To calculate the compression ratio, you may use the formula:
CR = (Vd + Vc) / Vc
So, let's suppose you are using the values of your petrol engine and follow the steps below.
Suppose the displacement volume(Vd) is 52 cc;
The compressed volume (Vc) is 8 cc;
Sum the values, and the result is 60;
Now divide 60 by 8 (Vc); and
The result is a compression ratio of 7.5:1.
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