re: elevated compression

[James Montebello <jamesm@domain.elided>]

On Wed, 13 Nov 2002, John & Deb Dohrmann wrote:
> 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).

All well and good, except you're about two orders of magnitude off on
the depth of the combustion chamber carbon layer in a coked-up engine.
Where the carbon really builds up is in the exhaust manifold and ports.
In the chamber itself, it's more like 0.01mm.  You'd have far bigger
problems than increased compression if the carbon build-up got so bad
it was 2.2mm thick, as the cold piston-to-head clearance is usually only
1.5mm or so in the squish area.

james montebello
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