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From: prins@cornell.UUCP (Jan Prins)
Newsgroups: net.auto.tech
Subject: compression tests
Message-ID: <1083@cornell.UUCP>
Date: Sat, 23-Nov-85 09:46:06 EST
Article-I.D.: cornell.1083
Posted: Sat Nov 23 09:46:06 1985
Date-Received: Sun, 24-Nov-85 06:42:41 EST
References: <633@bonnie.UUCP>
Reply-To: prins@cornell.UUCP (Jan Prins)
Organization: Cornell Univ. CS Dept.
Lines: 41

>BTW, the compression was between 170 and 180 psi on all cylinders; does
>anyone know what the spec is?  The shop manual doesn't seem to have it!!

I feel factory spec compression figures are not very useful as an engine
indicator, and I suspect that's why they're often not featured prominently
in service manuals.

Consider that these measurements are taken over a compression stroke
that is of vastly longer duration than an operating stroke.  Ring sealing
depends on combustion chamber pressure, which simply doesn't build very 
quickly at cranking rpms.  At the same time leakage past the rings takes 
place for a longer time.  Therefore the pressures observed are not the 
same as those you would see during operation.

Since the components involved are assembled to specified tolerances,
manufacturers are often able to give ballpark compression figures for
a new engine.  As the engine wears the compression figures drop but
are not a reliable indicator of power production ability.  When the 
compression figures fall below the "specs" they invite the conclusion
that the top end is "worn out", which is not necessarily true.  
 
Of course a serious problem like a stuck valve or a warped head can be
diagnosed with a compression test, but in that case the magnitude of the
figures involved is not important, just their difference.  And these
sorts of problems are pretty obvious to begin with.

I once saw a Toyota 18RC engine that wouldn't start.  The compression had
been measured between 30 and 50 psi on all cylinders.  The spec was,
as above, 170 psi.  Engine junk?  Well, not entirely.  The starting
procedure had to be adapted to give maximum compression - i.e. throttles
wide open.  Once started in this fashion the engine made normal power
and burned no oil.  Since then that car has piled up another 20,000 miles.

So it seems to me that a compression test only validates a claim that
an engine top end is relatively new.  I think their use in estimating 
the "life" of an engine is overrated.

   Jan Prins   UUCP:   {vax135, ihnp4, decvax}!cornell!prins
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               ARPA:   prins@gvax.cs.cornell.edu
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