Tony Rhodes wrote:
> I am not at all sure that you have to get better throttle response with an
> accellerator pump
> arrangement instead of the damper. I am not saying it isn't so, just that
> it is not a necessary
> acpect of the design differences.
>
> -Tony
Chris is correct in saying a fixed venturi carb will give better throttle
response.
In a conventional carb, when you rapidly go to wide-open-throttle (WOT), the
manifold
vacuum drops immediately to close to zero and there is very little flow
restriction
into the engine, meaning the engine has to do very little work to pull the
maximum
air/fuel charge into the cylinders. Since the vacuum that is necessary to get
the
fuel to discharge into the venturi is absent, the mechanical diaphragm
accelerator
pump operated off the throttle linkage provides the fuel for the mixture
enrichment.
In a CD carb like the SU, Zenith-Stromberg, or American made Predator carb,
going to
WOT does not result in maximum flow capability to the cylinder because the air
valve
is slow to rise due to the oil in the damper. The oil needs to be there to
function
like an accelerator pump by increasing the velocity of the air through the
venturi.
Since airflow and pressure are an inverse relationship, the greater airspeed
reduces
the pressure above the jet. Atmospheric pressure in the float bowl pushes the
fuel
out of the jet and into the engine. (Or if you prefer, the vacuum above the
jet sucks
the fuel up - its the same thing, just two ways of saying it.) The unfortunate
net
result is that the engine continues to pull vacuum, somewhat limiting the
amount of
air that can be drawn into the cylinders, while the least-restrictive condition
of the
air valve in the full-up position is not achieved for a few seconds.
All that said, the difference is pretty theoretical. I doubt you'd notice much
difference in the real world outside of a drag race - and even then, the
advantage
might only be a hundredth of a second...
Regards,
Brian Schlorff '61 TR-4 '64 TR-4 '72 TR-6 '79 Spit
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