Dan,
Not a dumb question, though it is sure to elicit some very emotive answers!
Carbs are one of my fave subjects!
Carbs are ultimately a huge compromise. Carbs are "narrow-band"
instruments, originally designed to deliver the correct air-fuel mixture at
a constant rpm. E.g., aircraft, boat and stationary engines. Carbs rely on
simple physics of air passing through a venture which creates a vacuum that
draws a flammable substance into the air stream. Clearly the flammable
substance (petrol/gas) will be most efficiently atomized when the petrol/gas
is of a certain viscosity, the venturi has a certain cross-section, and the
air is moving at a certain velocity. When these three factors are optimized
together, we have generated the optimum air-fuel mixture. Carbs are thus
"sized" for the engine and the volume of air it will consume (remember, a
4-stroke engine is just an air pump). Well that9s just great for a motor
running at a single speed!
Unfortunately, the car operates very differently, as the throttle is
constant exercised, from zero to full, and everywhere in between. This is a
"broad-band" application. And something carbs do not patently like. Over
the years, they have become more complex, not only in response to the
demands of driving habits, but also in response to increasing clean-air
legislation and the ability to start when cold and run smoothly when hot
(air density changes with temperature, which affects air flow,
which....well, you get it).
Enter fuel injection systems. These systems correctly monitor the demands
of the engine to correctly inject atomized fuel particles into the air
stream. Modern FI systems also monitor exhaust gasses, engine load,
temperature, humidity, etc. That9s why FI engines start when cold and idle
great. A modern FI system gives better economy, performance and clean air
than a carb system ever could.
Having said this, the Lucas PI is *not* a modern FI system. It is an
mechanical system, that perhaps does a better job than a carb, but it is not
in the same league as a modern FI system.
Having said all of this, I think you are being overly unkind towards the ZS
carb, and overly complimentary of the Lucas PI. The ZS is a good carb and
one of the last "simple" carbs that satisfied Federal requirements for fuel
consumption and emissions. Yet it can sit up and boogey and provide
PI-level satisfaction. The ZS is considerably simpler, more user-servicable
and cheaper than the PI.
Twin ZS's are great carbs. Triples are great for performance motors. I
don9t think assigning a "scale of serviceability" is fair. Horses for
courses. Those seeking the nirvana of British originality will seek the
Lucas PI. Those owner-mechanics who want performance will seek triple
carbs, and those seeking to extract maximum performance while making the car
easy to drive around town, will invest in a modern FI.
And personally, I think carbs have more "soul" than FI. Carbs engage the
driver in a way that FI cannot (though I am sure one could program a modern
FI to behave like a set of cranky carbs!). Carbs require a certain starting
procedure on a cold morning. They are clearly more impressive looking
(especially when the zinc is clean!), and when fitted with individual air
cleaners or trumpets, sound so much more visceral.
In parting, I have triple Webers, because that is what I grew up with, and
understand more then ZS' (or SU's).
Shane Ingate, now off his soapbox, in Maryland
Dan Malling asked:
> What is it about PI that provides a performance increase over
> carburetion?
> Is it a case where a theoretically very small carburetor could never
> provide enough fuel air mixture to a theoretically very large engine
> without running too rich -- I assume running too rich means that the
> fuel is not being atomized or vaporized correctly? On the other hand, a
> theoretically very large carburetor could not provide correctly mixed
> fuel/air to a theoretically very small engine because the theoretically
> very small engine can not produce enough air flow to make the
> theoretically very large carburetor work correctly?
> Hmmm.... Is that it? There has to be a balance between the "size" of the
> > carburetor and the "size" of the engine for things to work correctly?
> And even when working correctly (the best it can work), the carburetor,
> by its nature, still can not generate as much correctly mixed fuel/air
> as can the PI?
> If this is the case, then were does triple ZS fall in the scheme of
> things -- on a scale of 1-10, where 1 is dual ZS and 10 is PI? Maybe
> the answer is at low RPM triple ZS is at a 2 and at high RPM triple ZS
> is at 7?
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