COMPRESSION TEST :

A compression test is conducted to determine if the piston can compress the air-fuel mixture in the cylinder to a specified pressure which is essential for efficient combustion and maximum power output.

The compression test results are compared to the specifications. The compression pressure specifications will be listed, for example, as 160 pounds per square inch (psi), plus or minus 10 percent:

                                               160 psi ± 10%

The plus or minus tolerance is an important part of the specification since it limits the permissible variation between cylinder pressures. Only when ALL the cylinder pressure readings are within the limits of the specification, is smooth engine operation possible.

The figures in the chart table indicate and abnormally low reading in cylinder No.1. A ten percent reduction in the reading of cylinder No.2 (160−16−144) reveals that there is more than the permissible pressure variation between cylinder No.1 and 2. The cause of the low pressure in cylinder No.1 must be corrected before an effective tune-up can be performed.

                       

 

COMPRESSION  TEST RESULTS
CYLINDER	1	2	3	4	5	6
PRESSURE	130	160	155	157	152	158

                                                                       

Fig . Compression test

If the compression builds up quickly and evenly to the specified pressure on each cylinder and does not vary more than the allowable tolerance, the readings are normal. The engine can be considered acceptable for tune-up.

Worn piston rings will be indicated by low compression on the first stroke which tends to gradually build up on the following strokes.    A further indication is an improvement of the cylinder reading when about a tablespoon of motor oil is added to the cylinder through the spark plug hole with an oil can.

Valve trouble is indicated by a low-compression reading on the first stroke and does not rapidly build up pressure with succeeding strokes. The addition of oil will not materially affect the readings obtained.

Leaky head gaskets on two adjacent cylinders will produce the same test results as valve trouble. An additional indication of this particular trouble is the appearance of water in the crankcase.

Carbon deposits result in compression being considerably higher than specified. It is possible that carbon can hide a defect in the cylinder, as the deposit will raise the compression pressure of a cylinder to the extent which might compensate for leakage

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