Tested: Radium Engineering Coolant Expansion Tank

by Vince Illi


Recently, Radium Engineering began to release vehicle-specific kits for their universal billet aluminum coolant expansion tank. These kits include the expansion tank and associated bracketry and AN fittings to easily install the kit into your vehicle without having to perform any custom work or irreversible modifications. Radium sent us their kit for 2011-2014 S197 Mustangs to test out on our Grey Project Mustang 5.0. But before we get to the installation, let’s talk a bit about how a coolant expansion tank works!


Radium’s Coolant Expansion tank for ‘15+ S550 Mustangs. The kit for our ’11 S197 Mustang simply has a different bracket. Source: Radium Engineering.


How a Coolant Expansion Tank Works

As coolant heats up, it expands. If we limit the volume of this coolant (by sealing the system), we can build pressure in the system. A coolant system is pressurized to increase the boiling point of the water and squeeze any steam or air in the system into smaller bubbles to increase the available surface area of the coolant. The system’s pressure is limited by the pressure cap (also known as a radiator cap). The pressure cap consists of a spring and a gasketed “plunger.” When the system’s pressure exceeds that of the spring, the plunger is pushed up, and a small amount of coolant (or air or steam) escapes until the pressure comes back down to the designed limits.


The underside of Radium's pressure cap.

On older cars, the pressure cap was always on the top of the radiator (hence the term radiator cap).  This meant that the top of the radiator had to be above the cylinder heads of the engine to enable any air or steam in the system to escape. (Fun fact: the radiator cap on an old Ford Model A was high enough to be in the driver’s vision. It actually had a mercury thermometer on the back of it so the driver could monitor the engine’s temperature.)  As you can imagine, this somewhat limited the available designs of a car and is at odds with wanting to put weight as low as possible on the chassis.

To fix this issue, most modern cars have started using a Coolant Expansion Tank.  Essentially, the coolant expansion tank moves the “top” of the radiator to another location in the engine bay. This allows the use of a shorter radiator “below” the top of the engine while still enabling air and steam to be purged.

There are other advantages to a coolant expansion tank, as well. In order to maximize the surface area of the radiator being used, the radiator must be as full as liquid as possible. There is no room at all for air in this system. On a system with the pressure cap mounted on the radiator, if the maximum pressure is reached, liquid must be purged from it into an unpressurized container outside of the system (the overflow tank).  As the system heats and cools, it must continually add and remove liquid from the overflow tank. This is an inefficient design that reduces the available coolant when the engine is at its hottest!

In contrast, the expansion tank allows some air to be in the system, at the top of the tank. It allows the coolant to expand (hence the name) and purges air out of the system instead of coolant. (This deaeration leads to some manufacturers calling the expansion tank a De-gas Bottle.) This lets the radiator stay completely full of liquid coolant while allowing room for the coolant to expand.


A simplified cooling system with an expansion tank.

Three lines run to and from the expansion tank. The first is a steam line that comes from the highest point of the engine. This line removes air and steam from the engine and directs it to the expansion tank where it can be purged from the system. The radiator vent line comes from the highest point of the radiator and serves a similar purpose to the steam line; it purges air and steam from the radiator.  Finally, a third line—the return line—comes from the bottom of the expansion tank goes to the inlet of the coolant pump.

Now that we’ve discussed the purpose and operation of the coolant expansion tank, let’s take a look at Radium Engineering’s expansion tank!


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Tuesday, May 23, 2017 1:36 AM
Can you buy a radiator cap that doesn't open to pressure so you can build your own expansion tank type of setup? I saw a nice welded aluminum Honda radiator, but it has a radiator cap, and my current setup has an expansion tank.
Dusty Duster
Dusty Dusterlink
Tuesday, May 23, 2017 2:09 AM
You could fix that by using a higher-pressure cap on the radiator than on the expansion tank.

So you would put a 1.1-bar cap on the tank and a 1.5 on the radiator, for instance.
Tuesday, May 23, 2017 1:04 PM
Check out http://www.mazdatrix.com/b8.htm for RX-7 "Flat Caps." These are "radiator" caps without pressure relief. I can't recall if they fit Honda, but it's a good starting point.
Tuesday, May 23, 2017 5:06 PM
Thanks, y'all. Strangely, my expansion tank doesn't have a radiator cap, it's a VW type, I guess it somehow recirculates, so I'd have to go with the 'rawkus' solution.
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