Exhaust Flow Rate Calculator
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Exhaust Flow Rate Calculator
Calculated Results
Required Flow Rate:
– CFM
Metric Flow Rate:
– m³/min
Recommended Pipe Diameter (OD):
– inches
Exhaust Velocity Estimate:
– ft/s
Note: Pipe diameter recommendation is based on maintaining an exhaust gas velocity between 200-250 ft/s for optimal scavenging without excessive backpressure.
Understanding Exhaust Flow Rate
Calculating the exhaust flow rate is critical for designing an efficient exhaust system. If the tubing is too small, backpressure increases, robbing the engine of horsepower and increasing heat. If the tubing is too large, exhaust gas velocity drops, reducing the scavenging effect which is vital for low-end torque.
The Formula
The core formula for calculating the required Cubic Feet per Minute (CFM) of airflow for a 4-stroke engine is:
CFM = (RPM × CID × VE) / 3456
Where:
- RPM: Revolutions Per Minute (usually peak power RPM).
- CID: Cubic Inch Displacement.
- VE: Volumetric Efficiency (0.80 for street, 0.90+ for race).
- 3456: Conversion constant for 4-stroke engines.
Forced Induction Calculation
For turbocharged or supercharged engines, the air density changes significantly. The calculator adjusts the base CFM using the pressure ratio:
Pressure Ratio = (14.7 + Boost PSI) / 14.7
The final flow rate becomes: Base CFM × Pressure Ratio.
Pipe Sizing Guide
Once you know your CFM, you can select the appropriate pipe diameter. A general rule of thumb for street applications is to maintain a flow velocity of roughly 200 to 250 feet per second.
| Pipe Diameter (OD) |
Approx Max HP (Single) |
Approx Max HP (Dual) |
| 2.0″ |
~150 HP |
~300 HP |
| 2.25″ |
~200 HP |
~400 HP |
| 2.5″ |
~250 HP |
~500 HP |
| 3.0″ |
~350-400 HP |
~700-800 HP |
| 3.5″ |
~500-550 HP |
~1000+ HP |
| 4.0″ |
~700+ HP |
~1400+ HP |
Why Volumetric Efficiency Matters
Volumetric Efficiency (VE) represents how well the engine fills its cylinders with air compared to its static displacement. A standard street car might have a VE of 75-80%. A highly tuned naturally aspirated engine might reach 95-105%. Forced induction artificially increases VE above 100% relative to atmospheric pressure.
function toggleBoostInput() {
var type = document.getElementById('inductionType').value;
var group = document.getElementById('boostGroup');
if (type === 'fi') {
group.style.display = 'block';
} else {
group.style.display = 'none';
}
}
function calculateFlow() {
// 1. Get Inputs
var dispVal = parseFloat(document.getElementById('displacementVal').value);
var dispUnit = document.getElementById('displacementUnit').value;
var rpm = parseFloat(document.getElementById('maxRpm').value);
var ve = parseFloat(document.getElementById('volEff').value);
var induction = document.getElementById('inductionType').value;
var boost = parseFloat(document.getElementById('boostPsi').value);
var config = document.getElementById('exhaustConfig').value;
// Validation
if (isNaN(dispVal) || isNaN(rpm) || isNaN(ve)) {
alert("Please enter valid numbers for Displacement, RPM, and VE.");
return;
}
// 2. Convert Displacement to Cubic Inches (CID)
var cid = 0;
if (dispUnit === 'liters') {
cid = dispVal * 61.0237;
} else if (dispUnit === 'cc') {
cid = dispVal * 0.0610237;
} else {
cid = dispVal; // already CID
}
// 3. Base CFM Calculation (NA)
// Formula: (CID * RPM * VE) / 3456
// Note: VE input is percentage, so divide by 100
var baseCfm = (cid * rpm * (ve / 100)) / 3456;
// 4. Handle Forced Induction
var finalCfm = baseCfm;
if (induction === 'fi') {
if (isNaN(boost)) boost = 0;
// Pressure Ratio = (Atmospheric + Boost) / Atmospheric
// Standard Atmosphere = 14.7 psi
var pressureRatio = (14.7 + boost) / 14.7;
finalCfm = baseCfm * pressureRatio;
}
// 5. Calculate Metric Flow (m3/min)
// 1 CFM = 0.0283168 m3/min
var metricFlow = finalCfm * 0.0283168;
// 6. Calculate Recommended Pipe Diameter
// Logic: Target velocity ~250 ft/s (Street/Performance mix)
// Q (Flow in CFS) = CFM / 60
// Area = Q / Velocity
// Diameter = 2 * sqrt(Area / pi)
// Convert to inches
// We calculate per pipe. If dual exhaust, split flow in half.
var flowPerPipe = (config === 'dual') ? finalCfm / 2 : finalCfm;
var targetVelocity = 250; // ft/sec
var flowCFS = flowPerPipe / 60; // Cubic feet per second
var areaSqFt = flowCFS / targetVelocity;
var areaSqIn = areaSqFt * 144;
// Diameter = 2 * sqrt(A/pi)
var idealDiameter = 2 * Math.sqrt(areaSqIn / Math.PI);
// Round to nearest quarter inch for realistic pipe sizes, but minimum 2.0
// However, let's show the raw calculated ideal min diameter first, then maybe round text
var displayDiameter = Math.ceil(idealDiameter * 4) / 4;
if (displayDiameter < 2.0) displayDiameter = 2.0;
// Calculate actual velocity with recommended pipe
// Area = pi * r^2
var radiusFt = (displayDiameter / 2) / 12;
var actualAreaSqFt = Math.PI * Math.pow(radiusFt, 2);
var actualVelocity = flowCFS / actualAreaSqFt;
// 7. Update UI
document.getElementById('resCfm').innerHTML = Math.round(finalCfm) + " CFM";
document.getElementById('resCms').innerHTML = metricFlow.toFixed(2) + " m³/min";
var configText = (config === 'dual') ? " (Dual)" : " (Single)";
document.getElementById('resPipe').innerHTML = displayDiameter.toFixed(2) + '"' + configText;
document.getElementById('resVelocity').innerHTML = Math.round(actualVelocity) + " ft/s";
}