I cant tell you exactly what the difference is but i do know that annular booster are generally used when a car is over carbed as they tend to smooth out the idle. If your engine has low vacuum because of a big cam annular boosters give you back some driveability.
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Annular vs downleg boosters. If you pull decent vacuum from a modified engine the downleg style is the right choice. First the annulars larger size presents a slight airflow restriction reducing the carburetors total capacity compared to smaller downleg. Annular boosters allow the main circuit to start up earlier than a downleg. Annular boosters are quite a bit more sensitive and will have a tendency to have a fatter fuel curve. Of course annular boosters arent exactly a free ride. Carburetor boosters have evolved to keep pace with the changing demands of drag racing engines.
Downleg boosters especially the brasswell style with the step machined in them seem to be the industry favorite for multi purpose streetstrip applications these days. Annular boosters shown here is a straight leg booster design. Downleg boosters are similar in design to a straight booster but are positioned lower to improve signal. If you are using a carb that is a normal size use the down leg. A downleg booster starts the main system up earlier than a straight leg booster. Thats why they are generally not reccomended for typical street applications.
Yes if you take a carb that is flow rated with down leg boosters and replace them with annular boosters you will be reducing the cfm flow at 15 that the original carb was flow tested at. The bigger the banjo the more reduction. But if you buy a carb designed with annular boosters it should flow what the manufacturer rated it at.
Gallery of Annular Vs Downleg Boosters
