Optimize your engine's flow for maximum horsepower and torque
Single Exhaust
Dual Exhaust
Naturally Aspirated
Turbo / Supercharged
Mandrel Bent (Smooth)
Crush Bent (Restrictive)
Recommended Specifications:
Understanding Exhaust Pipe Diameter and Gas Velocity
Choosing the correct exhaust pipe size is a delicate balance between flow volume and gas velocity. Many enthusiasts mistakenly believe that "bigger is always better," but an oversized exhaust pipe can actually hurt performance on street-driven vehicles.
Velocity vs. Backpressure
The goal of a performance exhaust is to minimize backpressure while maintaining high exhaust gas velocity. High velocity creates a scavenging effect, where the movement of one exhaust pulse helps pull the next pulse out of the combustion chamber. If the pipe is too large, the gases cool down and slow down, destroying this scavenging effect and reducing low-end torque.
How Diameter is Calculated
The calculation is based on the volume of air an engine must move to produce a specific horsepower figure. Generally, for every 100 HP, an engine requires approximately 200-220 CFM (Cubic Feet per Minute) of exhaust flow.
Single Exhaust: Requires a larger single pipe to handle the combined flow of all cylinders.
Dual Exhaust: Splits the load between two pipes, allowing for smaller individual diameters while providing higher total flow capacity.
Forced Induction: Turbocharged engines benefit from slightly larger diameters because the turbo relies on a pressure differential to spin efficiently.
Practical Examples
Horsepower
Single Pipe (Avg)
Dual Pipe (Avg)
200 HP
2.25″
2.0″
400 HP
3.5″
2.5″
600 HP
4.0″+
3.0″
function calculateExhaust() {
var hp = parseFloat(document.getElementById('targetHP').value);
var config = document.getElementById('exhaustConfig').value;
var aspiration = document.getElementById('aspiration').value;
var material = document.getElementById('material').value;
if (isNaN(hp) || hp <= 0) {
alert("Please enter a valid horsepower figure.");
return;
}
var resultDiv = document.getElementById('exhaustResult');
var output = document.getElementById('diameterOutput');
var note = document.getElementById('logicNote');
// Base calculation logic
// Formula derived from CFM requirements (approx 2.2 CFM per HP)
// Area required = CFM / Velocity
// We use standard industry mappings for pipe sizes
var recommendedDiameter = 0;
var effectiveHP = (config === 'dual') ? (hp / 2) : hp;
// Add margin for forced induction (approx 15% more flow needed)
if (aspiration === 'forced') {
effectiveHP = effectiveHP * 1.25;
}
// Determine diameter based on flow capacity charts
if (effectiveHP <= 150) {
recommendedDiameter = 2.0;
} else if (effectiveHP <= 210) {
recommendedDiameter = 2.25;
} else if (effectiveHP <= 310) {
recommendedDiameter = 2.5;
} else if (effectiveHP <= 420) {
recommendedDiameter = 3.0;
} else if (effectiveHP <= 580) {
recommendedDiameter = 3.5;
} else {
recommendedDiameter = 4.0;
}
// Adjustment for crush bending (needs larger diameter to compensate for restrictions)
if (material === 'crush') {
recommendedDiameter += 0.25;
}
var configText = (config === 'dual') ? "Dual Pipes at " : "Single Pipe at ";
output.innerHTML = configText + recommendedDiameter.toFixed(2) + " inches O.D.";
var noteText = "This setup provides optimal flow for " + hp + " BHP. ";
if (material === 'crush') {
noteText += "Note: Crush bending reduces internal diameter at curves, requiring a larger starting size.";
} else {
noteText += "Mandrel bending maintains a constant internal diameter, offering superior flow characteristics.";
}
note.innerHTML = noteText;
resultDiv.style.display = 'block';
}