re: Compression checks

[Jon Pike <jhpike@domain.elided>]

> 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?).
Heh..  I know I'm not that lucky, at the moment!

My instinct was the same conclusion, that it would stay the same no
matter the altitude.  But, yes you do need compensation for altitude
gain..

I believe the changing factor to be, a thinner ambient air causing a
lower volumetric efficency..  i.e. the intake side flow restrictions,
like air filters, port size, valve opening, etc..  are taking a greater
toll on how much air ends up being breathed by the engine.

If this is so, I'd expect to cost you performance by changing the 
mixture faster than it might affect the compression.. though it should
have an effect there too.

Anyone have the definitive answer?

Jon
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