# Exhaust Diameter Calculator (Header Primary Tube)

URL: https://speedycalculator.com/calculators/physics/exhaust-diameter/
Last updated: 2026-10-30

> The exhaust diameter calculator estimates the inside and outside diameter of a header primary tube (exhaust pipe per cylinder) for a four-stroke engine from its displacement, number of cylinders and the rpm of peak torque.

## Inputs

- Engine displacement (L, in³ (CID), cm³ (cc))
- Number of cylinders
- Peak torque rpm
- Tube wall thickness (in, mm, cm, ft, m)

## Results

- Cylinder volume
- Required cross-section per cylinder
- Primary tube inside diameter
- Primary tube outside diameter

## Formula

```
Pipe cross-section (in²) = V_cyl (in³) × RPM / 88,200,  ID = √(4·area/π),  OD = ID + 2 × wall
```

## Explanation

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

FieldWhat to enterEngine displacementUnits: L, in³ (CID), cm³ (cc)Number of cylindersA numberPeak torque rpmA numberTube wall thicknessUnits: 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)2.851.1884.281.4555.71.6817.131.8798.552.05811.42.377

## 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.

## FAQ

**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.


## Sources

- [OpenStax – University Physics Volume 1](https://openstax.org/details/books/university-physics-volume-1)
- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)