Multiple Pass Radiators

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There’s the belief among some that the engine coolant can go through the radiator too fast and not have enough time to dissipate its heat or that it’s passing through the engine too fast to pick the heat up. This belief probably originated via the observation that removing the thermostat can cause the engine to boil over faster.

Adding a baffle into the thermostat housing to restrict coolant flow will then make boiling over less prominent. (The reality here is that a flow restriction at the coolant’s exit from the engine will raise the pressure within the engine which raises the boiling point of the fluid inside the engine cooling jacket.)

Another method of slowing the coolant flow that people use is a multiple pass radiator. They see it as a dual benefit of slowing the flow and providing the additional cooling of more radiator surface. It does decrease the radiator outlet temperature, but the temperature within the engine will actually go up because the fluid spends more time there as well.

Regardless of the fluid used, it is always easier to transfer heat between liquid and metal than it is between metal and air. The system’s heat transfer limit is at the radiator. The radiator is more efficient with a greater temperature difference between the metal and the air. If the air temperature is 90 and the radiator temperature is 95, not much heat will transfer compared to an air temperature of 90 and a radiator temperature of 250. (I’m using extreme temp numbers here just to make the point.) This means that the cooler end of a multiple pass radiator is less efficient than the hotter end.

A cooling system works best when there is a fast coolant flow and the radiator temperature remains high because it can shed heat to the atmosphere better. Water-based antifreeze puts a limit on the coolant flow rate because the pump can only turn so fast before it cavitates. Basically, the low pressure side of the pump has a lower boiling point and the fluid will vaporize; the pump can’t move a vapor.

Evans Coolant solves the boiling point problem allowing aggressive pump speeds and eliminating the need for engine coolant exit restrictors.