Carburetor Size Calculator

Carburetor Size Formula:

\[ CFM = \frac{\text{Engine Displacement} \times RPM \times \text{Volumetric Efficiency}}{3456} \]

Carburetor Size (CFM):

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1. What is the Carburetor Size Formula?

The carburetor size formula calculates the required airflow capacity (in cubic feet per minute - CFM) for an engine based on its displacement, operating RPM, and volumetric efficiency. This helps select the properly sized carburetor for optimal engine performance.

2. How Does the Calculator Work?

The calculator uses the carburetor size formula:

\[ CFM = \frac{\text{Engine Displacement} \times RPM \times \text{Volumetric Efficiency}}{3456} \]

Where:

Engine Displacement — Total cylinder volume in cubic inches
RPM — Maximum engine speed in revolutions per minute
Volumetric Efficiency — Engine's air pumping efficiency (typically 80-90% for street engines)
3456 — Conversion factor to get CFM

Explanation: The formula calculates the volume of air the engine needs at maximum RPM, adjusted for how efficiently it can fill its cylinders.

3. Importance of Proper Carburetor Sizing

Details: Correct carburetor sizing is crucial for engine performance. An oversized carburetor can cause poor low-end torque and throttle response, while an undersized carburetor can restrict top-end power.

4. Using the Calculator

Tips: Enter engine displacement in cubic inches, maximum RPM, and volumetric efficiency (85% is typical for street engines). All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What's a typical volumetric efficiency value?
A: Street engines: 80-90%, performance engines: 95-100%, race engines: 100-110% (with forced induction).

Q2: Should I round up or down when selecting a carburetor?
A: Generally round up to the nearest available size, but consider your application - street use can often use slightly smaller carbs for better drivability.

Q3: How does altitude affect carburetor sizing?
A: Higher altitudes require less CFM as air is less dense. Reduce CFM by about 3% per 1000 feet above sea level.

Q4: What about multiple carburetors?
A: Divide the total CFM by the number of carburetors you plan to use (e.g., two 300 CFM carbs instead of one 600 CFM).

Q5: How does this differ for fuel injection?
A: Fuel injection systems use different sizing calculations as they're not limited by venturi principles like carburetors.