Relative compression - petrol

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

PicoScope and PicoDiagnostics

Although it is possible to measure relative compression with PicoScope, it is easier in most cases to use the Relative Compression test built into PicoDiagnostics. PicoDiagnostics is supplied with all our diagnostics kits.

How to perform the test

Plug the current clamp, set to it high range if applicable, into Channel A.

The clamp should be positioned on one of the two battery connections (live or earth), whichever allows the easiest connection, as in Figure 1.

The current clamp needs to be switched on and facing the correct way. There is an arrow pointing to the battery positive (+) on one side and an arrow pointing to the battery negative (-) on the other side. Incorrect connection will lead to an inverted oscilloscope picture.

Slight adjustment to the timebase (ms/division) may be required to compensate for faster or slower cranking speeds.

With the example waveform displayed on the screen, you can now hit the space bar to start looking at live readings.

Example waveform

Waveform notes

The purpose of this waveform is twofold:

  • To measure the amperage required to crank the engine
  • To evaluate the relative compressions

The amperage required to crank the engine will largely depend on many factors including: the capacity of the engine, the number of cylinders, the viscosity of the oil, the condition of the starter motor, the condition of the starter's wiring circuit and the compressions in the cylinders.

The current for a typical 4 cylinder petrol engine is in the region of 80 to 200 amps.

In the waveform above, the initial peak of current (approx 460 A) is the current required to overcome the initial friction and inertia to rotate the engine. Once the engine is rotating, the current drops. It is also worth mentioning the small step before the initial peak, which is caused by the switching of the starter solenoid.

The compressions can be compared against each other by monitoring the current required to push each cylinder up on its compression stroke. The better the compression, the higher the current demand, and vice versa. It is therefore important that the current draws on all cylinders are equal.

The waveform below shows an engine with a loss of compression in one cylinder.

NOTE: when compression testing a petrol engine, it is advisable to isolate the ignition primary circuit to avoid stray high tension (HT) voltage damaging the electronic circuitry.

Technical information

It is essential to the running of the engine that it has sufficient compression. The compression provided by the rising piston will be determined by the swept area being compressed into the combustion area: this is called the compression ratio. The compression is also determined by the effectiveness of the seal between the cylinder's wall and the piston; this seal is maintained by the piston rings. The same applies to the seating of both the inlet and exhaust valves.

Piston rings are made of centrifugally spun cast iron, which produces a radial pressure forming the seal. Cast iron is used also for its excellent self-lubricating properties.

If a relative compression waveform highlights a problem, it will be necessary to perform a compression test.

When compression testing on a petrol engine, it is important to fully open the throttle to allow a larger quantity of air to pass into the cylinders.

A typical compression is between 120 and 200 psi. A low compression can be caused by:

  1. An ineffective seal between the cylinder and the piston
  2. Poor seating of the inlet and exhaust valves
  3. Broken or seized piston rings
  4. Incorrect camshaft timing
  5. An obstructed induction tract

All readings should be similar. If one is lower than the others, a 'wet' test can be performed by squirting a small amount of oil into the cylinder and re-testing the compression. The inclusion of the oil ensures a tight seal between the piston and the bore, so if the compression is regained the fault lies within the piston rings. If very little difference has been made, the fault lies within the valves.

There should not be more than 25% difference between the highest and lowest compression readings.

A higher than average compression can be caused by:

  1. Carbon build up in the combustion chamber (reducing its area).
  2. Excessive 'skimming' of the cylinder head.
  3. Incorrect thickness of the head gasket.

NOTE: It is advisable to disable the ignition's low tension circuit to avoid damaging the amplifier or the Electronic Control Module (ECM).


This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • 200 A / 2000 A (high amps) DC current clamp


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1 comment | Add comment

September 29 2016

Adding a sync source on another channel (ie:  inductive sync probe, ignition coil trigger signal, ignition coil current ramp, etc) adds a significant amount of helpful information.

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Guided test: Petrol Compression