Rex---I think that the designers of both the ZS and SU carburetors did a
fine job, using the Constant Depression method of metering fuel and air.
These are frugal with the fuel, AND can be modified for performance.
(That economy will suffer is a given) It is also as you stated, that the
stock setup correctly raises the air piston in response to the vacuum
created as the rpms increase, allowing proportionately more fuel to
accompany the increased air flow. Now here's where we may see things
differently...
When the gas pedal is "floored" at low rpms, the engine actually CAN use
more fuel and air. But since CD carbs do not have a power valve, or an
accelerator pump, (as most other carbs) there is no simple way* for the
fuel to match the air when the throttle is snapped open. Therefore the
piston is by necesssity restricted from rising quickly, by the spring
and damper mechanism, as you noted.
If the piston was allowed to rise quickly, the engine would falter
badly, from too lean a mixture. This doesn't mean the engine doesn't
WANT more air and fuel, it just can't get it quickly. If it could get
it, the acceleration 'down low' would be improved.
Surprisingly, the CD carburetor can be altered to (almost) keep up with
a design such as the Weber. *This is done by removing the air piston
spring, and the oil from the damper. With a custom needle providing the
proper taper (of which you would have to alter yourself, based on your
particular engine modifications) the fuel would then keep up with the
rapidly rising piston, and the A/F ratio could be correct for maximum
power.
NOT having a power valve to give that instant squirt of fuel would mean
that it will never quite match those carbs that do, and the altered
needle would most likely always be on the rich side of stoichio. NOT
what the designers intended.
There's no doubt in my mind that engines with Webers, properly sorted
out, respond better coming off idle, and any time the revs are low. If
you say the Weber "cleanly" flows more air by design than the CD at the
high end, I'll take your word for it.
If my explanation of the above left you wondering what the 4#!! I'm
talking about, better wording can be found in such books as "HOW TO
POWER TUNE YOUR SU's"
As always, I'm a believer in using an 02 sensor and A/F monitor when any
high performance mods are being carried out.
Dick Taylor
|