Each cylinder’s exhaust pulse needs a pipe big enough to flow freely but small enough to keep gas velocity high for good scavenging. This widely used hot-rodding rule ties the pipe area to the cylinder volume and the rpm where you want peak torque.
How to use the Exhaust Diameter Calculator
- Engine displacement
- Number of cylinders
- Peak torque rpm
- Tube wall thickness
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the engine displacement, the number of cylinders, the rpm of peak torque and the tube wall thickness (16 gauge = 0.065 in).
Inputs explained
| Field | What to enter |
|---|---|
| Engine displacement | Units: L, in³ (CID), cm³ (cc) |
| Number of cylinders | A number |
| Peak torque rpm | A number |
| Tube wall thickness | Units: in, mm, cm, ft, m |
What each result means
- Cylinder volume = [Engine displacement]÷[Number of cylinders]
- Required cross-section per cylinder = [Engine displacement]÷[Number of cylinders]×[Peak torque rpm]÷88200
- Primary tube inside diameter = sqrt(4×[Engine displacement]÷[Number of cylinders]×[Peak torque rpm]÷88200÷pi)×0.0254
- Primary tube outside diameter = sqrt(4×[Engine displacement]÷[Number of cylinders]×[Peak torque rpm]÷88200÷pi)×0.0254 + 2×[Tube wall thickness]
Example values
How the main result changes when Engine displacement changes and everything else stays at the example values above:
| Engine displacement (L) | Primary tube inside diameter (in) |
|---|---|
| 2.85 | 1.188 |
| 4.28 | 1.455 |
| 5.7 | 1.681 |
| 7.13 | 1.879 |
| 8.55 | 2.058 |
| 11.4 | 2.377 |
Formula
Pipe cross-section (in²) = V_cyl (in³) × RPM / 88,200, ID = √(4·area/π), OD = ID + 2 × wall
Worked example
Inputs — Engine displacement: 5.7 L; Number of cylinders: 8; Peak torque rpm: 4,500; Tube wall thickness: 0.065 in.
Results — Cylinder volume = 43.479 in³; Required cross-section per cylinder = 2.218 in²; Primary tube inside diameter = 1.681 in; Primary tube outside diameter = 1.811 in.
Key terms
- Header primary
- The individual pipe from each exhaust port to the collector.
Frequently asked questions
Why use peak-torque rpm?
The tube is sized to keep gas velocity high where you want the most torque.
Is a bigger pipe always better?
No – oversized pipes slow the gas and cost low-rpm torque.
What wall thickness is common?
16 gauge (0.065 in / 1.65 mm) for mild steel headers.
Does this size the collector or tailpipe?
No – only the primary tube for each cylinder.