This carburetor was the most often found carburetor in use during World War II on the larger, high-performance aircraft engines.
It is categorized as a floatless pressure carburetor that provides very accurate fuel delivery to the engine while at the same time preventing ice from forming in the carburetor as well as preventing fuel starvation to the engine during negative “G” and inverted flight. The prevention of negative “G” starvation was accomplished by the elimination of the customary float-controlled fuel inlet valve found on many other aircraft carburetors.
Unlike the float-type carburetor system that relies on venturi suction to draw fuel into the engine, a pressure carburetor only uses the venturi to measure the mass airflow into the engine and manages the flow of fuel that is continuously under pressure from a fuel pump to the spray nozzle.
Starting with the basics of fuel combustion, no matter what type of fuel system is used on a given engine, the carburetor’s sole job is to provide exactly the correct amount of finely atomized fuel into a given amount of air entering the engine. To be burnable, the air to fuel ratio must be within the stoichiometric range of between nine and sixteen pounds of air to one pound of fuel. Above or below this ratio, the fuel will not burn. Next, it is also a given that within that range of acceptable mixtures, there is but one ratio that is the ideal air-fuel ratio at that time, given the throttle position set by the pilot. In summary, it can be said that the ideal carburetor provides the correct air-fuel mixture ratio, as required by the engine, under all of its operating conditions.
Last, it is also a given that it takes exactly seven pounds of air passing through an engine to create one horsepower. It therefore takes 7,000 pounds of air to create 1,000 horsepower in a given engine. That 7,000 pounds of air requires at a minimum 437.5 pounds of fuel to a maximum of 777.8 pounds of fuel to be within the burnable range. The exact amount of fuel needed changes between the overly-lean lower limit of 16:1 and the overly-rich upper limit of 9:1 as the engine operating condition changes.