Compression Testing of Internal Combustion Engines - Direct Compression and Differential Compression Testing

Compression Testing of Internal Combustion Engines - Direct Compression and Differential Compression Testing
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Introduction to Engine Compression Testing Equipment and Techniques

The components of car internal combustion engines like all other engines will eventually require replacing, due to general wear and tear.

A simple test to access the performance of an internal combustion engine is to check the engine compression.

This can show the condition of the piston rings and cylinders, or the other components responsible for the engines compression.

This is an article on internal combustion engines performance testing and in particular engine compression testing equipment and techniques.

Differential Compression Testing

This is used mainly on small aero engines and the procedure is as follows:

  • Warm-up - The engine should be warmed up as per the normal pre-flight warm up.

  • Safety - The magnetos should be grounded, the master switch put to the “off” position, and the fuel supply isolated. Before changing the test gear to the next cylinder, the air supply must be isolated and pressure released from the equipment.

Differential Compression Test Equipment

The equipment consists of the following components:

  • Compressed air source (dry air) complete with regulating valve, the supply being capable of providing a continuous flow of air at 80 PSI.
  • Test kit containing test pressure gauge and cylinder pressure gauge with a calibrated orifice between them.
  • Hose connections and hoses for supply air and for the spark plug adapter.

How to Perform Differential Compression Testing

Engine cowlings, cooler baffles, and the spark plugs are removed from the engine.

The adapter is screwed into a spark plug hole and the propeller is turned to place the piston in the cylinder being tested at the top dead center position. This can be determined with a finger over the compression adapter by waiting for the “puff” as the piston completes the compression stroke.

With the propeller secured from rotating in either direction, the hose from the tester is connected to the spark plug adapter and pressure is applied. The left-hand gauge should be adjusted to 80 PSI, and the right-hand gauge will show the differential compression reading. It may be necessary to slowly rock the propeller back and forth slightly to “set the rings.”

Ideally the pressure at the cylinder gauge should equal the pressure at the regulator gauge. A reading of 60 PSI or above at the cylinder pressure gauge, however, is acceptable and is noted as 60/80. Any reading recorded at the cylinder gauge lower than 60 PSI indicates leakage of air past the components of the engine (rings, valves, or a crack in the cylinder) and requires further investigation, although a low reading by itself is not sufficient reason for rejecting a cylinder. You need to find out where the air is going and also consider whether the components are high time or if the engine has exhibited excessive oil consumption.

A system diagram of a differential compression tester and a compression test record card for a six cylinder engine are shown below. (Please click on the images to enlarge.)

Differential Pressure Testing Sketch

Direct Compression Testing

We shall examine the method of direct compression testing of a gasoline (petrol) engine, using the following sequence:

  • Warm-up - The engine is run to bring it up to working temperature.

  • Safety - The engine ignition is switched off and ignition key removed.

The spark plugs are then removed; remember to mark the leads with their relevant cylinder numbers.

The fuel supply should also be isolated, and the gas pedal held down in the fully open position to allow maximum air into the engine when cranking over.

When moving the test equipment from the cylinder; release the air pressure by opening the pressure relief valve on the gauge/adapter.

The ignition key can now be replaced in the ignition switch.

Compression Test – Dry Method

The compression adapter is screwed into the No.1 cylinder spark plug hole, and the engine cranked over at least six revolutions by an assistant.

The maximum compression reading is recorded and the process is repeated for the rest of the cylinders.

Compression Test – Wet Method

In this test a small amount of lube-oil (teaspoonful) is poured into the cylinders that had the lowest readings during the dry compression test.

The engine is turned over, and the readings taken as before and recorded.

A system diagram of a direct compression tester, and a compression test record card for a four cylinder engine are shown below.

Direct Compression Test Record Card

Direct Compression Testing Layout

Causes of Low Compression

Generally the causes of low compression are broken/worn piston rings or scored cylinders, usually due to high mileage or lack of oil changes.

Many years ago, I was traveling from Belfast to Invergordon in Scotland, to take up a position as engineer at an Aluminum Smelter. I was driving an old Vauxhall Viva of 1962 vintage, and it was burning a lot of oil.

I used a couple of gallons of oil on that journey and once I had a chance to check the problem, I lifted the head off and lo and behold there was a hole in one of the pistons about the size of a penny. A new piston cured the problem and the old car did us for a few years, albeit the toolbox was always in the boot!

Anyway, that is another cause of loss of engine compression. Some other causes are listed below.

Differential Compression Test

  • Exhaust valve passing – evident by listening for escaping air at exhaust pipe.
  • Inlet valve passing – evident by listening for air at the air breather filter inlet.
  • Piston rings/cylinder wear – evident by air coming from crankcase breather.

Direct Compression Test

If the direct compression test indicates low compression in the engine, further investigation of the cause can usually only be achieved by stripping the engine.

Compression Test Machines

There are numerous compression test machines available to the car maintenance person, for both the differential and direct compression methods. They are also compression test machines available for diesel engines.

A selection of these is shown below, with a few descriptive notes and prices.

Direct Compression Test Kits

1. Petrol Engine Compression Test Kit 6 pc Sealey CT955

Petrol Engine Compression Testing Kit from Just Off Base Website


  • Pressure gauge – 21/2”dia, calibrated to 300 psi and 20kg/cm.
  • Piping – push on connections, 400mm flexi.
  • Screwed Adapter – 12, 14 and 18mm deep threads.
  • Carry case – supplied
  • Price - £35 ($55)
  • Warranty – 1 year

2. Actron CP7828 Professional Compression Tester with Storage Pouch

Actron CP7828 Compression Tester


  • Gauge – 21/2” dia, calibrated to 300psi and 2100KPs
  • Adapters – 10,12, 14, deep threads
  • Piping – flexible
  • Carry case – pouch supplied
  • Price – £20 ($32)
  • Warranty – 1 year

Differential Compression Test Kits

1. ATS Differential Cylinder Pressure Tester

ATS Differential Compression Tester


  • Valves - On-off safety valve to relieve pressure
  • Orifice size- 0.40”
  • Gauges – 2 gauges supplied, calibrated to +/- 2%
  • Adapter – 18mm plug adapter
  • Piping – 24” flexi pressure hose
  • Price - £38 ($60)

The above compression testers are a selection that I have chosen because of their ease of use and price - motoring (or flying!) is costly enough without having to purchase top of the range digital testers; the manual ones are just as effective.

Please read the safety precautions in the operation manuals when purchasing a compression test kit, as these are very important for your safety. Some kits can be used without assistance, but it is much easier and safer to have another person to help you complete the test.

Reference Webs:

1. Luscombe: Differential Compression Testing

2. Justoffbase: Petrol Engine Compression Test Kit

3. Mityvac: Basic Compression Test Kit

4. ATS: Differential Compression Kit

5. Youfixcars: Direct Compression Test Procedure