Compression Ratio

By Norman Nock, British Car Specialist

Originally Published in the Austin-Healey Magazine


Left - Piston at BDC. Right - Piston at TDC. Compression ratio is equal to the volume of A divided by the volume of B.

When we suspected a too high compression ratio on a 100-4, we used this simple method to determine the ratio. After removing the spark plugs, we put # l cylinder to TDC by using the timing mark on the front cover. We filled #l cylinder with oil and carefully measured the amount of oil it took. It should take about 100 ml (c.c.). Then we then filled #2 cylinder, which is at BDC, and measured the amount. It should take about 800 ml (c.c). The compression ratio is: (800/100) = 8:1.


The compression ratio of an engine is the extent to which the combustible gases are compressed within the cylinder. It is calculated by dividing the volume existing within the cylinder with the piston at BDC with the volume in the cylinder with the piston at TDC. Up to a certain point, the more the fuel charge is compressed, the more power will be obtained.


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