# Given the compression ratio for a naturally aspirated engine, how do I calculate the expected compression pressure?

I've started learning more about cars, and I have a question. How do I know what compression pressure I should expect during a compression test?

With a naturally aspirated engine, I think I can use Boyle's Law. Assuming an naturally aspirated engine, I would expect the calculation to work like this.

Ptdc*Vtdc=Pbdc*Vbdc

Where P is pressure and V is volume. Tdc and bdc are top dead center and bottom dead center respectively.

Rearranging by dividing both sides by Vtdc:

Ptdc=Pbdc*Vbdc/Vtdc

I think the compression ratio is equivalent to Vbdc/Vtdc, so I can rewrite the expression as:

Ptdc=Pbdc*c

Where c is compression ratio.

It seems like Pbdc should be the same as atmospheric pressure in a naturally aspirated engine.

Okay, so let's make it concrete and assume that compression ratio is 8.8:1 and the atmospheric pressure is 1007mBar.

Ptdc=1007mBar * 8.8

Ptdc=8861mBar or 129psi

Assuming that a good compression test should measure at least 75% of that, I would expect 129psi*.75=97psi is the minimum to consider a cylinder in this engine to pass a compression test.

Is my thinking correct?