Glossary of Terms

Annular Fuel Discharge – Fuel discharged into the bore of the carburetor from a series of small holes around the perimeter of the booster venturi. This method of discharging fuel breaks up and partially atomizes the fuel for better throttle response, fuel economy, and power capability. It is the “state of the art” method of discharging fuel even today.

Calibration – A unique set of specifications for a particular engine size, vehicle line and transmission combination. In a carburetor calibration these unique specifications include: main jets, air bleeds, power valves, idle orificices, dashpot or solenoid, transmission kickdown, venturi size, etc.

CFM – Cubic feet per minute of air flow (when specifically speaking of carburetors it is an air/fuel mixture).

Manifold Vacuum – Vacuum created in a gasoline engine when the engine is running with the throttle plates closed or nearly closed (such as at idle). Pistons trying to take a big “gulp” of air on the intake stroke create this vacuum. With the throttle plates nearly closed the big “gulp” of air turns into a little “sip” of air. This restriction of the throttle plates is a low-pressure area or vacuum. At full throttle, when the throttle plates are wide open, there is ZERO intake manifold vacuum.

Mechanical Secondaries (Double Pumper) – The secondaries are opened via linkage fro the primary throttle shaft. The driver has control of opening them. This system requires a second accelerator pump, hence the name, “Double Pumper”. It should only be used in race applications.

Vacuum Secondaries – The secondaries are open only when the air velocity is high enough in the primary side of the carburetor to produce a vacuum signal to a diaphragm, which opens the secondary throttle plates. The driver has no direct control of the secondary opening.

Venturi Vacuum – A low-pressure area created just below the narrowest part of the throttle bore. By using a very high velocity (sometimes in excess of 20,000 feet per minute or 250 miles per hour), this low-pressure area (vacuum) can be relatively powerful. The more the throttle plates are open, the more air flows through the venturi at a higher velocity. The more velocity, the stronger the venturi vacuum. The stronger the venturi vacuum, the farther the secondaries will open.