Relative Compression Test is special test mode implemented in some scan tools or diagnostic scope software applications. This test monitors either minor changes in crankshaft angular rotation speed or in starter motor current. If engine has one or more cylinders with lower compression than other cylinders have it would result in higher than expected angular rotation speed or lower starter current when this cylinder goes over compression stroke. Software application takes into account firing order and cylinder 1 top dead center position in order to calculate number of the cylinder with lower compression.

This test quicker to do than traditional compression test with gauge but it only shows relative difference in cylinders compression expressed as percentage. If all of the cylinders have about the same loss of compression due to wear of cylinder bores, piston rings, valve seats etc. Relative Compression Test would be insufficient to detect this condition

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