B Strachan wrote:
>
> > The damper oil, variable venturi and dashpot act as a reverse
> > accelerator pump.
>
> I'm not sure I believe this. Looked at an acceleration pump on a Solex
> lately? They do exactly the reverse - dump a whole bunch of gas in the
> manifold the instant you open the throttle.
The thought is right, but "reverse accelerator pump" is probably not the right
term.
When the throttle is opened on a conventional car (like a Solex, Weber, Holly,
etc.) the vacuum drops* (not spikes) and the gas isn't pulled into the air
stream, so the engines bogs down. The throttle plate having been opened,
there's less restriction. But he air hasn't gotten moving yet, so vacuum
drops. Thus the need for a pump to squirt the fuel in till the airflow and
engine speed get going.
What the variable venturi does is to eliminate the NEED for an accelerator
pump. (In theory, anyway.) Because the venturi is small when the airflow is
low (low RPMs) the vacuum across the fuel inlet is high, permitting the flow of
fuel into the air stream. Drop the pedal on an SU equipped car and the dash
pot lifts (in response to increased airflow) to allow more air in, but its rise
is dampened by the oil, so doesn't open instantly. Thus the airflow increases
more slowly, so you don't have the bogging down problem. Also, the tapered
needle allows more gas flow with more airflow, so the air/fuel ratio remains
constant. THat's the basic theory, anyway. (That's why differetn tapers are
available - so you can tue the mixture for the kidn of response you want.) So
you can see the effect of a lighter oil is to allow the dash pot to rise more
quickly, giving a better throttle response. Get it too light and you have the
bogging down problem back. Get it too heavy and thrott!
le response suffers. Use 50 wt and you'll wonder where your horsepower went, I
imagine. If it's even drivable! Anybody tried it?
It's really a much more efficient carburetor design than a fixed venturi. If
the total opening (at wide open) were as big as the Solex it just might give
you the same kind of top end response.
*(Loss of vacuum with throttle opening is why water injection doesn't work well
without a pump. Most cheap water "injection" systems rely on manifold vacuum
to draw in the water vapor. But heavy throttle is right when you need water
injection.)
--
Mark van der Hoek
Houston
From bondage to spiritual faith
From spiritual faith to courage
From courage to freedom
From freedom to abundance
From abundance to selfishness
From selfishness to complacency
From complacency to apathy
From apathy to fear
From fear to dependency
From dependency to bondage
Ezra Taft Benson
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