Compression Test

["Paul Bayly" <thebaylys@domain.elided>]

My views: (Tenuous link, ALFA make aircraft components !!).

At altitude is there less cylinder pressure? YES
Will the gauge read lower?  NO IDEA !

For those assuming the engine will keep 'pumping up' until it reaches a
theoretical maximum regardless of altitude, just remember that the cylinder
pressure is dumped overboard once every second revolution.  The gauge
maintains the pressure as it has a check valve (non-return valve) in the
connection hose.   As far as I can see....14.7 psi @ sea level @ 15degC
multiplied by 10 (10:1 compression) would give a theoretical 'perfect'
pressure of  147 psi.   Thus the pressure should decrease proportionately at
altitude.   Problem I guess, is will a $10.00 compresssion tester show the
difference ?

The analogy with turbosupercharged piston aircraft engines is a good one.
The higher you go the less dense the air, even though the ambient air
temperature also decreases (7 deg F per 1000ft IIRC).  The forced induction
is needed to feed enough oxygen into the cylinder for X amount of fuel to be
burnt.
I don't believe that high compression is the thing which gives a bigger bang
 it's the amount of fuel you can expose to the right amount of oxygen.   The
smaller volume just makes flame propogation easier/better (hence Alfas
continued return to Twinspark rather than multi valve engines).

Apart from the obvious, Turbine engines are a bit different to pistons.   4
cycle piston engines have the four processes (suck, squeeze, bang, blow)
happening in the same place at different times.   Jets have the four
processes happening at the same time in 4 different places.


Beatle
Oz

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