What is engine compression?


One of the pieces of advice often given here is to have a compression test done on a car’s engine as a means of isolating and diagnosing problems.

I realize that the air/fuel mix fed into the cylinders is under compression. Is that what you are testing, how well sealed those cylinders are at keeping the air/fuel mix compressed to certain specs?

I’m just being insatiably curious.



That’s correct Marnet. The piston draws in the fuel on the downstroke and then the valves close near the bottom of the travel. The piston then goes back up compressing the air mixture. Near the top of the stroke the ignition fires and combusts the mixture. The piston then goes back down on the power stoke and comes back up pushing the exhaust out the exhaust port.

The ratio of the volume of air that a cylinder has at the bottom of the piston compared to the top of the piston travel is how you determine the compression ratio of the engine.


You’re close. The fuel/air is drawn into the cylinders but is not under compression at that point.
About the time the piston starts back up the intake valve is closing so this means that both the valves are essentially closed(3 or 4 valves in some cases).
The fuel/air mix is basically squashed at the top and cylinder pressures will go up.
The cranking compression will vary depending on the engine and the camshaft profile (a bit more complicated to explain), but will normally run from 160 to 200 pounds per square inch.

When checking compression, one looks for continuity in the readings. If normal compression is 180 PSI then you would hope they’re all in the 175-185 range. If you have one at 150 then you know you’ve got a problem.
With most engines, if the pressures are down in the 140, even all of them, then you can figure the engine is worn and on the way downhill although it may run for a long time like that.


Aha, that makes sense now. It’s a way to see if any particular cylinder has an issue. Also a way to see if the entire engine is getting worn out.

I presume that an engine that is getting fairly worn out will not only have lower compression numbers but tend to have both lessened gas mileage on average and possibly some loss of power, especially on acceleration?





Thank you for the explanations. Nice to learn something new everyday. G



As for the air/fuel mix entering the combustion chamber under compression, this is only true on turbocharged or supercharged engines. And this compression is a fraction of what takes place in the combustion chamber.