Understanding the Constant Depression Carburetor
Carburetors have been around since
the beginning of the automobile. Over the decades, carburetors have varied from
the simple to the very elaborate. All carburetors share one common property:
they mix gasoline with air in the proper ratio for combustion over a wide range
of conditions, including engine load, altitude, engine temperature, and air temperature.
Generating a useable air-fuel mixture has been accomplished in a wide variety
of ways. In the early days, the RPM range of internal combustion engines was
limited so a wide range of an air/fuel mixture was not as important and
therefore carburetors for such engines were often very simple devices. Early
carbs relied on moving air to draw fuel out of an orifice where it then
traveled through the manifold to the combustion chamber. The fuel openings (called “jets”) were fixed
so a certain amount of air flow was necessary to draw the fuel out. The larger
the fuel openings, the more air flow necessary to start the fuel delivery.
As engines became larger and the
RPM ranges increased, larger amounts of air/fuel was necessary. At idle, the
air speed was too slow to draw fuel from the jet so idle circuits were added;
these were usually small openings near the throttle plate. These idle “jets”
would supply fuel until larger throttle openings allowed sufficient airflow to
cause the main jets to come into play. As the need for more air/fuel increased,
it was found that the main jets could not be sized to supply sufficient fuel at
high revs without compromising the effectiveness of the carburetor at low revs,
so power valves were added to provide additional fuel under conditions of high load
and/or power demand. These added extra fuel that the main jet was not able to supply
during high load conditions. As ever-larger, more powerful engines came into
being, it became clear that single throat (or barrel) carburetors could not
provide a proper mixture throughout the speed range. This situation led to the
development of multiple-throat carburetors such as two-barrel, four barrel, and
even multiple carburetors as well as other features designed to accommodate
wide engine speed ranges, such as mechanical and vacuum-operated secondary
throttles for high demand conditions.
It was also found that when the
throttle plate was opened quickly, a small amount of time elapsed before the
fuel flow “caught up” with the increased airflow, causing a lean air/fuel
mixture. This could be observed by the engine sputtering or stumbling. Carburetor
designers resolved this issue by incorporating an accelerator pump to squirt
additional fuel into the air stream whenever the throttle was opened.
Suffice it to say that as time went
on, carburetors became very complicated devices. A typical late model American
Carburetor can have up to 200 pieces and any one of them can cause a problem if
defective or out of adjustment. The Weber carburetor is popular with a lot of
high performance cars whether as stock or added on by the owner. The Weber does
have the advantage of being very tunable and when correct can give very good
results, especially with very high performance engines that don’t have a very
smooth vacuum. A disadvantage of this carburetor on the Jaguar is that there is
a carburetor throat for each cylinder. Changing a jet, air corrector, choke,
accelerator pump needle, or most anything requires 6 or 12 of the item. Therefore
the Weber is usually used on high performance engines, and since there is no
standard setup for non Weber equipped engines, the results is a lot of trial
and error to see what works. . This can get rather expensive and time
consuming.
One carburetor designed under
different principles is the SU carb. This carburetor operates on what is known
as the Constant Depression principle. Depression is Brit-speak for vacuum and thus
SUs and related carburetors can also be called constant vacuum carburetors. (The
Zenith Stromberg carburetor is also a constant depression carb.) These carburetors
are designed such that the throat or opening varies with engine load, resulting
in a constant depression or vacuum being maintained at the jet opening. Most CD
carburetors are considerably simpler in design than traditional carbs. Generally
a tapered needle attached to a moveable piston is drawn out of the fuel jet as
the piston rises due to engine demand as sensed by the changing volume of air
into the engine. As the needle is drawn out of the main jet, more fuel flows
into the airstream.
Incorporated into these carburetors
is an oil filled dampener which slows the rise of the piston to keep the
depression at the jet constant which does away with the need for an accelerator
pump. The same jet/needle is used from idle to full load and in between. Carburetor
opening size varies depending on engine size, although in practice there is a
limit to the amount of air that a single CD carburetor can flow. For the
larger, more powerful engines, like the Jaguar XK engine, multiple carburetors
are used.
Another advantage of the SU carburetor
design, i.e. varying the carburetor opening due to engine demand, is that full
throttle can be applied at low speeds without the causing the engine to “bog”, as
is usually the case with conventional carburetors. There is also a spring that
pushes down on the dampener piston in this style of carburetor. This is used to
dampen the induction pulses so the dampener does not bounce up and down
creating an unstable mixture.
The Zenith Stromberg is considered by
many to be an emission carburetor design. This is somewhat of an unfair label,
as my 1964 Morgan had a pair of Zenith Strombergs before emission laws even
existed and the engine did not lack for power. The Zenith Stromberg carburetor
is built with tighter mixture controls which helped these carburetors to
address the emission laws of the late 60s. Just to mention one design feature: on
the Zenith Stromberg carburetor, the fuel needle is spring loaded towards the
engine side of the main jet (rather than being centered in the jet as on most
SUs) so that fuel is drawn out on the “upstream” side of the needle, which was
found to provide improved fuel atomization. Zenith Stromberg carburetors also
feature a temperature-controlled bypass valve. When the under hood temperature
is higher than normal (such as in heat soak conditions after the car is parked
when hot), this valve opens to allow air to bypass the dampener, thus weakening
the mixture until the temperature comes down. There are also over-run valves that
allow air/fuel mixture to bypass the throttle plate under high vacuum conditions,
such as closed throttle on deceleration. This keeps the engine from coming back
down to idle too fast and also keeps high vacuum conditions from occurring.
Both of these devices are for emission control. When properly adjusted, Zenith
Stromberg carburetors offer very nice performance with better emission control.
When Grand Tourismo Jaguar was racing an XKE and had to replace the triple SU
setup with triple Zenith Strombergs, they actually picked up additional horsepower.
There are numerous variants of both
the SU and Zenith Stromberg carburetors. Some of the SU carburetors have
external float bowls and some are built into the base of the carb. One version
of the SU carburetor even has a temperature controller that adjusts mixture
depending on the temperature of the fuel. Some of the SU carbs have a built-in choke
mechanism and others have an external choke/starting carb. The Zenith Stromberg
carburetors have built-in chokes which add fuel and set the throttle opening,
rather than moving the jet as on many of the SU carburetors. For emissions and
proper running, the height of the jet relative to the bridge of the carburetor
(or bottom of the opening where the jet is located) is critical if proper
mixture control is to be achieved. On SU carburetors, the needle is fixed to
the dampener piston and the main jet is adjusted up or down. The Zenith
Stromberg carburetor has a fixed main jet and the needle is positioned up and down
in the piston to set the mixture. The relative position of the top of the jet and
the level of fuel are critical in proper mixture control. Having the jet in a
fixed position relative to the bridge gives better mixture control.