🚗 Carburetor CFM Calculator
Calculate required carburetor CFM for your engine
Total engine displacement in cubic inches
Maximum engine RPM
Typical: 75-85% for stock, 85-95% for modified, 95-100%+ for race engines
How to Use This Calculator
Enter Engine Displacement
Input your engine's total displacement in cubic inches (CID). This is the total volume swept by all pistons. Common values: 302 CID (5.0L), 350 CID (5.7L), 454 CID (7.4L), etc.
Enter Maximum RPM
Enter the maximum RPM where you want peak performance. This is typically your engine's redline or maximum operating RPM. Street engines: 5000-6000 RPM, performance: 6000-7000 RPM, race: 7000+ RPM.
Enter Volumetric Efficiency
Input the volumetric efficiency as a percentage. Stock engines: 75-85%, mildly modified: 85-90%, well-built performance: 90-95%, race engines: 95-100%+. Default is 85% for typical street engines.
Calculate Required CFM
Click "Calculate CFM" to get the required carburetor CFM rating. Round up to the nearest standard carburetor size. Common sizes: 600, 650, 700, 750, 800, 850, 950 CFM.
Formula
CFM = (CID × RPM × VE) / 6912
Where:
- CFM = Cubic Feet per Minute (required airflow)
- CID = Engine Displacement (Cubic Inch Displacement)
- RPM = Maximum Revolutions Per Minute
- VE = Volumetric Efficiency (as percentage, e.g., 85 for 85%)
- 6912 = Constant (3456 × 2, accounts for 4-stroke cycle)
Example Calculation:
For a 350 CID engine, 6000 RPM, 85% volumetric efficiency:
CFM = (350 × 6000 × 85) / 6912
CFM = 178,500,000 / 6912
CFM = 25,832
CFM ≈ 600 (round up to nearest standard size)
For a 454 CID engine, 5500 RPM, 90% VE:
CFM = (454 × 5500 × 90) / 6912 = 670 CFM
Note: This formula is for 4-stroke engines. The 6912 constant accounts for the conversion from cubic inches per minute to CFM and the fact that only half the cycles are intake strokes in a 4-stroke engine. Always round up to the nearest standard carburetor CFM rating.
About Carburetor CFM Calculator
The Carburetor CFM Calculator determines the required airflow capacity (in cubic feet per minute) for a carburetor based on engine displacement, maximum RPM, and volumetric efficiency. Selecting the correct CFM rating is crucial for optimal engine performance - too small restricts airflow and limits power, while too large can cause poor low-end response and fuel economy issues.
When to Use This Calculator
- Carburetor Selection: Choose the right CFM rating when buying a new carburetor
- Engine Modifications: Recalculate CFM requirements after engine upgrades or modifications
- Performance Tuning: Verify if your current carburetor is properly sized for your engine
- Engine Building: Plan carburetor requirements during engine design and build
- Restoration Projects: Determine correct carburetor size for vintage engine restorations
Why Use Our Calculator?
- ✅ Accurate Formula: Uses the standard carburetor sizing formula for 4-stroke engines
- ✅ Volumetric Efficiency: Accounts for engine efficiency in the calculation
- ✅ Quick Sizing: Instantly determine required carburetor CFM
- ✅ Step-by-Step Display: Shows the complete calculation process
- ✅ Free Tool: No registration required, works on all devices
Understanding Carburetor Sizing
Selecting the correct carburetor CFM is critical for engine performance. A carburetor that's too small will restrict airflow at high RPM, limiting horsepower. A carburetor that's too large will provide poor low-end throttle response, reduced fuel economy, and potential stalling issues. The general rule is to choose a carburetor that matches or slightly exceeds the calculated CFM requirement, rounding up to the nearest standard size.
Common Applications
Street Performance: Street-driven vehicles typically use carburetors sized at 600-750 CFM for small-block engines (302-350 CID) and 750-850 CFM for big-block engines (400+ CID).
Race Engines: High-performance race engines with increased RPM limits and higher volumetric efficiency may require 850-1000+ CFM carburetors for optimal power output.
Restoration: Classic car restorers use this calculator to match original equipment specifications or upgrade to modern carburetors with appropriate sizing.
Tips for Best Results
- Always round up to the nearest standard carburetor CFM rating (600, 650, 700, 750, 800, 850, 950 CFM)
- For street use, slightly larger is better than too small - provides headroom for future modifications
- Be honest about your volumetric efficiency - stock engines are typically 75-85%, not 100%
- Consider your driving style - if you rarely hit max RPM, you may size slightly smaller
- For dual quad (two 4-barrel) setups, divide total CFM requirement by 2 per carburetor
- Consult with engine builders or carburetor manufacturers for critical applications
Frequently Asked Questions
What happens if I use a carburetor that's too small?
A carburetor that's too small will restrict airflow at higher RPM, limiting top-end horsepower. The engine may feel strong at low RPM but will "run out of breath" at higher speeds. You'll lose power and the engine may not reach its potential RPM range.
What happens if I use a carburetor that's too large?
An oversized carburetor can cause poor low-end throttle response, reduced fuel economy, hesitation, stalling, and a "bog" when you step on the throttle. The large venturis can't maintain proper air velocity at low engine speeds, leading to poor fuel atomization and mixture quality.
How do I determine my engine's volumetric efficiency?
Volumetric efficiency varies by engine type and modifications. Stock engines: 75-85%, mildly modified (headers, intake, cam): 85-90%, well-built performance engines: 90-95%, race engines with optimized everything: 95-100%+. When in doubt, use 85% for a conservative estimate or consult with your engine builder.
Does this work for fuel injection too?
The CFM concept applies to airflow requirements, but fuel injection systems are rated differently (pounds per hour or liters per hour of fuel flow). However, the airflow requirement calculation is similar. For fuel injection, you'd use this to determine throttle body size requirements.
Should I round up or down on CFM?
Always round up to the nearest standard carburetor size. It's better to have slightly more CFM capacity than needed (within reason) than to restrict your engine. However, don't go more than 100-150 CFM larger than calculated for street use, as this can hurt low-end performance.