Carburetor CFM Calculator

Carburetor CFM Requirements

A carburetor must flow enough air to meet engine demand at peak RPM. CFM (cubic feet per minute) sizing depends on engine displacement, maximum RPM, and volumetric efficiency, the percentage of theoretical air capacity the engine actually ingests.

Carburetor CFM Formula

$$\text{CFM} = \frac{\text{displacement}_{\text{ci}} \times \text{RPM} \times \text{VE\%}}{3456}$$

Displacement is in cubic inches, RPM is peak engine speed, and VE% is volumetric efficiency (typically 80–90% for naturally aspirated street engines). Example: a 350 ci engine at 6000 RPM with 85% VE needs about 517 CFM.

Frequently Asked Questions

What is volumetric efficiency?

Volumetric efficiency is the ratio of actual air drawn into the cylinders to the theoretical maximum at a given RPM. Street engines typically achieve 80–90%; race engines can exceed 100% with forced induction.

Should I oversize the carburetor?

Slight oversizing (10–15%) is common. Too large a carburetor causes poor throttle response and fuel economy because air velocity through the venturi drops.

Why is 3456 in the formula?

It converts cubic inches per minute to cubic feet per minute. Two revolutions per intake stroke for a four-stroke engine gives displacement × RPM / (2 × 1728) = displacement × RPM / 3456.

Does this work for turbocharged engines?

This formula targets naturally aspirated engines. For forced induction, multiply by boost pressure ratio or use a higher effective volumetric efficiency.

What VE should I use?

Use 80% for conservative sizing, 85% for typical street builds, and 90% or higher for well-tuned performance engines. When unsure, 85% is a reasonable starting point.

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Carburetor CFM Requirements

Carburetor CFM Formula

Frequently Asked Questions

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