# 20% Less At Altitude!

*Dear Ralph:
I believe there is one important factor affecting compression that you neglected to mention in your Winter Issue “Dear Ralph” column: ALTITUDE. Here in Idaho Falls, (4600 ft above sea level) we get compression numbers approximately 20% less than the sea level numbers published in the shop manuals. This is because we have less pressure in the cylinder before compression begins. At sea level the absolute pressure is 14.7 psi where here it is on the order of 12.4 psi. Assuming isentropic compression (no heat transfer to or from the cylinder walls) of an ideal engine, the final absolute pressure in the cylinder will be some factor mutiplied by the initial absolute pressure. Therefore your final absolute compression number will be proportional to your initial absolute pressure. To find the equivalent compression gauge reading at altitude one must know the local atmospheric pressure and make compression gauge corrections according to the following relationship: “Absolute pressure = gauge pressure + local atmospheric pressure”. Doing the math on this indicates that a sea level compression value of 120 psi ‘gauge pressure’ will be equivalent to a 5000 ft elevation value of approximately 100 psi ‘gauge pressure’. As a rule of thumb the shop manual compression value should be reduced by about 1% for every 250 ft above sea level. Perhaps this should be mentioned in a future column before some hapless soul in Denver tears down their ‘worn out’ 100 psi compression engine.*

*Ted R. Reed
Idaho Falls, ID*

Thanks for pointing out such accurate, very useful information.

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