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Premium Member
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Discussion Starter #1
Compression testing of petrol engines​

Apologies for the length of the posting, its split into three posts so you can either jump straight into the method in brief or you can follow the method in step-by-step detail. The third post deals with examining the results. Enjoy the read, happy testing and may the force (pressure) be with you.

When to carry out a compression test

If the engine is running rough or is suffering from a lack of power or performance and no faults can be found in the ignition circuit, fuel system, crankcase ventilation system, or the exhaust system. Carry out a compression test to confirm or eliminate worn piston rings/cylinder bore, worn valve gear/camshaft or blown head gasket/cracked head.

If you’ve just taken delivery of a ‘new’ car, OK in reality it may be 10+ years old with 100K+ miles on the clock. Carrying out a compression test will give you a good idea of the overall condition of the engines internal components, the bits you don’t notice when walking round and kicking wheels.

If you’ve just ragged it quarter after quarter at a Santa Pod RWYB day, or maybe you’ve just put in a personal best at a Snetterton track day, you might just want to check you’ve not done any internal damage and it’ll still get you to work and back on Monday.

If you’ve added some performance parts such as camshafts, a skimmed or modified cylinder head, modified pistons and the like. Carry out a compression test to give yourself a reference point of what sort of pressure the combustion chamber is producing. All these things will have some effect, modified camshafts may lower the pressure, skimmed or modified heads may raise it.

Incorporate a compression test into your normal service routine when you change the spark plugs, half the works done once the plugs are out and it’s not so messy changing that fuel filter when its empty! This will give you an early indication of any internal wear and you may just pick up on an early sign of a blown head gasket before it becomes a cracked cylinder head. If you do this keep records of your tests showing date and mileage for future comparison.

Tools required

Calculator, pencil & paper, yellow green & red PVC tape (optional), 5mm hex key (if your plugs have a cover), spark plug socket, extension and tee bar or ratchet, 50ml of engine oil if you need to wet test, and, of course, a compression tester. I recommend having a workshop manual to hand for reference. You will also need a helping hand when it comes to spinning the engine over.

If you haven’t got a compression tester a trip to your local motor factor with £15-£20 will get you sorted. The screw in type are the easiest to use and offer greater accuracy over the rubber bung type. They comprise of a pressure gauge calibrated to about 20 Bar (or 300 psi), non-return valve (which leaks a little), rubber hosepipe, one-way valve (also leaks a little) and varying sized spark plug hole adaptors.

The method in brief

Calculate the pressure figures, note to carry out a proper test you will need to take three or four pressure readings for each cylinder. Working on a warm engine with a fully charged battery, disable the fuel system, isolate the ignition circuit and remove the spark plugs. Screw the compression tester into the No1 cylinder spark plug hole, spin the engine on full throttle, note and record the pressures. Repeat the test on the remaining cylinders, noting and recording the pressures. Examine the results, wet test if required. Re-fit the spark plugs; connect the HT & LT leads, re-fit fuel pump fuse. See below for step-by-step detail

Time allowed 20 minutes, allow another 10 minutes if you carry out a ‘wet’ test too.

Premium Member
2,359 Posts
Discussion Starter #2
The method in step-by-step detail​

Calculating maximum pressure from the compression ratio

First you need to determine the compression ratio of your engine do this by reference to the workshop manual. Then you need to calculate what the maximum pressure within the combustion chamber should be, to do this multiply the compression ratio by 1.38 to work in Bar, alternatively multiply the compression ratio by 20 if you want to work in psi. You will also need to calculate the figures for 66% and 80% maximum pressure to do a comprehensive test.

Example (MP = CR x 1.38, 60% MP = MP x 0.66, 80% MP = MP x 0.8)
If the compression ratio is 10.8:1 the three calculations are:
10.8 x 1.38 = 14.9 giving a maximum pressure figure of 15 Bar
14.9 x .8 = 11.92 giving an 80% pressure figure of 12 Bar
14.9 x .66 = 9.83 giving a 66% pressure figure of 10 Bar
Or, if you want to work in psi (MP = CR x 200, 60% MP = MP x 0.66 and 80% MP = MP x 0.8)
10.8 x 20 = 216 giving a maximum pressure figure of 215 psi
216 x .8 = 172.8 giving an 80% pressure figure of 170 psi
216 x .66 = 142.56 giving a 66% pressure figure of 145 psi
Note: pressure figures are rounded for ease of reading the calibration gauge.


Ensure the battery is in good condition and fully charged and that the air filter element is in a serviceable condition, you need a fast spin over of the engine and a good flow of air intake. Warm up the engine to a normal operating temperature of around 88 degrees Celsius.

Safety first, de-pressurise and disable the fuel system by turning the ignition off and removing the fuel pump fuse, then start the engine and allow it to idle until it runs out of fuel and stalls, repeat this three times to ensure all fuel in the system is spent. Isolate the ignition circuit by disconnecting the ignition module wiring plug or LT lead and move it clear of the connector to ensure the circuit remains isolated. This is to protect the exhaust, catalytic converter, electrical components and the ECU. It also reduces the risk of fire, unburnt fuel and arcing HT leads are just a too risky combination for me!

Remove the spark plug cover if fitted. Disconnect the HT leads and lay them to one side out of the way. Starting at No1 cylinder, No1 cylinder being at the timing/camshaft belt end of the cylinder head, remove the spark plugs one at a time noting which cylinder they came from and their general overall condition. This information could be very useful when examining the compression test results, for example wet oily plugs are an indication of worn piston rings/cylinder bores or worn valve guides. This could be very relevant to the test results, refer to the workshop manual for other spark plug conditions.

The compression test

You will need to mentally note three or four pressure readings for each cylinder in quick succession and then write them down. As the engine spins over the needle on the pressure gauge will jump each time the piston reaches TDC (top dead centre of stroke, one revolution), make a mental note of the pressure readings on the second, fourth and the sixth revolution, if the gauge has not registered maximum pressure by the sixth revolution take an additional pressure reading at the tenth revolution, then write these down. I find it useful to cut a slither of yellow, green and red PVC tape and stick these on the rim of the dial, just outside the calibration marks so you can still read them, to highlight the calibration at the 66%, 80% and maximum pressure marks.

Select the correct adaptor to fit the diameter of your spark plugs (screw in type tester), de-compress the compression tester by pressing the little button found on the non-return valve just below the dial. Screw the adaptor, finger and thumb tight, into the No1 cylinder spark plug hole so that the dial can be seen. Holding the gauge away from the engine, get your helper to fully depress the accelerator pedal (this allows for maximum air intake) and spin the engine over via the ignition key for a minimum of 10 revolutions, you will need a little communication here. Watch the dial and make a mental note of the pressure readings on the second, fourth and sixth revolution (the needle will jump with each revolution), i.e. No1 12 14 15. If the gauge has not registered maximum pressure by the sixth revolution take an additional reading at the tenth revolution. Write down all three/four pressure readings.

Remove the compression tester from No1 cylinder, de-compress it by pressing the little button and then repeat the test on the remaining cylinders in turn, writing down the pressure readings for each cylinder. See below to examine the results.

Premium Member
2,359 Posts
Discussion Starter #3
Examining the results​

A good compression test result will show a quick build up of pressure, around 66% by the second revolution, 80% pressure on the fourth revolution and maximum pressure on the sixth revolution, with no more than a 10% variance across all cylinder (some prefer to use 1 Bar (15 psi) for the variance figure). The higher the figures, providing they don’t exceed the maximum pressure figure, and the lower the variance across all cylinders the better. If that’s the case all’s well, re-fit the spark plugs, connect the HT leads, ignition module plug or LT lead and re-fit the fuel pump fuse and your done.

An acceptable borderline compression test result will show a slow build up of pressure, less than 66% pressure on the second revolution and/or less than 80% pressure on the fourth and sixth revolutions, and 80% or more by the tenth revolution, with no more than a 10% variance across all cylinders. This is a sign of piston ring/cylinder bore wear or camshaft lobe wear, a wet compression test will confirm which – see below. Providing there is no tapping, chattering or knocking from the cylinder head/block and that the engine normally runs smoothly there’s probably not too much to worry about. Do another test next time you change the spark plugs, or if the engine starts running rough or losing power.

If any cylinder shows more than the maximum pressure suspect a build up of carbon in the combustion chamber, head removal and de-carbonising may be required. If ignored excessive pressure could lead to a blown head gasket or a cracked cylinder head, check the spark plug condition before deciding if the cylinder head needs to come off. Note on a good engine you could see as much as maximum pressure +20% by the tenth revolution this is due to the non-return valve on the tester preventing the pressure from dropping within the gauge.

If adjacent cylinders show low pressure (greater than a 10% variance) this could be due to an internally cracked cylinder head or an internally blown head gasket between those cylinders. This can be confirmed by the presence of coolant in the oil and will be recognised by a whitish colouring to the engine oil (check the dip stick) and a build up of suds (a whitish oil and water mix, sometimes referred to as froth or mayo) inside the oil filler cap.

If any cylinder shows less than 80% maximum pressure a major engine overhaul is on the way, but don’t panic too soon, many engines have been known to run fairly well with 70% or less pressure, just don’t race them. Carry out a wet compression test.

Wet compression test

A wet compression is carried out the same way as a normal test except that you pour 5ml of engine oil down each spark plug hole just prior to fitting the compression tester, allow about 30 seconds or so for the oil to settle around the piston rings and form a seal. Don’t forget to write down all three or four pressure readings for each cylinder. Compare the wet compression test results with the first set of test results.

An increase in pressure indicates worn piston rings and/or worn cylinder bores, as the additional oil has acted as a seal.

No increase in pressure points towards the camshaft or the valve gear and could mean any combination of: pitted or worn valve/valve seat, bent valve, worn valve guide, burnt valve or a worn camshaft lobe.

No increase in pressure may also be due to an externally cracked cylinder head or externally blown head gasket this would be a little obvious though, as you should be able to hear it hissing/popping or see it bubbling.

When your done re-fit the spark plugs, connect the HT leads, ignition module plug or LT lead and re-fit the fuel pump fuse.
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