One more issue about porting and polishing intakes. It's not just a "less
friction over a smooth surface=more flow" relationship. Once again - true in
high flow situations.
At part throttle/low rpm, a semi-rough surface in the intake port creates a
"boundary layer" of turbulence at the very outside of the flow that encourages
a higher overall velocity of the mixture. Strange but true. Another good reason
to leave the intakes alone... Has to do with "laminar flow" theory way beyond
me.
> The beauty of the SU is that it is a variable choke carb - thus,
>velocity changes have much less effect on fuel/air ratio because the choke
>diameter "self adjusts" according to the vacuum developed as the charge flows
>through the restriction created by the carb's piston and the ridge in which the
>jet is mounted.
True about the carb. Too bad it don't work that way for the port itself. Once
the mixture gets past dash pot the velocity drops, the fuel that was nicely
atomized has plenty of time to form bigger droplets and puddle...
The importance of maintaining or increasing the VELOSCITY of the mixture
through the port can be seen in all modern engine design. SMALLER ports/valves
and more of them rather than bigger ports/valves. 4 or 5 small valves per
cylinder. This is why Porsche finally went to water cooling. They couldn't
design an air-cooled, multi-valve head!
(A moment of silence for the air-cooled Porsche...)
Christopher M.Lillja
Marketing Associate
Princeton University Press
Fax:609 258 6305
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