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Old 05-24-2003, 10:28 PM
Duke2.6 Duke2.6 is offline
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Join Date: Mar 2003
Location: Southern California
Posts: 2,055
The ratio of atmospheric pressure to cranking compression pressure is not a computation of compression ratio i.e. 180/14.7 = 12.24. As air is compressed it heats up so there is not a linear relationship between volume ratio and pressure ratio.

Guys fret endlessly about compression readings - too high, too low, whatever. Actual cranking compression pressure is a complex interaction of compression ratio, valve timing, and cranking speed. Absolute numbers don't mean that much. The thing to look for is consistency across all cylinders.

The twelve pound variation between your high and low readings are "okay", indicating the engine is probably in "good" mechanical condition, not "great" not "fresh" - the old gal has some miles on her, but with good maintenance and good driving habits, she's probably got plenty more.

Take more readings in the not too distant future and see how they compare. The average over the counter compression gage is not a laboratory precision instrument, and you might be surprised how much the readings may be change at another point in time.

Compression tests should be done with the engine at operating temperature, the throttle blocked open with the ignition and fuel system disabled, and sufficient compression strokes should be run - generally about five if not six, for the gage to achieve a peak reading. Then this should be repeated two more times for each cylinder to verify consistent readings.

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