re: Compression checks

[bearbvd@domain.elided (Greg Hermann)]

At 11:37 PM 11/13/02, Jon Pike wrote:
>> 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?

In the gas equation, n changes at altitude, because the intake air is less
dense (less moles of air in the same intake volume), so the compression
test indicated will be lower, approximately in proportion to the air's
densty ratio (to sea level) at the test altitude.

The fact that the air IS heated by the compression process has a _VERY _
significant effect on the test pressure, AS does the fact that the process
in the engine is NOT perfectly adiabatic (some heat IS lost).

Greg
>
>Jon
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