Water Pump and Thermostat. These items are common wear items and its best to address them now or you will be paying for this job twice after a Water Pump failure. When performing this service, we also replace your timing cover seals, timing cover bolts and your front oil cooler crossover pipe as the are highly susceptible to the elements of New England. We also strongly urge our customers to take this service one step further and replace their Camshaft Seals (Camshaft Sprockets need to be removed) to prevent possibly oil leaks into the timing cover. Your radiator is drained during the Timing Belt Service and is replaced with quality long life engine coolant.

Other possible items that we may recommend need replacing during this service are your upper and lower radiator hoses. The upper coolant hose on turbo model Subaru’s especially are prone to ballooning with age and making contact with the power steering pump pulley or the radiator fan assembly. The accessory belts for both power steering and air conditioning will also be inspected and recommended for replacing if deemed necessary.

This service is completed with the engine in-vehicle and is performed same day as drop-off. The service typically averages out to 3-4 hours depending on your vehicles configuration (SOHC, DOHC). Our Timing Belt Service starts at $799 for most Subaru platforms and is all inclusive to keep your Subaru running to its 200k mile potential.

If your Subaru suffers from an engine failure, you may also need to replace your timing components and water pump when replacing with a used engine from a supplier like LKQ for warranty purposes. Labor times are adjusted for situations like this and generally include additional services like valve cover gaskets etc..

My goal with this article was to shed some light on common Subaru maintenance and some industry trends that are starting to become more prevalent.

Subaru EJ25 SOHC Timing Belt Component Kit
Subaru EJ25 SOHC With Timing Belt Tensioner Failure

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