As further clarification, keep in mind one thing; all carbs, including the
variable venturi, meter the amount of fuel coming out of the fuel bowl via
the main jets throught use of vacuum, which is variable throughout the rpm
range and throttle settings of the engine.
The compromise in a "normal" carburetor reflects the imperfect metering
via vacuum. The SU and other variable venturi carbs further refine the
metering by adjusting the size of the venturi, thus eliminating the
weakness of using pure vacuum as a metering tool. Try sucking through a
large straw sometime, then suck the same liquid through a smaller one. You
will probably find you can actually get more liquid throught the smaller
straw (within certain limitations).
Variable venturis were though to be the wave of the future at one time.
Indeed, they were able to survive into the fuel injection age. Ford and SU
each had different mechanisms to accomplish the same thing. Fuel injection
rendered the design moot in the late 70s and 80s.
Those of you who are contemplating changing out your SUs for Solexes or
Webers should consider carefully what they want to accomplish. Large
Webers are almost undriveable on the street with anywhere near the correct
jets. Most of the time, you would be cussing them out :).
Marcus A. Pryor
For a new age of reason
|