re: Compression checks

[Stacy Faught <slfaught@domain.elided>]

This may be over-simplifying the issue a bit....

Idea gas law says that PV=nRT....so if the right-hand side
of the equation remains the same throughout the test (which it should...),
then P and V will change at the same ratio, no matter what the starting P is.
Since a compression gauge measures relative compression,
the reading should be the same regardless of altitude.  But the 'real' pressure
in the cylinder head would be lower at high altitude--hence altitude adjustors that lean
the fuel mix at high altitude.  

Of course this assumes you have no leakage of the volume chamber--a perfectly
sealed cylinder head and piston rings in other words....(could we all be so lucky?).  In real life, 
most cylinder heads bleed a little air/fuel under the compression stroke, and the
lower absolute pressure at high altitude would force less air past the seals, so under 
real circumstances, the relative compression would be higher....but I doubt a
compression gauge would accurately measure this.


At 1:29 AM +0000 11/13/02, Fred wrote:
>Date: Tue, 12 Nov 2002 17:35:21 -0500
>From: Ferdinando Di Matteo <aroctech@domain.elided>
>Subject: Commpression checks
>
>Here is one to kick around, if I get a high reading at sea level, then
>drive up to  a 14,000 foot mountain top.  would I get a higher or lower
>reading then?   Fred
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