You didn't mention the elevation where you live. For those of us who
have lived at higher altitudes, the difference in ambient pressure will
also affect compression readings.
At sea level, standard pressure is 14.7 psi. It drops about 1 psi per
2000 ft altitude, which may not seem too significant, but consider that
at 5000 ft (Denver), ambient pressure might down 2.5 psi. Multiply that
times 6.5 (compression ratio), and the compression reading is down by 16
psi!
The airplane mechanics have a much better way to test compression, using
a "differential" testor. They apply a known pressure (80 psi
typ.)through the sparkplug hole on compression stroke, and see how much
the cylinder will retain, for example 76/80. Also makes diagnosis of
problems easier. Hiss heard in the carburator is intake valve, hiss in
the exhaust pipe is exhaust valve, and hiss in the oil filler tube is
rings...
-Rob Root
1942 GPW
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