On 10 May 95 at 1:12, Glenn Schnittke wrote:
> What I'd like to know is; once I get the old head off again, how do I
> determine the ACTUAL ratio and capacity?
Well, you need measuring implements and formulas. lets see-
The cylinder volume you can calculate:
Vcyl - cylinder volume= stroke * ((bore/2)^2) * pi
(remember- this is the volume of just 1 cylinder, not the whole engine!)
The combustion chamber volume, you need to measure, by CC'ing- measuring
how much fluid it takes to fill it. (Usually done with a burrette, and flat
plastic plate with a hole in it.)
To the measured Vcc, you need to add some other small items- the volume around
the rings, the head gasket, and add/subtract for dished/domed and
recessed/protruding piston tops, and any other bits, like cutouts, etc. You
can often find stock numbers listed in reference books, or you can break out
your burrette again.
Total Vcc = V head + V rings + vgasket +/- V piston
Formulae for Compression ratio, and combustion chamber volume (Vcc):
****************************
* CR = (Vcyl + Vcc) / Vcc *
* or Vcc= Vcyl/(CR-1) *
****************************
So, you can either find the C/R of your engine, or determine the needed
combustion chamber volume for a target C/R.
> And more importantly, but less
> immediate; why does it run so well with a cracked head and bad rings?
Because the B-series engines are stout, reliable engines, which will run under
almost any adverse conditions and get you home!
Also- don't forget, if you increase the C/R, you'll need better petrol.
planing the head = decreased volume = increased C/R
bigger bore = increased C/R
grinding/polishing in the combustion chamber = decreased C/R
Dished pistons instead of flat = decreased C/R
You get a power gain from bigger bore, and maintain the same C/R with a very
small increase in chamber volume, or dished pistons, and avoid high octane
petrol.
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/ _ \ Roger Garnett (Roger-Garnett@cornell.edu)
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