re: elevated compression

["John & Deb Dohrmann" <jdohrmann@domain.elided>]

After reading the recent posts of very high numbers from compression tests,
I got curious about how sensitive compression is to carbon buildup.  So I
fired up Excel and played with some numbers.
Compression is the ratio between the volume of the cylinder with the piston
down (and the intake valve closes) and the volume at top dead center.  The
resulting pressure is the compression ratio times the starting pressure
(probably a little below atmospheric).
In the 2L, each cylinder has a max volume of about 490 ml.  At 9:1
compression, I get a TDC combustion chamber volume of 54.4 ml.  Since the
area of the top of the piston is 55.5 square cm, the effective height (or
thickness) of the combustion chamber at TDC is about 11 mm.  And 9 times
atmospheric pressure gives a pressure of 132 psi.
If you reduce the height of the TDC combustion chamber by 2.2 mm (carbon
buildup?) the effect is to make the combustion chamber enough to make the
compression ration 10:1 and the pressure 147 psi.
You could reach 200 psi at 14:1 compression, which would only take reducing
the height of the combustion chamber by 4.7 mm.

NOTICE:  This is a very rough model, assuming perfect ring sealing, etc.
The point is, the combustion chamber is very thin to begin with and it
doesn't take a lot of buildup to reach compression ratios of 13:1 or even
15:1 and pressures up to 220 (15:1).

John Dohrmann
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