If your car overheats or gets very hot in traffic or while cruising fairgrounds, but is fine out on the highway, you probably don't have a cooling problem, but an airflow problem. You're simply not getting enough air passing through the radiator and likely not able to let hot air escape from the engine compartment either. Engine-driven fans are great at their job, but when the motor is idling, the fan simply isn't turning fast enough to keep the radiator cool. Options here include an improved mechanical fan, electric fan or fans, or a combination of both.

If a mechanical fan is your choice, the options are a flex fan, which are noisy and rob horsepower, a temperature-controlled viscous fan, the speed of which increases as the temperature of the coolant increases, or a thermostatic viscous fan, which switches on or off at a specified temperature, only running when needed. If you chose to go for an electric fan, it should ideally cover 70 percent of the radiator core, usually meaning two fans will be required. There's also the option of push or pull fans. There's almost always more room in front of a radiator to fit a fan, where a "push" type can be installed, but pulling is more efficient, plus a fan on the front of the radiator in an early car isn't visually appealing, and will itself block airflow.

It's probably rarely considered, but fans not only pull or push air, but also move it radially as they spin. A shroud will ensure the air is all drawn through the fan. Shrouds also enlarge the air draw area, and even shield the radiator from engine heat to a certain extent. Shrouds will work for both electrical and mechanical fans, but if used with the latter, ensure the fan blades are placed halfway into the shroud opening for maximum efficiency and to prevent air escaping, with no more than 1-inch clearance between the tips of the blades and the shroud. You can choose to manually switch on an electric fan, or wire it so it runs all the time the ignition is on, but that's a little pointless when you can get sensors that will either turn the fan on at a specified temperature, or others like Flex-a-lite's Variable Speed Control that set the fan speed from 60 percent to 100 percent depending on the built-in fan thermostat setting, offering greater control over the engine's temperature.

The ideal operating temperature varies depending on who you speak to, but is generally in the 190-210 degree F range, though modern engines are designed to run even hotter, with some not turning their fans on until above 200 degrees.

It is the thermostat's job to maintain a constant temperature in the engine, as well as to allow the engine to heat up to its operating temperature as quickly as possible. It does this by regulating the amount of water that passes through the radiator. Until operating temperature is reached, the thermostat remains closed, re-circulating coolant through the engine. Depending on the thermostat's setting, it will open somewhere between 180 and 195 degrees F, though high-flow thermostats are available that open at 160 degrees F. The coolant will now pass through the radiator at a rate depending on how far the thermostat is open.

These days more than plain water is required in a cooling system, as aluminum (think radiator, heads and intake manifolds) can be damaged without an anti-corrosion additive. Commonly referred to as anti-freeze, ethylene glycol in a 50/50 mix with water will prevent corrosion, lower the freezing point and raise the atmospheric boiling point to around 220 degrees F from 212 degrees F. However, coolant temperatures can reach over 250 degrees F at times, and would therefore still boil, so the cooling system is pressurized to further raise the boiling point. For every pound of pressure the boiling point of water will rise by 3 degrees. Most cooling systems have a pressure limit of 15 psi, and it is the radiator cap that creates the pressure. As the coolant heats, it builds pressure, and the cap is a pressure release valve in essence, as it is the only part of the system where that pressure can escape. When the pressure reaches 15 psi, or whatever the cap is rated at, the valve opens and coolant escapes, usually flowing through the overflow tube to an overflow, or recovery, tank. This keeps air out of the system, and when the coolant temperature drops, a vacuum is created which opens another valve and allows the coolant to return to the radiator from the overflow tank. Which is why a recovery tank is a vast improvement over the overflow tube venting on to the floor! While we're on the subject of keeping air out of the system, it's worth mentioning that the radiator cap should be the highest part of the cooling system, to prevent air pockets.

The only part of the cooling system we haven't talked about is the water pump. Usually, or at least if your radiator is correctly sized and you have the correct thermostat installed, an OE water pump will be adequate, but Edelbrock and Stewart offer high-flow water pumps, with the latter offering four stages for Ford, Chevy and Chrysler applications. The most extreme is capable of pumping 160 gallons-per-minute. The water pump speed should match the crankshaft speed, though by a careful selection of pulleys it can be driven faster than the crank, though any more than 10 percent faster isn't recommended.

If you are having cooling issues, hopefully some of the above may be of use, though of course it is assumed that your engine is tuned correctly and running perfectly to begin with. Apart from the stress induced by a car that runs hot, you risk holed pistons, damaged rings, blown head gaskets, warped heads, premature failure of other components owing to excessive heat, and even a seized motor. It may not be as glamorous or exciting as some other aspects of building a car, but a cooling system that works correctly makes for great peace of mind and increased confidence in your rod or custom's usability.

Walker Radiator
Street Rod Headquarters
Cool Craft
Mattson Radiator
Vintage Air
10305 I.H. 35 North
San Antonio
TX  78233
Summit Racing
P.O. Box 909
OH  44309-0909
Speedway Motors
P.O. Box 81906
NE  68501
Yogi's Inc
US Radiator
Southern Rods & Parts
Cooling Components