Compression Pressure and Ratios

["Georg and Claire Taht" <m.five@xxxxxxxxxxx>]

Some thoughts on compression pressure and ratios.

The ideal Gas Law states the following:

PV=nRT where

P=Pressure
V=Volume
n=number of molecules
R= constant depending on what unites are used
T= Temperature

Now in an ideal world, nRT is constant and hence
P1V1=P2V2,
P1=Initial Pressure
V1=Initial Volume
P2=Final Pressure
P2=Final Volume
Knowing any of the three you can solve for the fourth.

But an engine doesn't behave that way.
You have blow by, therefore the number of molecules in the
cylinder are decreasing.  Also there may be associated
temperature rise, which will again change the equation.

Lastly,  for the above equation to work the cylinder
would have to be completely filled and the valves closed
at the bottom of the compression stroke.  Due to cam timing
the valves may be open, even as the piston starts to come up
and the effective compression ratio may actually be
different.

I believe this discussion started because of wanting to know
the compression pressures and were they valid.

My 87 535is shows pressures between 150 and 155.  I consider
this
to be good. Note I had an 79 Honda Accord with the same 8:1
compression
ratio as my 535is would provide pressures between 175 and
180.

Again note that evenness is more important.  I wouldn't even
worry about
it until you are below 120psi.  Once pressures start to drop
from
an established norm for the engine there are probably other
things like
increased oil consumption happening.

Georg
87 535is AKA "MOCK 5"
m.five@domain.elided

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