What is good compression for an engine?
The short answer: A good rule of thumb says that each cylinder in a mechanically sound engine should have compression of 130 psi or higher. A good rule of thumb is no more than 10% variation between any of the cylinders.
Engine compression = engine power. A simple equation even we non-engineers can understand. In this post, we’ll look at how to test engine compression.
Video transcript: How to Do an Engine Compression Test
Featuring John Gardner, ASE Master Certified technician, automotive technology instructor and host of Motorhead Garage, with AMSOIL Director of Product Marketing Len Groom (author profile).
John Gardner: Wouldn’t it be cool if you could understand the mechanical integrity of your engine? Well, you can. Welcome to this AMSOIL tech tip on taking a compression reading. How do you do it?
Real simple. Just go down and get you an inexpensive compression gauge. Here’s what you do. You warm up your engine.
Once you warm up your engine, be careful, don’t burn yourself. Blow out all the spark plug holes and then remove all the spark plugs. Then you can go ahead and block the throttle wide open so you get full airflow into the engine. Once you do that, go ahead and insert the gauge into one of the cylinders.
Then disable the ignition. You want the car to crank but not start. Then what you want to do is you want to turn it over for four compression strokes. That’s four cycle strokes.
Nuh nuh nuh nuh nuh nuh nuh nuh nuh nuh nuh nuh And once it does that, go ahead and watch your gauge.
Ten to fifteen difference between the cylinders. As long as you have your manufacturer specification, you’re fine. All cylinder low? Well, you have burnt valves, you have a timing belt issue or just a plain and simple tired engine.
One or more? Usually just a cylinder issue. Two adjacent cylinders, well that’s probably a head gasket problem. Len, I’d rather not get to this compression portion because I think preventative maintenance, we’re all about that.
We can solve a lot of these problems prior to having a compression issue. Is that right?
Len Groom: That’s correct. You want to run a simple test first right before you have to get more invasive with a compression test and that can come in the form of fuel additives.
John Gardner: And fuel additives. Let’s address the cylinder itself. Do you have something down in there?
Len Groom: Well true. You’re to bring it in the same way generally through the fuel but what will happen is our performance improver or our upper cylinder lubricant is gonna do the same job. It’s gonna help clean deposits away that could be performance robbing or could be causing a ring stick that would be a loss of compression and what we’ve actually seen is carbon buildup in the actual crosshatches in the cylinder.
John Gardner: Wow, that’s interesting. How can we prevent it all together? I mean maybe get it before it even gets into the cylinder.
Len Groom: Right, you’re gonna add it to the fuel. That’s the easiest way to get the additives in there.
Put it in the fuel and go from there. Fairly simple.
John Gardner: Well There you go. You don’t even have to take a compression reading, but now you know how to. Make sure you check them out on AMSOIL.com.
What is engine compression?
First, however, let’s define our terms.
Engine compression refers to the pressure your engine generates inside the cylinders while it’s running.
How much pressure the engine produces and how well it converts that pressure into usable work influence your engine’s efficiency and power.
How it all works and how wear and deposits can erode compression (i.e. horsepower) over time are interesting topics you can read more about here. But today, we’re talking about how to test engine compression.
For this example, I used my 1998 Toyota* Corolla. Don’t laugh. I paid cash for it and it runs as smooth as a sewing machine. I also sought the help of Pat Burgraff, one of the techs in our Mechanical Lab.
Check out the video or the step-by-step directions to find out how to test engine compression.
Tools you need for testing compression
- Compression tester (available at most parts stores)
- Ratchet & extensions
- Spark-plug socket
How to Test Engine Compression
Total Time: 30 minutes
Ensure the vehicle won’t start when you crank it over
Testing compression requires you to crank the engine several revolutions, and you don’t want it to fire in the process. Remove the fuel-pump and fuel-injection fuses so you’re not dumping gas into the cylinders each time you crank the engine. Then, disconnect the coil packs. Bear in mind the process for your vehicle may be different from the images here.
Pull the spark plugs
Label the plug wires so you return them to the correct positions. Otherwise, your vehicle won’t start when you’re done. Thread the compression gauge into a spark plug opening. Take care not to cross-thread it. You can get a compression tester for less than $50 at most auto parts stores.
Crank the engine
Have a helper crank the engine 5-10 times, or until the needle on the compression gauge stops ratcheting up. Note the psi and move to the next cylinder.
Record results
Write down the results for each cylinder so you can compare and identify if compression is too low in one cylinder.
What’s considered “normal” engine compression?
Here’s where things grow murky. “Good” compression depends on the engine. Unfortunately, engines don’t come with their proper compression stamped on the outside.
But a good rule of thumb says that each cylinder in a mechanically sound engine should have compression of 130 psi or higher.
While I’ve seen some people claim 100 psi is sufficient, the gearheads and other sources I’ve consulted consider that too low.
In addition, you want consistency from one reading to the next.
Again, a good rule of thumb is no more than 10% variation between any of the cylinders.
That’s not to say 15 or 20% variation in one cylinder means your engine is junk. But a good, healthy engine should demonstrate minimal variation.
My trusty Corolla nailed the test, making between 165-175 psi in each cylinder.
A quick test if one cylinder has low compression
If one cylinder has low compression, try pouring about a teaspoon of oil into the spark-plug hole and retesting. If compression increases, it’s likely the rings are stuck or worn. The oil acts as a seal and helps close the gap between the rings and the cylinder wall through which the cylinder is losing pressure.
If that doesn’t work, it’s possible the valves or valve seals are worn.
If you suspect stuck rings, try an engine flush designed to clean deposits, such as AMSOIL Engine and Transmission Flush.
You can also try a fuel additive that cleans pistons, like AMSOIL P.i.
Word to the wise: you may illuminate your check engine light performing this test, as I did. It went off by itself after driving a few miles, though.
When low compression points to a valve
If a cylinder reads low and squirting a little oil into it doesn’t raise the reading, the rings aren’t the problem; the leak is at the top of the cylinder. Common causes are a burned exhaust valve, a bent valve from a timing-belt failure, a valve held open by carbon or tight valve clearance, or a blown head gasket. At 3,000 rpm, each valve in a four-stroke engine opens and closes 25 times per second, so a valve that doesn’t seat fully overheats quickly. A cylinder leak-down test shows exactly where the air escapes.
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Brad Nelson is an AMSOIL Technical Writer II who develops content about lubricant technology, product performance and vehicle maintenance. He works with engineers and product specialists to translate testing, application requirements and approved performance claims into clear customer guidance. Nelson’s work includes AMSOIL Blog articles, AMSOIL Magazine features and technical explanations concerning motor oils, gear lubricants, vehicle systems and maintenance practices.
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