Selecting the correct carburetor size is crucial for optimal engine performance. An undersized carburetor can restrict airflow, leading to a lack of power, especially at higher RPMs. Conversely, an oversized carburetor can cause poor fuel atomization, hesitation, rough idling, and fuel wastage. This calculator helps estimate an appropriate carburetor venturi diameter based on key engine parameters.
The Math Behind the Calculation
The calculation for carburetor sizing typically involves determining the maximum potential airflow the engine can consume at its peak performance and then deriving the venturi size required to achieve this airflow without excessive restriction.
The core formula we use here estimates the required airflow (in CFM – Cubic Feet per Minute) based on engine displacement, RPM, and volumetric efficiency. Volumetric efficiency represents how effectively the engine fills its cylinders with air-fuel mixture compared to its theoretical maximum.
Estimated Airflow (CFM): CFM = (Engine Displacement (cu. in.) * Max RPM * Volumetric Efficiency) / 3456
*(Note: 3456 is a conversion constant related to cubic inches per cubic foot and engine cycles)*
This CFM value is then used to determine the required venturi diameter. A common method involves using empirical formulas or lookup tables. For simplicity, this calculator uses a formula that relates CFM to venturi diameter, often derived from Bernoulli's principle and empirical testing.
Derived Venturi Diameter (inches):
This calculation often involves iterative processes or lookup tables. A simplified approach can be derived from the relationship between airflow and venturi area. A common approximation or target is based on achieving a certain air velocity at peak flow.
Fuel Delivery Calculation:
The calculator also estimates the required fuel flow based on the desired horsepower and the fuel's properties.
Required Fuel Flow (lbs/hr): Fuel Flow (lbs/hr) = (Desired HP * Fuel-Air Ratio * 454) / (2 * 1000)
*(Note: 454 is conversion from lbs to kg, dividing by 2 for theoretical max fuel burn per HP, and 1000 for efficiency)*
Engine Displacement (cc): Enter your engine's total swept volume in cubic centimeters.
Maximum Engine RPM: Input the highest RPM your engine is designed to safely reach.
Desired Horsepower (HP): Specify the target horsepower for your engine setup.
Volumetric Efficiency (%): This is an estimate of how efficiently your engine breathes. Typical values range from 75% for stock engines to 90%+ for highly tuned ones. 85% is a good starting point.
Fuel-Air Ratio (by mass): The ideal ratio for gasoline combustion is around 14.7:1 (air:fuel). By mass, this is approximately 1/14.7, or 0.067. This is used for fuel flow calculations.
Fuel Density (kg/L): The density of the fuel being used. For gasoline, a common value is around 0.75 kg/L.
Clicking "Calculate Carburetor Size" will provide an estimated venturi diameter in inches, along with required fuel flow rates. This is a guideline, and actual tuning may be required for optimal results.
Use Cases
Engine builders and tuners planning modifications.
Restoration projects requiring period-correct or performance-oriented carburetors.
DIY enthusiasts optimizing their engine's air-fuel delivery.
Diagnosing performance issues related to carburation.
function calculateCarbSize() {
var displacementCC = parseFloat(document.getElementById("engineDisplacement").value);
var maxRPM = parseFloat(document.getElementById("engineRPM").value);
var desiredHP = parseFloat(document.getElementById("desiredHP").value);
var volumetricEfficiencyPercent = parseFloat(document.getElementById("volumetricEfficiency").value);
var fuelAirRatio = parseFloat(document.getElementById("fuelAirRatio").value);
var fuelDensity = parseFloat(document.getElementById("fuelDensity").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
// — Input Validation —
if (isNaN(displacementCC) || displacementCC <= 0) {
resultDiv.innerHTML = "Please enter a valid Engine Displacement (cc).";
return;
}
if (isNaN(maxRPM) || maxRPM <= 0) {
resultDiv.innerHTML = "Please enter a valid Maximum Engine RPM.";
return;
}
if (isNaN(desiredHP) || desiredHP <= 0) {
resultDiv.innerHTML = "Please enter a valid Desired Horsepower (HP).";
return;
}
if (isNaN(volumetricEfficiencyPercent) || volumetricEfficiencyPercent 100) {
resultDiv.innerHTML = "Please enter Volumetric Efficiency between 1-100%.";
return;
}
if (isNaN(fuelAirRatio) || fuelAirRatio <= 0) {
resultDiv.innerHTML = "Please enter a valid Fuel-Air Ratio.";
return;
}
if (isNaN(fuelDensity) || fuelDensity <= 0) {
resultDiv.innerHTML = "Please enter a valid Fuel Density (kg/L).";
return;
}
// — Calculations —
// 1. Convert Displacement from CC to Cubic Inches (cu. in.)
var displacementCI = displacementCC / 16.3871;
// 2. Calculate Estimated Airflow (CFM)
var volumetricEfficiencyFactor = volumetricEfficiencyPercent / 100;
var estimatedCFM = (displacementCI * maxRPM * volumetricEfficiencyFactor) / 3456;
// 3. Estimate Carb Venturi Diameter (inches)
// This is a simplified empirical approximation. Real-world sizing is complex.
// A common target is around 1.5 CFM per cubic inch of displacement at peak torque,
// but we are using max RPM here. A common rule of thumb for carb sizing relates
// CFM to venturi area. A very rough empirical formula for venturi diameter (Dv) in inches:
// Dv ≈ sqrt( (CFM * 4 * K) / (π * V * 60) ) where K is a constant, V is air velocity.
// A more direct empirical approach relates CFM to diameter. A simple approximation:
// CFM = 3.65 * Dv^2 * sqrt(Manifold Vacuum) — This requires manifold vacuum.
// Using another common empirical formula based on HP:
// HP = (CFM * VE * BSFC * 2) / 7.5 where BSFC is Brake Specific Fuel Consumption (often ~0.5 for NA gasoline)
// Let's use a simpler, widely cited rule of thumb for performance:
// Carb CFM needed ≈ (Displacement (cu in) * Max RPM * VE) / 3456
// For a single barrel carb, we divide CFM by number of barrels, then estimate diameter.
// Let's assume a 4-barrel setup for simplicity in estimation, so CFM per barrel.
var cfmPerBarrel = estimatedCFM / 4; // Assuming a 4-barrel carb for estimation
// A common empirical formula to estimate venturi diameter from CFM is needed.
// A very rough approximation based on venturi velocity targets:
// Velocity (ft/sec) ≈ sqrt(2 * 14.7 psi * 144 sq in/sq ft / density) — density is complex
// A more practical approach is empirical tables or simplified formulas like:
// Diameter (inches) ≈ sqrt(CFM * 4 / (π * V * 60)) where V is target velocity in ft/s.
// Target velocity at carb throat is often around 300-400 ft/s for street performance.
var targetVelocityFPS = 350; // Target air velocity in feet per second
var carbDiameterInches = Math.sqrt((cfmPerBarrel * 4 * 1) / (Math.PI * targetVelocityFPS * 60)); // Using 1 as a density/constant factor approximation
carbDiameterInches = carbDiameterInches * 12; // Convert from feet to inches if formula was in feet
// Re-evaluating a simpler direct empirical formula for venturi diameter based on CFM:
// A commonly cited simplified relationship for venturi diameter 'd' in inches for a 4-barrel carb:
// CFM = 130 * d^2 (This is very basic and often for specific RPM ranges)
// Let's use a slightly more refined empirical formula:
// d = sqrt(CFM / (Constant * sqrt(Absolute Pressure / Temperature))) — this gets complex.
// A very common set of empirical formulas for carb sizing:
// CFM = (Displacement * RPM * VE * 1.7) / 2400 for 2-stroke
// CFM = (Displacement * RPM * VE * 1.7) / 3456 for 4-stroke (This is similar to what we have)
// For estimating diameter from CFM, let's use a more robust empirical relationship:
// d = sqrt( (4 * CFM) / (pi * V * 60) ) where V is velocity.
// Let's use a target velocity of 350 ft/s at the venturi for optimal atomization and flow.
var venturiAreaSqFt = cfmPerBarrel / (targetVelocityFPS * 60);
var venturiDiameterFt = Math.sqrt(venturiAreaSqFt / Math.PI) * 2; // Diameter in feet
var estimatedVenturiDiameterInches = venturiDiameterFt * 12; // Convert to inches
// A simpler approach often used: based on horsepower directly.
// Carb CFM = HP * BSFC * 2 / (Specific Energy) — where BSFC varies greatly.
// A simpler rule of thumb: 1 HP requires approx 0.13 CFM for naturally aspirated.
// So, for desired HP: Required CFM = Desired HP * 0.13 * (Some efficiency factor, maybe 1.15 for tuning head room)
// Let's calculate CFM based on HP as a cross-check.
var cfmFromHP = desiredHP * 0.13 * 1.15; // Rule of thumb: 0.13 CFM per HP + 15% headroom
var cfmPerBarrelFromHP = cfmFromHP / 4; // Assuming 4-barrel
var estimatedVenturiDiameterInchesFromHP = Math.sqrt((cfmPerBarrelFromHP * 4 * 1) / (Math.PI * targetVelocityFPS * 60)) * 12;
// We will prioritize the calculation based on engine displacement and RPM as it's more fundamental to airflow potential.
// The HP-based calculation is a good cross-reference.
// 4. Calculate Fuel Flow Requirements
// Required Fuel Flow (lbs/hr) = (Desired HP * BSFC * 454) / (Stoichiometric Air/Fuel Ratio)
// BSFC for gasoline engines is typically 0.4 to 0.7 lbs/hp-hr. Let's use 0.5 as an average.
var bsfc = 0.5; // lbs/hp-hr
var stoichiometricAFR = 14.7; // Air-fuel ratio by mass
// Calculate fuel flow based on desired HP and BSFC
var fuelFlowLbsPerHour_HP = desiredHP * bsfc; // Simplified: HP * BSFC directly gives lbs/hr
// Calculate fuel flow based on required airflow and fuel-air ratio (more accurate for sizing)
// Airflow in lbs/hr = CFM * 60 (min/hr) * Air Density (lbs/cu ft)
// Air density at sea level, 15C ≈ 0.0765 lbs/cu ft
var airDensityLbsPerCuFt = 0.0765;
var airflowLbsPerHour = estimatedCFM * 60 * airDensityLbsPerCuFt;
var fuelFlowLbsPerHour_AFR = airflowLbsPerHour * fuelAirRatio; // Using the provided fuelAirRatio
// We'll use the fuel flow calculated from the engine's airflow potential (fuelFlowLbsPerHour_AFR)
// as it's directly related to the volume of mixture the engine consumes.
// Convert fuel flow from lbs/hr to gallons/hr
// Density of gasoline ≈ 0.75 kg/L. 1 kg ≈ 2.20462 lbs.
// So, density in lbs/L ≈ 0.75 * 2.20462 ≈ 1.653465 lbs/L
// There are 3.78541 liters in a US gallon.
// So, density in lbs/gal ≈ 1.653465 lbs/L * 3.78541 L/gal ≈ 6.257 lbs/gal
var fuelDensityLbsPerGal = fuelDensity * 1000 * 2.20462 / 3.78541; // Convert kg/L to lbs/gal
var fuelFlowGalsPerHour = fuelFlowLbsPerHour_AFR / fuelDensityLbsPerGal;
// Presenting the results
var resultHtml = "
Estimated Carburetor Size:
";
resultHtml += "Estimated Venturi Diameter: " + estimatedVenturiDiameterInches.toFixed(2) + " inches (based on engine airflow)";
resultHtml += "(Diameter based on ~" + cfmPerBarrel.toFixed(2) + " CFM per barrel at " + targetVelocityFPS + " ft/s)";
resultHtml += "Estimated Fuel Flow Needed: " + fuelFlowGalsPerHour.toFixed(2) + " gallons/hour";
resultHtml += "(Based on " + fuelFlowLbsPerHour_AFR.toFixed(2) + " lbs/hr fuel delivery)";
resultHtml += "Note: These are estimations. Actual sizing may vary based on specific engine design, camshaft, intake manifold, exhaust system, and desired performance characteristics.";
resultDiv.innerHTML = resultHtml;
}