Compression check

[Andries Scholtz <AndriesS@domain.elided>]

For compression tests, the reading at altitude will be lower , for the
simple reason the air is sucked into the engine is limited by the valves,
timing etc, air cleaner etc but mostly by the atmospheric pressure, so:
 if you leave the engine at bdc with an inlet vale open the amount of air at
sea level will be x  at a pressure of 14.7 psi (101kPa)  ,  at altitude the
volume will still be x but the pressure will be less (at 4500 ft  will be
15% less).

Since PV=nRT for a given gas , temp etc 

We are  measuring compression which  means that V1/V2  is constant  which
leaves us with 2 paramateres that can changes T and P, i.e

V1= nRT1/ P1 Before
And 
V2= nRT2/P2 After

V2/V1  =  nR(T2/T1)(P1/P2)

We are measuring P2  ;  V2/V1 , n R are constant - thus C= (T2/T1) (P1/P2)
if T2/T1 is also constant it means that P2 needs to drop if P1 dropped, i.e.

If at sea level P1 =3  and we take C as .5  it means  P2  (measured value)
will be 6
 At high altitude P1=2 (lower pressure) and we take C as 0.5 it means P2
will be 4

Now fluid heats up as it compresses so  that effects the story as well , but
that is thermo dynamics 101.

If you have a turbo and you can actually generate pressure by turning the
engine fast enough 2500rpm for a decent high pressure turbo ,you're losses
due to altitude will be less but getting the turbo to blow will be very
unlikely


For tires, the same apply but you have a sealed system so PV=nRT, volume is
fixed, n is fixed R is fixed, so as temp raises P will raise. And 42Psi at
sea level should be 42Psi at high altitude , if not the gauge is stuffed, so
a spring plate gauge is good, not depended on atmosphere 

My understanding of an absolute gauge is measuring pressure inside the
system and relative gauge is relative to  the atmospheric pressure.


Capish,

Andries
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