The radiator's job is to remove the heat from the fluid so it can return to the water block cool. It achieves this by transferring the heat to the air that is flowing past it.

There are four main variables affecting the performance of a radiator:

1. Material
2. Size
3. Fluid flow rate
4. Air flow rate

Material

When selecting a Radiator it's best to select one that is made from a metal with a high W·m⁻¹·K⁻¹. The higher this rating the lower the C/Ws of your radiator will be(all else being the same).

Common metals used in radiators:

• Copper (300 K) 401 W·m⁻¹·K⁻¹
• Aluminum (300 K) 237 W·m⁻¹·K⁻¹

As you can see it’s best to have a radiator made interlay of copper. Some manufactures will use a brass copper combination. Others will produce theirs with Aluminum. When selecting a radiator it’s important to consider what your water block is made of. If it’s made of copper it’s NOT recommended that you use an aluminum Radiator as you will have trouble with galvanic corrosion. Obviously the same is true with an aluminum water block and a copper radiator. Galvanic corrosion is covered more under the Fluids section of this site

Size

In most cases its best to have the biggest radiator possible. The bigger the radiator, the more surface area there is for the fluid to transfer its heat to the radiator and the bigger the area the air has to remove the heat from the radiator.

Fluid flow rate

When a radiator is designed the manufacturers are trying to improve performance by altering the path fluid flows and in doing so transferring more heat to the radiator. Some designs will achieve lower C/W by sacrificing flow though the Radiator. This makes selecting a radiator more complicated than simply selecting one that produces the lowest C/W at specific flow rate. This is because resistance to flow created by the radiator can affect flow through it and therefore affect the radiators C/W.

Air fin design

The heat is ultimately removed from your system by air flowing past your radiators fins. Fin design will affect how efficiently heat can be removed from your radiator and how much noise will be created by the air passing the fins.