Ken-
You say that you have figured out a way to compare compression readings and
compression ratio, and you are asking if there are any known flaws in your
methodology...
Well one major variable comes to mind, cam timing. Joe mentions the fact that
the valve will be open for some period of the compression stroke in his prior
statement. While this is an obvious trueism, it must be taken further. If all
the engines being tested had the same camshaft and the same clearances, the
test would be valid. However, any change in camshaft timing will produce a
large and noticeable difference in the compression reading, but not the
compression ratio. The longer that the intake valve stays open, as on any
higher performance cam, the less time/distance the piston has to make the
compression. As an example lets say a stock cam closes the intake valve at 30
degrees after bottom dead center, and a mild cam closes it at 50 degrees. Now
lets say that the stock cam closing works out to be where the piston is .5"
up the bore, and the other closing works out to be 1" up the bore from bottom
dead center. It should be obvious that the compression reading is going to be
different with these two cams, and indeed you will often find that a hot rod
cam will show lower compression readings
than a stock cam will during a compression test. The reason for this is that
the engine is turning too slowly on the starter and the
"compression" is going right back out of the carbs. Once the engine is up to
speed, inertia of the air/fuel mass will overcome this and indeed will get a
larger charge (more "compression") into the engine. As an aside, you can
often see this on a race motor as fuel standoff. What this is, is the
fuel/air mixture being forced back out of the carbs at low engine speed due
to the piston forcing it out of the still open intake valve. This is the main
reason full race motors do not "idle" worth a darn, and in fact usually
hardly even run at slow speeds.
If we apply this scenario to the extreme in the field, we can see that in
reality a worn stock camshaft might give the highest compression readings of
all!
The best use for compression readings, is as a comparison between different
cylinders in the same engine. (Of course now you know to check your cam lobes
if one or more of the readings is outside of the expected result...)
Jeez, this is hard without using my hands to explain...
Nick in Nor Cal
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