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Radiators Aren’t the Only Thing Needed to Keep Cool

Posted in Features on February 4, 2016
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Most internal combustion engines are water cooled and use a radiator as a heat exchanger to cool engine coolant with ambient temperature air directed across radiator fins. But, there's far more to a properly operating cooling system than just a large radiator. Ignoring the other pieces to the equation may leave you with an engine that runs hot despite use of a substantial radiator.

There are a couple of common overheating scenarios. First is the one where a normal idle temperature gradually increases into the red as you load the engine at speed. The water pump is spinning at sufficient speed and airflow across the radiator is high due to the vehicle motion. In this case, your radiator may be insufficient for your application. But, there are other things to consider as well.

In the second scenario, overheating occurs more at idle but the engine temp returns to a more normal range at highway speeds. When this is the case, the radiator is typically sized large enough and not your problem. The problem is often more one of airflow to effectively use the radiator capacity. This is where proper fans and shrouding come into play. It's also possible this scenario could be an issue with low pump coolant flow, and in some cases an improved aftermarket water pump may be part of the solution.

Airflow incident on the front of the radiator is best if it is ambient air coming from in front of the vehicle and not from any of the heated underhood air. As radiators are set further back from the front grille, this can start to pose an issue. Consider where the air getting to the front of your radiator is coming from.

When dealing with an overheating problem, whether it be a stock system or a custom mix of components on an engine swap, start with the simple items such as clogged radiator fins, a collapsed radiator hose constricting flow, a non-working electric fan or worn-out clutch on a mechanical fan, a failing thermostat, or radiator cap that no longer holds proper pressure in the system.

Whether you run a mechanical fan or electric fan it must be well aligned to the radiator to effectively pull cool air through its fins. Other heat exchangers such as air conditioning condensers or various oil coolers placed in front of the radiator all block airflow to the radiator, preheat the air getting to the radiator surface, and generally impede efficient cooling action of the radiator. Granted, many times these items are needed on the vehicle, but beware how and where you mount them and their effect on engine cooling. Blades of mechanical fans should be spaced fore/aft so that one-third of the leading edge of the blades lies inside the shroud while two-thirds of the trailing edge lies rearward outside the shroud as shown here.

A big radiator won't work to its best potential if it’s not mated with an effective shroud. The purpose of the shroud is to efficiently couple the fan airflow to the surface area of the radiator to force the airflow through the radiator fins, and not around the radiator. Use of a well-fitted shroud will help direct the airflow not just over the surface area of the fan, but also force airflow through the four corners of the radiator as well. This can significantly increase cooling capacity that can be derived from a fan of given size. While this truck runs cool enough, there is some loss of cooling efficiency with a shroud that does not cover its full radiator fin surface.

Use of an infrared thermometer to monitor temperature may be helpful. However, remember that these tools only measure surface temperature of the object you're lasing. It may or may not give you a very accurate temperature reading of the coolant inside the object you're looking at, depending on material thickness, conductivity, etc. But, an infrared thermometer can be useful to see if hot water is flowing in a hose or distinguish if some portion of a radiator is clogged and not flowing heated water through it.

When refilling a coolant system, air pockets can develop and lead to overheating issues. One trick we like to use when installing new thermostats is to drill about a 1/8-inch hole near the outer edge to serve as a bleed hole when refilling the coolant system and trying to purge air pockets. This is especially useful on closed systems that use a fill/overflow bottle but no radiator cap.

More airflow never harms the system's ability to lower the temperature of the coolant. However, it is possible to run the coolant too quickly through the radiator to maximize heat transfer there. That is why it's often a poor idea to completely remove a thermostat hoping to increase coolant flow and system cooling. Most systems are designed to cool best with some restriction in the flow at this point.

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