Compression ratio

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Atmospheric pressure at sea level is 14.7 psi and changes with altitude.

Compression ratio is always multiplied by your current atmospheric pressure. So at sea level if your system has a 6:1 compressor ratio, it means your system has 6 times the pressure of 14.7.

So your systems pressure is 14.7 * 6 = 88.2 psi

&

Your systems absolute pressure is 88.2-14.7 = 73.5 psi . Absolute pressure is simply because if you walk around with a pressure gauge in your hand, its sitting on the table, threaded into an empty pressure vessel, etc it will not show 14.7 psig (psi on the pressure gauge), regardless that atmospheric pressure at sea level is 14.7 psi.

Another example:

I have 100:1 compression ratio for my motor. So my motor is capable of producing 14.7 * 100 = 1470 psi, and the absolute pressure will be 1470-14.7 = 1455.3 psia (absolute pressure).