How to Calculate Your Engine's Compression Ratio

By Max. D Gray. Updated: January 16, 2017
How to Calculate Your Engine's Compression Ratio

We often wonder if our vehicle is in good working order, and if it will be able to withstand a long journey and everything we use it for on a daily basis. Beyond getting a basic car service and opting to have the recommended servicing according to its mileage, there is an easy way to check the state of our engine, so at OneHowTo we show you how to calculate your engine's compression ratio effectively.

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Steps to follow:

For this you will need a pressure gauge to measure the engine's compression. You can buy this in shops specializing in car parts and accessories or on the Internet.

How to Calculate Your Engine's Compression Ratio - Step 1

Start the car and let the engine run until it reaches its normal temperature. You shouldn't carry out this check on a cold engine, so it is best to do it after having driven a short distance.


Switch off the ignition and once the car is off, disconnect the wires located in the spark plugs, always focusing on what you're doing because later you will have to reconnect them in the same positions. To avoid any uncertainty, you can take a picture of the spark plugs before disconnecting the cables.

You may also want to take advantage to clean the spark plugs if they are dirty.

How to Calculate Your Engine's Compression Ratio - Step 3

Unscrew a spark plug and place the tip of the gauge in the hole where the spark plug is inserted. It is very important that the gauge's nozzle is fully inserted inside this hole.


Now you have to ask someone else to start the engine and accelerate the car for about four seconds, this way you can measure the engine compression of the spark plug. Turn off the engine and repeat the above steps with each of your car's spark plugs.

How to Calculate Your Engine's Compression Ratio - Step 5

Each spark plug should have the same pressure, and it is important that they match the pressure recommended by the car manufacturer in the handbook. If you have a petrol engine the difference between cylinders can be up to 1.5 bar, which is completely normal.


If you don't know the manufacturer's guidelines regarding the suitable compression for your car's engine, then rely on the compression ratio of your engine (whether petrol or diesel) and add up those numbers. For example in a compression ratio of 14:1 the sum result is 14+1, so 15 should be the pressure value as indicated on the manometer.


If there is a problem with your car's engine compression, it is best to take it for a service check to determine the source of the leak and allow professional to fix it.

If you want to read similar articles to How to Calculate Your Engine's Compression Ratio, we recommend you visit our Car Maintenance and Repair category.

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