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.