What Is Inside a Radiator April 13 2022

You probably know that radiators pass engine coolant through them to cool down the coolant and send it back to the engine. But what is inside a radiator, and how do they work?

BMW E46 M3 stock radiator.

This is the stock radiator out of our BMW E46 M3. It has an aluminium core with plastic end tanks. Crimped tabs hold the plastic end tanks onto the aluminium core. The hot coolant from the engine enters the upper inlet, floods the inlet end tank and the coolant makes its way through the radiator core where it is cooled by the air passing over it. The coolant then spills into the outlet end tank where it is then sucked out of the outlet by the water pump. On every radiator I've ever seen, the inlet is positioned high and the outlet positioned low. The low placement of the outlet is so that if there is low coolant in the system and the radiator is only partially full, the engine will still have coolant to draw from. Because the outlet is placed low, placing the inlet high allows for better and more even flow through the radiator. If both the inlet and the outlet were mounted low, there would be little flow through the top of the radiator, as most of the flow would occur through the bottom of the radiator, reducing efficiency.

Screwdriver prying up tabs on radiator that hold the end tank on.

We took an angle grinder with a cutoff wheel and scored the tabs. This allowed us to pry up on the tabs without much resistance.

Radiator with the end tank removed.

Inside, you can see the core is just a series of flattened tubes that carry coolant from one end to the other. There is also a gasket that seals the plastic end tank to the aluminium core. Some aftermarket radiators have aluminium end tanks welded onto the aluminium core. The main benefit of this is that the radiator is less susceptible to leaks, because it is a welded construction and there are no gaskets to fail. Aluminium end tanks are also more durable than plastic end tanks because plastic end tanks get brittle over time and crack.

Closeup of the inside of an end tank.

The inside of each of the plastic end tanks is just one continuous volume. However, some performance end tanks are more complex internally because they have multiple cores stacked in front of each other, with the coolant flowing from one core to the next core. For example, in a triple pass radiator, the end tank on the outlet side has to route incoming coolant from the first core into the second core, and then receive the incoming coolant from the third core to route it to the radiator outlet.

Radiators with multiple cores can provide more cooling capacity, but they are usually thicker and may require modifications to the vehicle in order to fit. They are usually more expensive as well.

So there you have it. Now you have a basic understanding of how radiators work, and maybe you have a little more info to inform your decisions the next time you are shopping for a radiator.

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