Q&A / Tech

Mailbag: How to Choose a Carburetor for Supercharged Engines

 

(image © superchevy.com)

Q: I am getting ready to pull the trigger on a supercharger for my 1966 Chevelle. What should I do about my carburetor? Do I need to go larger to accommodate the increased airflow requirements of the supercharger? What else can you tell me about achieving the right carb setup for my application?

A: To make maximum power and boost you’ll usually need a larger carburetor(s). That’s because a blown engine can require 50 percent more air than a naturally aspirated engine when operating at full throttle. However, if your supercharged engine is primarily driven on the street at moderate engine speeds (under 4,000 rpm), you don’t necessarily need a larger carburetor.

Typically the carburetors(s) will need to be enriched by 5 to 10 percent on the primaries and 10 to 20 percent on the secondaries. The idle mixture screws may need to be enriched by one or two turns. In either case, the carburetors need to be jetted properly to prevent a lean condition, which can lead to detonation. If you notice glowing red headers, an audible popping, or engine surging, your carburetor is too lean.

For initial start-up, it is actually better to have a slightly rich condition to help prevent the engine from overheating. After initial start-up, check the spark plugs for proper reading and adjust the carburetor(s) accordingly. You want to see a medium to dark tan color.

If you run one or more Holley carburetors, also be aware they contain power valves. Power valves provide additional fuel when there is no vacuum at the base of the carburetor. However, in a blower application, there will always be some vacuum, so the power valves will not function properly. You will need carburetors that have a “boost referenced” power valve circuit.

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