> My question is, how could it NOT be that atmospheric
> pressure is being multiplied by the compression ratio to end up
> at the psi I see on my compression gauge?
> Please, scientists, if you could keep it easy, I would like an
> explanation.
Jim, the primary reason the pressure is higher than you expect is that air
expands when you heat it, and compressing it heats it. Roughly speaking,
pressure times volume divided by temperature remains a constant.
The primary reason it's lower than expected from the above rule is that the
advertised CR assumes both valves are closed at BDC, but they aren't. The
intake valve stays open a fair amount past BDC, which has the effect of
lowering CR at low rpm (like cranking speed).
So, as always, compression readings should be used only as a relative
indicator of cylinder health. As long as they are consistent within your
engine, it's very unlikely you have any significant wear or damage. And if
they are inconsistent within your engine, chances are good you have
significant damage.
Oh, a few other jokers : CR can vary depending on how much the plug
displaces, most compression gauges don't displace as much (again depending
on which plug you are using). And they aren't all the same, either.
And since compression readings are only to be used as a relative indicator,
the gauges themselves need not be all that accurate. I have two different
gauges, which give results different by 20-30 psi.
Randall
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