Swirl Pot Sizing: Why Capacity Matters Less Than You Think

When choosing a coolant swirl pot, capacity is often the first specification people focus on. Bigger is assumed to be better. More volume must mean better cooling.

In reality, capacity is one of the least important factors in how a swirl pot performs.

A swirl pot’s effectiveness comes from how it is designed and integrated into the cooling system — not from how much coolant it can hold.

What a Swirl Pot Actually Does

A swirl pot has a very specific role within a cooling system:

Separate air from circulating coolant

Maintain a stable, air-free feed back into the system

Provide a reliable high point for bleeding

It is not a storage tank and not an expansion reservoir. Once air is consistently removed from the system, additional capacity offers diminishing returns.

Why Bigger Isn’t Better

Oversizing a swirl pot can introduce disadvantages rather than improvements.

Unnecessarily large swirl pots often result in:

Slower engine warm-up due to added thermal mass

Increased system volume with no functional benefit

Packaging compromises in tight engine bays

Longer and more awkward bleeding during filling

In some cases, a large pot can make the cooling system feel less responsive, not more stable.

What Matters More Than Capacity

1️⃣ Vertical Height

The height of the coolant column is far more important than total volume. Vertical separation allows air bubbles to rise naturally and exit the system without being dragged back into circulation.

2️⃣ Outlet Position

The lower outlet must always remain fully submerged. If air is allowed back into the feed, swirl pot capacity becomes irrelevant — the system is compromised regardless of size.

3️⃣ Flow Behaviour

Coolant should slow and stabilise as it enters the pot. Controlled internal flow encourages air separation rather than turbulence that keeps bubbles suspended.

A smaller, well-designed pot will outperform a larger one if these conditions are met.

Typical Swirl Pot Sizes (and When They Make Sense)

500–600 ml

Suitable for compact layouts and systems that already bleed well but lack a proper high point.

750–900 ml

A practical balance for performance road and track cars, offering sufficient height without unnecessary bulk.

1 litre and above

Usually only justified for very large cooling systems, remote mounting, or unusual plumbing layouts.

Beyond this, gains are marginal.

Why Some Swirl Pots Have Radiator Necks (and Others Don’t)

Some swirl pots are fitted with a radiator-style filler neck and pressure cap, while others are sealed units with no cap at all. This is not a quality difference — it reflects system design.

Swirl Pots With a Radiator Neck

A swirl pot with a radiator neck is designed to act as:

Pressure control point

Fill and bleed point

Highest point in the system

In this configuration, it effectively replaces the radiator cap. This layout is common when:

The radiator cap is not the highest point

The radiator is mounted low or remotely

Engine swaps or custom setups remove a conventional fill location

Swirl Pots Without a Radiator Neck

A swirl pot without a radiator neck is intended to work alongside a separate header or expansion tank.

In this case:

Pressure management occurs elsewhere

The swirl pot’s role is purely air separation

Coolant continuously passes through rather than being filled directly

This approach keeps functions separated and avoids unnecessary complexity when a header tank is already present.

Which Design Is Better?

Neither design is inherently better.

A swirl pot should only include a radiator neck if the cooling system requires it. Adding one “just in case” introduces extra hoses, redundancy, and potential leak points.

As with capacity, correct integration matters more than features.

In Summary

Swirl pot performance comes from geometry, placement, and system design, not from litres.

A properly sized, well-positioned swirl pot — with or without a radiator neck — will always outperform an oversized unit added without consideration.

Capacity matters.

Just far less than most people think.

If you’re considering adding a swirl pot to your setup, our current range can be found in the shop.