The RPM at which you want to calculate maximum flow.
~80-90% for stock NA, 95-105% for tuned NA, >110% for Turbo/Supercharged.
Estimated air temperature entering the engine.
Estimated EGT. ~1200-1400°F for NA, ~1500-1600°F for Turbo.
Calculation Results
Intake Airflow (Cold):–
Expansion Ratio:–
Exhaust Flow Rate (Hot):–
Min. Suggested Pipe Diameter (Single):–
Min. Suggested Pipe Diameter (Dual):–
function calculateExhaustFlow() {
// Get Inputs
var liters = parseFloat(document.getElementById("engineDisplacement").value);
var rpm = parseFloat(document.getElementById("maxRPM").value);
var ve = parseFloat(document.getElementById("volumetricEff").value);
var intakeT = parseFloat(document.getElementById("intakeTemp").value);
var exhaustT = parseFloat(document.getElementById("exhaustTemp").value);
// Validation
if (isNaN(liters) || isNaN(rpm) || isNaN(ve) || isNaN(intakeT) || isNaN(exhaustT)) {
alert("Please enter valid numbers for all fields.");
return;
}
// 1. Convert Liters to Cubic Inches (CID)
// 1 Liter = 61.0237 CID
var cid = liters * 61.0237;
// 2. Calculate Base Intake CFM (Cubic Feet per Minute)
// Formula: (CID * RPM) / 3456 * (VE / 100)
// 3456 comes from converting RPM to strokes (RPM/2) and Inches to Feet (1728).
// Standard simplified is CID * RPM / 3456.
var baseCFM = (cid * rpm) / 3456 * (ve / 100);
// 3. Calculate Temperature Correction (Charles's Law approximation)
// Use Rankine scale: F + 460
var intakeRankine = intakeT + 460;
var exhaustRankine = exhaustT + 460;
var expansionRatio = exhaustRankine / intakeRankine;
// 4. Calculate Actual Exhaust Flow (Hot CFM)
var exhaustCFM = baseCFM * expansionRatio;
// 5. Estimate Pipe Sizing
// A common rule of thumb for street/performance is maintaining ~200-250 ft/s velocity
// OR simpler: ~2.2 CFM per square inch of cross-sectional area for zero restriction is too low.
// A common tuning metric: 115 CFM per square inch of area is a safe limit for flow.
// Let's use the flow limit formula: Required Area (sq in) = Exhaust CFM / 115
var requiredAreaSingle = exhaustCFM / 115;
// Diameter = 2 * sqrt(Area / PI)
var singleDiameter = 2 * Math.sqrt(requiredAreaSingle / Math.PI);
// For dual exhaust, split the CFM in half
var requiredAreaDual = (exhaustCFM / 2) / 115;
var dualDiameter = 2 * Math.sqrt(requiredAreaDual / Math.PI);
// Display Results
document.getElementById("resIntakeCFM").innerText = Math.round(baseCFM) + " CFM";
document.getElementById("resExpansion").innerText = expansionRatio.toFixed(2) + "x";
document.getElementById("resExhaustCFM").innerText = Math.round(exhaustCFM) + " CFM";
document.getElementById("resSinglePipe").innerText = singleDiameter.toFixed(1) + " Inches";
document.getElementById("resDualPipe").innerText = dualDiameter.toFixed(1) + " Inches";
// Show result box
document.getElementById("resultsArea").style.display = "block";
}
Understanding Engine Exhaust Flow Rate
Calculating the exhaust flow rate is a critical step in automotive performance tuning, particularly when sizing exhaust systems, selecting turbocharger turbine housings, or designing emissions control systems. While intake airflow determines potential horsepower, the exhaust system must be capable of evacuating the expanded hot gases efficiently to prevent backpressure, which robs the engine of power.
Why Hot Flow Matters: Exhaust gas is significantly hotter than intake air. According to the Ideal Gas Law, as temperature rises, volume expands. This means your engine must push out a volume of gas several times larger than what it sucked in.
How the Calculation Works
This calculator uses a multi-step physics-based approach to estimate your required exhaust flow:
Displacement Conversion: Converts engine size from Liters to Cubic Inches (CID).
Base Intake Volume: Uses the standard formula (CID × RPM) / 3456 adjusted for Volumetric Efficiency (VE) to find how much air enters the engine.
Thermal Expansion: Applies Charles's Law using the ratio of absolute temperatures (Rankine scale) between the intake air and the exhaust gas.
Estimating Volumetric Efficiency (VE)
Volumetric Efficiency represents how effectively the cylinders fill with air compared to their static volume. Use these guidelines for the calculator:
Stock Economy Engines: 75% – 85%
Performance Street Engines: 85% – 95%
Race Engines (Naturally Aspirated): 95% – 110%
Forced Induction (Turbo/Supercharger): 110% – 250%+ (Enter the effective VE, which roughly equals boost pressure ratio × engine VE).
Exhaust Pipe Sizing Guide
Once you know your Exhaust CFM, sizing the piping correctly is the next step. If the pipe is too small, backpressure increases, choking the engine at high RPM. If the pipe is too large, exhaust gas velocity drops, reducing the scavenging effect which creates low-end torque.
The recommendation provided by this calculator is based on a flow limit of approximately 115 CFM per square inch of cross-sectional pipe area. This generally maintains a gas velocity conducive to high performance without inducing restrictive backpressure.