Relative compression (cranking current)

Figure 1

Figure 2

How to perform the test

The first task is to immobilise the vehicle by disabling either the fuel or the ignition circuit (without causing any damage or illuminating a malfunction indicator lamp). This task may be manufacturer-specific and may need to be given some consideration.

In our example the motorcycle is fitted with a combined fuel injection and ignition fuse mounted conveniently beside the battery, and we have chosen to remove this as a form of immobilisation, as in Figure 1.

Channel A

Plug the BNC connection of the high-amp current clamp into Channel A of the scope. Connect the current clamp around the battery cable (either cable, which ever is easier) as in Figure 2. The clamp does, however, need to be facing the correct way: there is an arrow pointing to the battery positive (+) on one side and the negative on the other side (-). Incorrect connection will invert the waveform.

Switch the current clamp on. With the example waveform displayed, hit the space bar to begin taking live readings. Check that the clamp's auto zero is correct before relying on the measurements.

Example waveform

Waveform notes

The purpose of this particular 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 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 engine is in the region of 80 to 180 amps. This will, however, vary according to the mechanical resistance of the engine and the compression ratio.

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 draw on each cylinder is equal, as is nearly the case in the waveform above.

Technical information

It is essential to the running of the engine that it has sufficient compression. The compression provided by the rising piston is determined by the ratio of two volumes: the volume swept by the cylinder as it compresses, and the volume remaining in the combustion chamber at top-dead-centre. This ratio is called the compression ratio.

The compression is also determined by the effectiveness of the seal between the cylinder's wall and the piston, and 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 also used 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 with the piston rings, but if very little difference has been made, the fault lies with the valves.

It is generally accepted that there should not be more than 25% difference between the highest and lowest compression readings.

A higher than average compression can be caused by:

carbon build-up within the combustion chamber (reducing its volume)
excessive 'skimming' of the cylinder head
incorrect thickness of the head gasket

Faulty waveforms - troubleshooting

This test is only a comparison between cylinders and is not a substitute for a physical compression test with a suitable gauge.

Pin data

Not applicable.

AT101-2