ci
Enter engine displacement (cubic inches or liters).
Cubic inches or liters (select unit below)
RPM
Enter max RPM between 1,000 and 12,000.
Highest RPM the engine will reach under load
How efficiently your engine fills cylinders with air/fuel
Affects recommended carb sizing buffer
Calculated CFM Required
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📋 Carburetor Size Recommendations
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Sources & Methodology
CFM formula verified against Holley Performance carburetor tech resources, Edelbrock carburetor sizing guides, and the standard industry CFM calculation used by engine builders.
Holley Performance — Carburetor Tech Center
Industry-standard CFM sizing formula, volumetric efficiency guidance by engine build level, and carburetor selection guides for street and race applications
Edelbrock — Tech Reference and Carburetor Selection
Carburetor CFM recommendations by engine displacement and application, VE benchmarks by cam profile and cylinder head flow
Methodology: CFM = (Displacement in ci x Max RPM x VE) / 3,456.
Liters converted to ci by multiplying by 61.024.
The constant 3,456 = 2 (four-stroke cycle) x 1,728 (ci per cubic foot).
Recommended carb sizes calculated at 90% (street), 100% (street/strip), and 110% (race) of calculated CFM,
then rounded up to the nearest standard carb size (390, 450, 500, 600, 650, 750, 800, 850, 1050 CFM).
⏱ Last reviewed: April 2026
How to Calculate Carburetor Size (CFM)
Selecting the correct carburetor CFM rating is one of the most important decisions in engine building. An undersized carburetor starves the engine of airflow at high RPM, limiting peak power. An oversized carburetor hurts low-speed drivability by reducing air velocity through the venturi, weakening the fuel signal and causing flat spots and poor idle quality.
The CFM Formula
CFM = (Displacement x Max RPM x Volumetric Efficiency) / 3,456
Where:
Displacement = engine size in cubic inches
Max RPM = peak RPM under load
VE = volumetric efficiency as a decimal (0.80 = 80%)
3,456 = 2 (four-stroke factor) x 1,728 (cubic inches per cubic foot)
Example: 350 ci x 5,500 RPM x 0.80 = 1,540,000 / 3,456 = 446 CFM required
Displacement = engine size in cubic inches
Max RPM = peak RPM under load
VE = volumetric efficiency as a decimal (0.80 = 80%)
3,456 = 2 (four-stroke factor) x 1,728 (cubic inches per cubic foot)
Example: 350 ci x 5,500 RPM x 0.80 = 1,540,000 / 3,456 = 446 CFM required
Volumetric Efficiency Guide
Volumetric efficiency is how completely the engine fills each cylinder on the intake stroke. A perfectly efficient engine would be 100% VE. Real engines are limited by intake restriction, valve timing, and cylinder head flow. The VE you use in the formula has the largest impact on the result, so it is important to estimate it correctly for your build.
| Build Level | Typical VE | Description |
|---|---|---|
| Stock / Mild Street | 75-78% | Stock cam, stock heads, stock intake |
| Street Performance | 80-82% | Mild cam, stock or lightly ported heads |
| Performance | 85-88% | Aftermarket cam, ported/polished heads |
| High Performance | 90-93% | Aggressive cam, CNC heads, performance intake |
| Race / All-Out | 95-100% | Full race build, maximum head flow |
| Supercharged / Turbo | 100-115% | Forced induction exceeds naturally aspirated limits |
Standard Carburetor CFM Sizes and Applications
| CFM Rating | Typical Application | Common Engines |
|---|---|---|
| 390 CFM | Small 4-cyl, economy engines | 2.3L-2.8L four cylinders |
| 450 CFM | Small 6-cyl street engines | 3.5L-4.2L inline-six |
| 500 CFM | Mild small block V8 | 302/305 ci street builds |
| 600 CFM | Popular street small block | 350/360 ci mild performance |
| 650 CFM | Mild-performance small block | 350-383 ci performance |
| 750 CFM | Performance big block / stroker | 383-454 ci performance |
| 800 CFM | Big block street/strip | 454-496 ci high performance |
| 850 CFM | Big block race | 496-540 ci race engines |
| 1050 CFM | All-out race engines | 540+ ci race, supercharged |
💡 Pro Tip: When in doubt between two sizes, choose the smaller carburetor for a street engine. An undersized carb can be corrected with jetting and throttle body modifications. An oversized carb causes drivability problems that are difficult to tune around. The Holley rule of thumb is to never exceed 110% of the calculated CFM for any street-driven vehicle.
Liters to Cubic Inches Conversion
To convert engine displacement from liters to cubic inches, multiply liters by 61.024. Common conversions: 2.0L = 122 ci, 3.5L = 214 ci, 4.6L = 281 ci, 5.0L = 305 ci, 5.7L = 350 ci, 6.2L = 378 ci, 7.0L = 427 ci, 7.4L = 454 ci. The calculator handles this conversion automatically when you select liters as the input unit.
Frequently Asked Questions
CFM = (Engine Displacement x Max RPM x Volumetric Efficiency) / 3,456. Displacement is in cubic inches, VE is a decimal (0.80 = 80%). The divisor 3,456 accounts for the four-stroke cycle and unit conversion. Example: 350 ci x 5,500 RPM x 0.80 / 3,456 = 446 CFM required.
A mild street 350 at 5,500 RPM with 80% VE needs about 446 CFM. A 600 CFM carburetor is the most popular choice, providing a comfortable buffer. For a performance 350 revving to 6,500 RPM with 85% VE, the calculated need is 560 CFM, making a 600-650 CFM carb ideal.
Volumetric efficiency is the ratio of air-fuel mixture actually entering the cylinder versus the theoretical maximum. Stock street engines: 75-80%. Performance builds with cam upgrades: 85-90%. Race engines with ported heads: 95-100%. Using too high a VE estimate results in an oversized carburetor with poor street manners.
A carburetor more than 10-20% larger than calculated causes poor throttle response, rough idle, and flat spots. For street use, stay at 90-100% of the calculated CFM. Performance builds can use up to 110%. Going more than 20% over the calculated value typically hurts street drivability significantly due to reduced air velocity through the venturi.
Multiply liters by 61.024 to get cubic inches. Common conversions: 2.0L = 122 ci, 3.5L = 214 ci, 4.6L = 281 ci, 5.0L = 305 ci, 5.7L = 350 ci, 6.2L = 378 ci, 7.0L = 427 ci, 7.4L = 454 ci. The calculator handles this automatically when you select liters as the input unit.
A 454 at 5,500 RPM with 80% VE needs about 578 CFM. A 650-750 CFM carburetor is the typical choice for a mild street 454. For a performance 454 at 6,500 RPM with 85% VE, the requirement is 726 CFM, making a 750-800 CFM carb appropriate.
CFM = (Cubic Inches x RPM x VE) / 3,456. The divisor 3,456 = 2 (four-stroke fires each cylinder once every two revolutions) x 1,728 (cubic inches per cubic foot). This gives the airflow in cubic feet per minute required at the specified RPM and efficiency level.
Not necessarily. A correctly sized carburetor produces the most power for a given engine. An oversized carb reduces air velocity, weakening the fuel signal and causing a lean condition at low RPM. An undersized carb restricts peak airflow at high RPM. The right size maximizes both low-end response and top-end power.
A 383 stroker at 6,000 RPM with 85% VE requires about 566 CFM. Most 383 stroker street builds use a 650-750 CFM carb. For a mild street 383 at 5,500 RPM with 80% VE, the calculated need is 487 CFM, where a 600 CFM carb would be appropriate.
For engines needing over 250 CFM, a 4-barrel carburetor is almost always the better choice. 2-barrel carbs top out around 350-400 CFM and suit only smaller displacement engines. 4-barrel carbs provide better idle quality, fuel economy at cruise (only primaries open), and much higher peak airflow capacity.
Higher compression improves volumetric efficiency by drawing more charge into the cylinder. An engine at 10:1 compression has higher effective VE than the same engine at 8.5:1. When calculating CFM for a high-compression build, increase the estimated VE by 3-5% over a stock compression baseline.
For street/strip use, a vacuum secondary 4-barrel carb works best. Vacuum secondaries open progressively based on engine demand, providing good street manners while still delivering full power at wide-open throttle. Holley 600-750 CFM, Edelbrock 600-750 CFM, and Quick Fuel 650-750 CFM are popular street/strip choices.
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