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Street Race Truck More... The Toolbox

Cooling System Principles


The purpose of your cooling system is to allow the engine to warm up to the required operating temperature as rapidly as possible and continue to maintain that temperature under all driving conditions. The cooling system should be able to do this when ambient temperature is as low as -30F and as high as 110F.

The peak combustion temperatures on an engine cycle can range from 4000F to 6000F and average between 1200F and 1700F. Continued temperatures as high as this would damage or weaken engine parts if the heat were not removed.

In a conventional cooling system the coolant flows through the block first, through the heads, then after the thermostat opens, through the radiator. Low-temperature coolant leaves the radiator from the outlet and is then pumped back through the engine block where it absorbs heat and continues to re-circulate.

The coolant temperature will increase as much as 15*F as it passes through the engine, and then cools back down as it continues to cycle through the radiator. The flow rate may be as high as one gallon per minute per horsepower depending on your water pump and pulley ratio.

The Thermostat

The thermostat's function is to control the minimum operating temperature of your engine. It’s typically designed as an encapsulated wax-based, plastic pellet heat sensor that will swell as heat increases. The temperature rating on the thermostat will indicate the temperature at which it should begin to open. Typically it’s fully open 20F higher than its rated temperature. For example: a thermostat rated at 180F is fully open at 200*F.

The Pressure Cap

The cooling system is pressurized in order to raise the boiling point of the coolant. The boiling point will increase by about 3F for each pound of increase in pressure. At atmospheric pressure, water will boil at 212F. Under 15psi, water will then boil at 257F. With the proper antifreeze/water mixture, the boiling point should exceed 270F when under 15psi of pressure.

The Recovery System

Excess pressure usually forces some coolant from the radiator through an overflow. Most systems connect the overflow to a plastic or steel reservoir to hold excess coolant when the system is hot. When the system cools, the pressure decreases and a partial vacuum forms. This vacuum will then pull the coolant from the overflow back into the cooling system, keeping the system full.

Speedy Tip

If you notice your radiator hose is collapsed when your engine cools, don’t assume the hose is bad. The collapsed hose may be a result of a defective radiator cap. A properly operating cap should draw coolant from the radiator overflow container back into the radiator and not form a vacuum in the system. We also suggest using a spring inside the lower radiator hose to prevent any chance of this happening.

Coolant Mixture

You can check for proper coolant mixture by using a coolant hydrometer, supplied at most any local auto parts stores. This will measure the density of the coolant, which in turn measures the boiling and freezing point of your coolant. The higher the density, the more concentration of antifreeze to water.

It’s recommended to use distilled or de-ionized water with your coolant mix. That fact remains that distilled water may be at an advantage because it does not contain minerals. If the drinking water you mix with contains minerals, over enough time it can lead to deposit build-up that could prevent proper heat transfer. Speedway Motors offers a premium ready-to-use 50/50 antifreeze and de-ionized water blend from Afco, under part number 106100001.

Coolant additives can also help improve cooling by lubricating and eliminating the possibility of bubbles or foaming. Additives like Red Line’s WaterWetter, part number 91015709, have been dyno tested to substantially improve heat transfer and reduce the possibility of corrosion. We also offer great options from both Pro-Blend and Afco.

To learn more about the importance of a fan/shroud with cooling performance and the difference between downflow and crossflow radiators, check out the related articles tab!

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