Testing Demon Carburetors on a Small Block Chevy - Size Matters
4. In order to speed up the air and increase the vacuum that draws the fuel out of the boosters, carb tuners will use several different styles and diameters of booster. According to BG Fuel Systems, here's why:
Boosters are usually produced as zinc diecastings and are available in three configurations: down leg, annular, and straight leg. The straight-leg style is rarely used. To get boosters to function effi- ciently, they need to be placed precisely in the middle of the carburetor's main venturi. This is because the bores of main venturi have a narrow waist, which causes the airflow to increase in speed. As the air speed increases, a pressure drop occurs (a depression) at the narrow waist. The boosters, which also have a narrow waist to increase the depression further, draw fuel from the carburetor's fuel bowls through the metering circuits and out into the air stream.
The style of booster chosen is usually determined by air speed: The bigger themain venturi area, the slower the air speed. To increase the air speed in a main venturi that is operating with a down-leg booster, a carburetor manufacturer or tuner might switch to an annular style. Because of its larger size, the annular booster creates a bigger restriction in the main venturi and therefore increases air speed.
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