Carburetor CFM Calculator
Enter your engine displacement, maximum RPM, and volumetric efficiency to find the carburetor airflow rating your engine needs. The calculator uses the standard automotive formula and recommends the next commercially available carburetor size so you can order with confidence. Switch between cubic inches and litres for displacement input.
What is carburetor CFM and why does it matter?
CFM stands for cubic feet per minute - the volume of air-fuel mixture a carburetor can flow at wide-open throttle. Matching CFM to your engine is critical: an undersized carburetor chokes the engine and limits peak power, while an oversized one starves each cylinder of the vacuum signal needed to draw fuel, causing a flat, unresponsive throttle, poor idle, and worse fuel economy. The ideal carb is the smallest size that keeps airflow from being the limiting factor at your target RPM.
The carburetor sizing formula explained
The standard formula is: CFM = (Engine displacement in in³ x Max RPM x Volumetric efficiency) / 3456. The constant 3456 comes from two sources: 12 cubed (1728) converts cubic inches per revolution to cubic feet, and the result is divided by 2 because a four-stroke engine only draws in mixture every other revolution (the intake stroke). So 1728 x 2 = 3456. Volumetric efficiency (VE) accounts for the fact that real engines never fill their cylinders perfectly - a stock engine might fill only 72-75% of its theoretical volume, while a heavily ported race engine with a hot camshaft can exceed 100% in its power band.
Choosing the right carburetor size for street vs race
Street engines benefit from a slightly smaller carb than the calculated maximum. A carb running closer to its rated airflow maintains strong venturi vacuum at part-throttle, which improves fuel atomisation, idle quality, and response from a stop. Race-only engines can run closer to, or even slightly above, the calculated CFM because they spend most of their time near wide-open throttle where signal strength is not a concern. As a rule of thumb, street builds should stay within 10% of the calculated CFM, while strip-only cars can go up to 20% over if the inlet track and camshaft support it.
Standard carburetor sizes and how to match them
Carburetors from Holley, Edelbrock, and other manufacturers are available in fixed CFM steps - commonly 350, 390, 450, 500, 550, 600, 650, 700, 750, 800, 850, 950, and 1050 CFM. When the formula gives you a number between steps, always round up to the next standard size. For example, a calculated 620 CFM requirement would call for a 650 CFM carb, not a 600 CFM unit. Note that CFM ratings are measured at different vacuum levels by different manufacturers - a Holley rating (at 1.5 in Hg) is not directly comparable to an Edelbrock rating (at 3 in Hg), so a 750 CFM Edelbrock flows more air than a 750 CFM Holley rating implies when tested at the same pressure.
Volumetric efficiency by engine type
| Engine type | Typical VE range | Notes |
|---|---|---|
| Stock factory engine | 70-75% | Cast iron heads, stock cam, restrictive intake |
| Mildly built street | 75-85% | Aftermarket carb, mild cam, aluminium heads |
| Street-strip build | 85-95% | Performance heads, hot cam, ported intake |
| Race / full-build | 95-110% | Full port work, race cam, individual runners |
| Supercharged / turbo | 100-130% | Boost adds air mass above atmospheric fill |
Typical VE ranges used in carburetor sizing. Forced-induction engines can exceed 100% effective VE.
Frequently asked questions
What does CFM mean on a carburetor?
CFM stands for cubic feet per minute - the maximum volume of air-fuel mixture the carburetor can deliver to the engine at wide-open throttle. A higher CFM rating means more airflow capacity, which allows the engine to produce more power at high RPM. However, a carb that is too large will hurt low-speed drivability by reducing the vacuum signal that draws fuel into the airstream.
What volumetric efficiency should I use for a stock engine?
A stock factory engine typically runs 70-75% VE. This accounts for modest cam timing, restrictive cast-iron heads, and a stock intake manifold. Using a higher VE for a stock engine will oversize the carburetor and cause poor throttle response and idle quality.
Can volumetric efficiency exceed 100%?
Yes. Naturally aspirated engines with highly tuned intake tracts, aggressive camshaft overlap, and resonance tuning can exceed 100% VE in a narrow RPM band - the intake pulse timing momentarily pressurises the cylinder above atmospheric. Forced-induction engines (supercharged or turbocharged) routinely operate at effective VE values of 110-130% because the blower or turbo pushes additional air mass in.
Is a bigger carburetor always better for power?
Not at all. A carburetor only improves power if airflow is currently the bottleneck. On a mildly built street engine, a carb significantly larger than the calculated CFM will hurt throttle response, fuel economy, and idle quality without adding measurable power. The rest of the induction system - intake manifold, cylinder heads, and camshaft - must support the airflow before a larger carb can help.
What is the difference between a 2-barrel and 4-barrel carburetor?
A 2-barrel carburetor has two throttle bores and is sized for smaller or lower-output engines. A 4-barrel unit has four bores: the primary two are small for efficient part-throttle and idle, while the secondary two open at high throttle to deliver full airflow. The CFM rating covers the total flow of all bores at wide-open throttle, so a 4-barrel 600 CFM and a 2-barrel 300 CFM carb technically flow the same total air - but the 4-barrel delivers much better drivability because it keeps the working bores small at low load.
How does the formula constant 3456 come from?
The number 3456 is derived from two unit conversions. First, 1728 cubic inches equals one cubic foot (12 x 12 x 12 = 1728). Second, a four-stroke engine only takes an intake stroke every two crankshaft revolutions, so actual air consumption per revolution is half the displacement. Combining these: 1728 x 2 = 3456. Dividing displacement (in³) x RPM by 3456 gives airflow in cubic feet per minute.