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Dean-
The compression email has been kicked around a few times.
I have attached a lenghy diatribe on the subject, the gist of which is that
you can not estimate your compression ratio from figures obtained in a
compression check. As long as the numbers are about the same for each
cylinder, the test has done all it can do.
For those of you who want to know more, download the attached file.
The Car Curmudgeon
Nick in Nor Cal
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Subj: Re: Compression Check Numbers... =
Date: 06/18/96
=0D
Ken-
You say that you have figured out a way to compare compression readings a=
nd 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 f=
urther. If all the engines being tested had the same camshaft and the sam=
e clearances, the test would be valid. However, any change in camshaft ti=
ming will produce a large and noticeable difference in the compression re=
ading, but not the compression ratio. The longer that the intake valve st=
ays open, as on any higher performance cam, the less time/distance the pi=
ston has to make the compression. As an example lets say a stock cam clos=
es the intake valve at 30 degrees after bottom dead center, and a mild ca=
m 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 work=
s 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 compr=
ession readings
than a stock cam will during a compression test. The reason for this is t=
hat 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 wil=
l 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 sp=
eed 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 f=
act 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 reading=
s of all!
=0D
The best use for compression readings, is as a comparison between differe=
nt cylinders in the same engine. (Of course now you know to check your ca=
m lobes if one or more of the readings is outside of the expected result.=
=2E.)
=0D
Jeez, this is hard without using my hands to explain...
Nick in Nor Cal
=0D
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