Carburetor CFM
Find the ideal carburetor size for your engine. Enter displacement, max RPM, and volumetric efficiency to get the exact CFM rating with interactive charts and breakdowns.
Stock engines: ~80%, Rebuilt street: ~85%, Race engines: 95-110%
About This Calculator
The Carburetor CFM Calculator determines the ideal carburetor size (airflow capacity in cubic feet per minute) needed for any internal combustion engine. Whether you are building a street performance engine, restoring a classic car, or tuning a race engine, choosing the right carburetor is critical for optimal power, drivability, and fuel efficiency. An incorrectly sized carburetor can cost 5-15% horsepower and cause drivability issues.
The calculator uses the standard formula CFM = (Engine Displacement in in³ × Max RPM × Volumetric Efficiency) / 3456. The constant 3456 converts cubic inches of displacement into cubic feet per minute while accounting for the four-stroke engine cycle (one intake event every two revolutions). Volumetric efficiency (VE) represents how effectively the engine fills its cylinders — stock engines typically achieve 80%, rebuilt street engines reach 85%, performance builds hit 90%, and race engines can exceed 100% with tuned intake and exhaust systems. The result is the theoretical airflow demand at the specified RPM, giving you the minimum carburetor CFM rating required.
Choosing the Right Carburetor Size
A common rule of thumb is to select a carburetor that matches your calculated CFM at peak torque RPM rather than peak horsepower RPM, as this ensures good throttle response where you drive most often. For street applications, slightly undersizing (within 20-30 CFM) can improve low-end torque and fuel atomization. For race applications, sizing at peak power RPM is standard. Modern four-barrel carburetors are typically rated at 1.5 inHg vacuum drop — the same standard used in this calculator's formula. Always consider intake manifold compatibility and whether you need vacuum secondary, mechanical secondary, or EFI conversion for your specific application.
Frequently Asked Questions
What is the formula for calculating carburetor CFM?
The carburetor CFM formula is CFM = (Engine Displacement in cubic inches × Max RPM × Volumetric Efficiency) / 3456. The constant 3456 converts cubic inches to cubic feet (12 × 12 × 12 × 2 = 3456, accounting for the four-stroke engine's two revolutions per intake cycle).
What is volumetric efficiency (VE) and what values should I use?
Volumetric efficiency measures how effectively an engine fills its cylinders with air compared to the theoretical maximum. Stock street engines typically have 80% VE, rebuilt street engines around 85%, performance street builds 90%, and full race engines can range from 95% to 110% (supercharged or highly tuned naturally aspirated engines).
What happens if my carburetor CFM is too large?
An oversized carburetor causes poor throttle response, reduced low-end torque, and rough idling because the airflow velocity through the venturi is too low to properly atomize fuel. This leads to a lean condition at low RPMs and drivability issues. Always size your carburetor to match your engine's actual airflow needs, not larger.
What happens if my carburetor CFM is too small?
An undersized carburetor restricts airflow at high RPM, acting as a bottleneck that prevents the engine from reaching its full power potential. The engine may feel strong at low RPMs but run out of breath at higher RPMs. The restriction causes a pressure drop across the carburetor, reducing volumetric efficiency and peak horsepower.
How is the constant 3456 derived in the carburetor CFM formula?
The constant 3456 comes from converting cubic inches to cubic feet and accounting for the four-stroke cycle. A cubic foot is 12 × 12 × 12 = 1728 cubic inches, but a four-stroke engine intakes air only once every two revolutions, so 1728 × 2 = 3456. This converts in³/min to CFM for a four-stroke engine.
Can I use this calculator for two-stroke engines?
For two-stroke engines, the formula changes because they have an intake event every revolution instead of every two. Use CFM = (CID × RPM × VE) / 1728 instead (half of 3456). Two-stroke engines also typically have different VE characteristics, often ranging from 70-90% depending on the design.
How do I convert engine displacement from liters to cubic inches?
To convert liters to cubic inches, multiply by 61.0237. For example, a 5.7L engine = 5.7 × 61.0237 = 348 cubic inches (rounds to 350). For cc to cubic inches, divide cc by 16.387 or multiply liters by 61.0237. Common engine sizes: 2.0L = 122 in³, 3.0L = 183 in³, 5.0L = 305 in³, 6.0L = 366 in³, 7.0L = 427 in³.
What is a good carburetor CFM for a 350 engine at 6000 RPM?
For a 350 cubic inch engine at 6000 RPM with 85% volumetric efficiency: CFM = (350 × 6000 × 0.85) / 3456 = 516 CFM. A 500 CFM or 525 CFM carburetor would be ideal. For a stock 350 at 80% VE and 5500 RPM: (350 × 5500 × 0.80) / 3456 = 445 CFM — a 450 CFM carburetor would work well.