Carburettor Size Calculator

Use this calculator to determine the optimal carburettor size (in Cubic Feet per Minute – CFM) for your engine based on its displacement, maximum RPM, and volumetric efficiency.

Engine Displacement (Cubic Inches): Maximum Engine RPM: Volumetric Efficiency (%):

function calculateCarburetorCFM() { var displacement = parseFloat(document.getElementById('engineDisplacement').value); var rpm = parseFloat(document.getElementById('maxRPM').value); var efficiency = parseFloat(document.getElementById('volumetricEfficiency').value); if (isNaN(displacement) || isNaN(rpm) || isNaN(efficiency) || displacement <= 0 || rpm <= 0 || efficiency <= 0) { document.getElementById('carburetorCFMResult').innerHTML = 'Please enter valid positive numbers for all fields.'; return; } // Convert volumetric efficiency from percentage to decimal var efficiencyDecimal = efficiency / 100; // CFM = (Engine Displacement in CID * Max RPM * Volumetric Efficiency) / 3456 // The constant 3456 is derived from: // 1728 (cubic inches per cubic foot) * 2 (revolutions per power stroke for a 4-stroke engine) var cfm = (displacement * rpm * efficiencyDecimal) / 3456; document.getElementById('carburetorCFMResult').innerHTML = '

Calculated Carburettor Size:

' + 'Based on your inputs, the estimated optimal carburettor size is:' + '' + cfm.toFixed(2) + ' CFM' + 'This value represents the Cubic Feet per Minute of airflow required for your engine at its maximum RPM.'; } .carburetor-size-calculator { font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif; background-color: #f9f9f9; padding: 20px; border-radius: 8px; box-shadow: 0 2px 10px rgba(0, 0, 0, 0.1); max-width: 600px; margin: 20px auto; border: 1px solid #ddd; } .carburetor-size-calculator h2 { color: #333; text-align: center; margin-bottom: 20px; font-size: 24px; } .carburetor-size-calculator p { color: #555; line-height: 1.6; margin-bottom: 15px; } .calculator-inputs label { display: block; margin-bottom: 8px; color: #333; font-weight: bold; } .calculator-inputs input[type="number"] { width: calc(100% – 22px); padding: 10px; margin-bottom: 15px; border: 1px solid #ccc; border-radius: 5px; font-size: 16px; } .calculator-inputs button { background-color: #007bff; color: white; padding: 12px 20px; border: none; border-radius: 5px; cursor: pointer; font-size: 18px; width: 100%; transition: background-color 0.3s ease; } .calculator-inputs button:hover { background-color: #0056b3; } .calculator-result { margin-top: 25px; padding: 15px; background-color: #e9f7ef; border: 1px solid #d4edda; border-radius: 8px; text-align: center; } .calculator-result h3 { color: #28a745; margin-top: 0; font-size: 20px; } .calculator-result .result-value { font-size: 28px; color: #007bff; font-weight: bold; margin: 10px 0; } .calculator-result p { margin-bottom: 5px; color: #333; }

Understanding Carburettor Sizing for Optimal Engine Performance

A carburettor is a crucial component in internal combustion engines, responsible for mixing air and fuel in the correct proportions before the mixture enters the engine cylinders. Proper carburettor sizing is paramount for an engine to achieve its full potential, whether it's for daily driving, racing, or heavy-duty applications.

Why Carburettor Size Matters

The size of a carburettor is measured in Cubic Feet per Minute (CFM), indicating the maximum volume of air it can flow. An incorrectly sized carburettor can lead to a host of performance issues:

Too Small: A carburettor that is too small will restrict airflow, effectively "starving" the engine at higher RPMs. This results in a significant loss of horsepower and torque, especially at wide-open throttle. The engine will feel sluggish and unable to breathe properly.
Too Large: Conversely, a carburettor that is too large can lead to poor throttle response, especially at lower RPMs. The air velocity through the venturi (the narrowest part of the carburettor) will be too low to create sufficient vacuum to draw fuel effectively from the main jets. This often results in a rich condition, poor fuel economy, hesitation, and a "bogging" sensation.

The goal is to match the carburettor's airflow capacity to the engine's maximum airflow demand, ensuring efficient fuel atomization and optimal power delivery across the engine's operating range.

Key Factors in Carburettor Sizing

The calculator above uses three primary factors to determine the ideal carburettor size:

1. Engine Displacement (Cubic Inches – CID)

This refers to the total volume displaced by all the pistons in an engine's cylinders during one complete revolution. A larger displacement engine naturally requires more air to fill its cylinders, thus demanding a higher CFM carburettor. While often measured in liters or cubic centimeters, for carburettor sizing, cubic inches (CID) is the standard unit.

2. Maximum Engine RPM (Revolutions Per Minute)

This is the highest rotational speed at which you expect your engine to operate. The faster an engine spins, the more frequently its cylinders need to be filled with air and fuel. Therefore, an engine designed to rev higher will require a carburettor capable of flowing more air per minute.

3. Volumetric Efficiency (%)

Volumetric efficiency (VE) is a critical metric that describes how effectively an engine fills its cylinders with air/fuel mixture compared to its theoretical maximum. It's expressed as a percentage:

100% VE: Means the engine is filling its cylinders completely with air at atmospheric pressure.
Below 100% VE: Common for naturally aspirated engines, as intake restrictions, valve timing, and exhaust backpressure prevent a full fill. A stock street engine might have a VE of 75-85%.
Above 100% VE: Achievable with forced induction (turbochargers, superchargers) or highly optimized naturally aspirated engines (e.g., race engines with tuned intake/exhaust systems and aggressive cam profiles) that can create a ram-air effect.

Accurately estimating your engine's volumetric efficiency is crucial for precise carburettor sizing. If you're unsure, common estimates are:

Stock/Mild Street Engine: 75-85%
Performance Street Engine: 85-90%
Mild Race Engine: 90-95%
Dedicated Race Engine: 95-105%+ (depending on setup)

The Carburettor CFM Formula Explained

The calculator uses the widely accepted formula for estimating carburettor CFM for a 4-stroke engine:

CFM = (Engine Displacement in CID * Max RPM * Volumetric Efficiency) / 3456

Let's break down the constant 3456:

1728: This is the number of cubic inches in one cubic foot. We need this to convert engine displacement from cubic inches to cubic feet.
2: For a 4-stroke engine, it takes two complete revolutions of the crankshaft for each cylinder to complete one full combustion cycle (intake, compression, power, exhaust). This means the engine only "breathes" once every two revolutions.

So, 1728 * 2 = 3456. This constant effectively converts the engine's air demand per minute into cubic feet per minute.

How to Use the Calculator

Enter Engine Displacement: Input your engine's total displacement in cubic inches (e.g., 350 for a small block Chevy).
Enter Maximum Engine RPM: Input the highest RPM you expect your engine to reach under full load (e.g., 6000 RPM).
Enter Volumetric Efficiency: Input your estimated volumetric efficiency as a percentage (e.g., 85 for a mild street engine).
Click "Calculate Carburettor CFM": The calculator will instantly provide the recommended CFM.

Example Calculation

Let's say you have a 350 CID engine that you want to rev to 6000 RPM, and you estimate its volumetric efficiency at 85%.

Engine Displacement (CID): 350
Maximum Engine RPM: 6000
Volumetric Efficiency (%): 85 (or 0.85 as a decimal)

CFM = (350 * 6000 * 0.85) / 3456

CFM = 1,785,000 / 3456

CFM ≈ 516.49

In this scenario, a carburettor around 500-550 CFM would be a good starting point.

Important Considerations Beyond the Formula

While this calculator provides an excellent starting point, several other factors can influence the final carburettor choice:

Intended Use: A street car might benefit from a slightly smaller carburettor for better low-end throttle response and fuel economy, while a race car might tolerate a slightly larger one for maximum top-end power.
Camshaft Profile: Aggressive camshafts can significantly alter an engine's volumetric efficiency and RPM range, requiring adjustments to the calculated CFM.
Intake Manifold Design: Single-plane vs. dual-plane manifolds affect air velocity and distribution, which can influence optimal carburettor size.
Altitude: Engines operating at higher altitudes experience lower atmospheric pressure, reducing the actual air density. This effectively lowers the engine's volumetric efficiency, and a slightly smaller carburettor might be more appropriate.
Fuel Type: While not directly affecting CFM, the type of fuel (gasoline, E85, methanol) can influence carburettor tuning and jetting.
Personal Preference: Some builders prefer a slightly undersized carburettor for crisp street manners, while others lean towards slightly oversized for maximum potential on the track.

Always use the calculated CFM as a guideline. Fine-tuning and real-world testing (e.g., dyno runs, track performance) are often necessary to achieve truly optimal results.