Compression ratio and compression test readings.

[Don DRI05 Ricciardiello]

Consider valve timing/overlap at the rpm your taking readings at. The
ability of the engine to consume air varies with rpm when the cam timing is
fixed.

Don





From: Stephen Webb <swebb at netlab.uky.edu>@diy-efi.org on 16/10/2001 23:44
      AST

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cc:
Subject:  Compression ratio and compression test readings.



I've often noticed that the pressure spec for perfomring a compression
test is higher than what I would expect.

For instance, on a 10:1 engine, I would expect a pressure of 10 times the
ambient pressure, which would be 9 bar on a gauge, or 10 bar
absolute.  But, the factory says I should expet between 10 and 13 bar for
this particular engine.  (1.8 16v VW engine)

>From my reasoning that's 1-4 bar higher than I would think.

Is my reasoning wrong?  Do I just heat up the air when I'm compressing it?
I'm not sure if I can apply the ideal gas law like I am about to do, but
here goes:

P1V1=nrT1

P2V2=nrT2

P1V1/T1=P2V2/T2

V1=10*V2
P1/T1=P2/10*T2

1/T1=13/10*T2

T1=0.77T2

Lets say T1=300K (about 80 F I think)

T2=390K  =~ 240 F

So I go from about 80 deg F to 240 deg F by compressing the air?

Is this really what's going on?

-Steve





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