Compression ratio and compression test readings.

[Seth]

Yep, you heat the air when you compress it, so the reading is higher
than if the temp of the compresed air was the same as uncompressed. 

-Seth

Stephen Webb wrote:
> 
> 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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