The other day somone wrote in about the differences in the performance
obtained with the Zenith-Stromberg carbs compared to Webers. It was
mentioned that the real failing with the ZS carbs is that their pistons
don't open up quickly when the throttle is suddenly opened wide open,
hurting performance from a stop, and that their performance is basically
equal to Weber carbs when the ZS carbs' pistons are all the way up. Well,
this is contrary to my understanding of these carbs. The
constant-depression carbs or variable venturi carbs as they are sometimes
called (both Zenith-Stromberg and SU carbs) are unique in that they allow
larger carburetors on an engine than one would be able to use with fixed
venturi carbs. As an example, my son's IROC Camaro uses a two-barrel
throttle body with twin 1-7/8" barrels - only slightly more area than the
twin 1-3/4" holes that feed a stock Six, and the IROC engine is twice as
large at 5 litres! Anyway, the reason the British engines are able to use
such large carbs for their relatively small displacement is that the
constant-depression carbs are "small" when the engines need small
carburetors and only get their maximum size at conditions of both high RPM
and high load. At conditions of low engine speed and high load (i.e.
flooring the pedal at a stoplight) the engine can't use a large throat
carburetor until the revs rise up considerably, and the CD carbs handle this
situation nicely. If the needle profiles are correct and the correct piston
return springs are utilized, the carbs' venturi areas increase smoothly as
the engine climbs up it's horsepower curve. What I am inferring by all this
is that a properly set up ZS or SU carb should be superior to any fixed jet
carb (including Webers) at slow engine speeds and at part throttle
conditions. However, the real failing of the CD type carbs IMHO is at wide
open conditions they don't flow nearly as cleanly as a Weber. If you
compare the airflow passage of a ZS carb with the piston up and the throttle
butterfly open to that of a Weber, there is no comparison. The "bridge" and
clunky throttle spindle assembly of the ZS cannot flow as well as the smooth
venturi and throttle of the Weber, meaning that at high RPM/ wide open
throttle conditions, the Weber carbs cannot be equaled by the variable
venturi carbs. But, this is also why the Webers require such careful
tuning. In order to enjoy the wonderful WOT performance of the Webers, the
engine must have sufficient performance enhancements (cam, head, headers,
ignition) to be handle these large carbs at slower speeds, and the special
"driveability" enhancements of the webers (emulsion tubes, etc.) have to be
carefully set up to work well at moderate and slow speeds. The backyard
hobbyists usually find that they can make a ZS or SU carb setup work quite
well with a minimum of tuning due to their variable nature, while the Webers
may require the use of a rolling road to really get them correct.
Rex Townsend
|