Coolant Swirl Pot vs Header Tank – What’s the Difference and Which Do You Need?

When upgrading or modifying a performance car, cooling reliability is just as important as power. Two components often discussed are the coolant swirl pot and the header tank. Although they’re sometimes confused or used interchangeably, they serve different purposes.

In this journal entry, we’ll break down the differences and explain which one makes sense for your setup.

What Is a Coolant Swirl Pot?

A coolant swirl pot is designed to remove air from the cooling system. Air pockets are a common cause of overheating, inconsistent coolant temperatures, and poor heater performance.

A properly designed swirl pot:

Encourages air bubbles to rise to the top

Returns deaerated coolant back into the system

Provides a stable point for bleeding the system

This makes swirl pots especially popular in:

Track cars

Motorsport applications

Engine swaps

High-revving or turbocharged setups

At The Alloy Shed, our swirl pots are hand-fabricated from aluminium, TIG welded, and designed to be compact, functional, and durable under hard use.

What Is a Header Tank?

A header tank (also known as an expansion tank) manages thermal expansion of coolant as engine temperature rises.

Its main roles are:

Allowing coolant to expand and contract

Maintaining system pressure

Feeding coolant back into the system as it cools

Header tanks are typically positioned higher than the radiator and are common in OEM setups as well as road-going performance cars.

Unlike a swirl pot, a header tank does not actively remove air, unless paired with specific hose routing or bleed lines.

A header tank relies on gravity to collect air, while a swirl pot uses controlled coolant flow and centrifugal forces to actively separate and remove air.

Do You Need Both?

In some setups — especially race or hillclimb cars — you may use both a swirl pot and a header tank.

The swirl pot handles air separation

The header tank manages expansion volume

For most track-focused builds, a swirl pot alone is enough when installed correctly. For road cars, a header tank may be sufficient.

Why Custom Aluminium Makes Sense

Plastic tanks crack, discolour, and fail over time — especially around the neck and hose outlets.

All Alloy Shed tanks are:

Hand-built in the UK

TIG welded from aluminium

Pressure-tested

Designed with proper hose engagement and beading

Built to be functional first, not just shiny

Every weld and detail is made in a small workshop — not mass-produced overseas.

Final Thoughts

Choosing between a swirl pot and a header tank depends on how the car is used, not just what looks good in the engine bay.

If you’re building a serious performance or motorsport car, a properly designed coolant swirl pot can make a real difference to cooling stability and reliability.

Interested in a Custom Swirl Pot or Header Tank?

Take a look at our latest builds or get in touch to discuss your setup.

Every tank from The Alloy Shed is built with the same goal: reliability, quality, and no compromises.