Carburetors: How to Tell What Size You Need

BY Thomas S.

E=Mc2 may be the more famous equation, but for a gearhead like you, the much more important equation is CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3,456. That’s because this formula is the key to choosing the right-sized carburetor for your specific application.

You may have thought that bigger is better and, therefore, wanted a bigger carburetor for more air and greater power. But, that’s not always the case. In fact, an oversized carburetor can lead to poor drivability and the choking of engine performance. Ideally, you’d proportion the carburetor to your engine’s volumetric potential for optimal performance.

For example, most ordinary street engines boast a volumetric efficiency of about 80%, whereas a modified street engine can average about 85 percent. Fully modified race engines can achieve 95% efficiency or more. With a 355 CID engine and 5,500 max RPM specifically, you’d take 355 x 5,500 = 1,952,500 x .85 = 1,659,625 ÷ 3,456 = 480 CFM. Even taking into account marginal room for error, a 500 CFM carburetor suits this engine perfectly and is sure to optimize your performance.

The next step is to determine if a vacuum secondary or mechanical secondary carb is best-suited for your needs. It should be noted that vacuum secondaries typically are ideal for heavy vehicles, street gearing, automatic transmissions and engines built for low-end torque. Mechanical secondaries, on the other hand, pair well with lighter vehicles, strip gearing (4.11 or higher), manual transmissions and engines built for top-tier horsepower.

Finally, you’ll need to decide on the type of choke you want. Conveniently, most performance carburetors come with a manual choke or an electric choke that can be converted with the proper kit should your needs change.

Now that you have a better understanding of how to choose the best carburetor, you’re ready to browse our extensive selection of top-of-the-line carburetors so you enjoy maximum performance for years to come.