Airflow is the most critical factor in the cooling system and affects a radiator's cooling efficiency the most. A vehicle's speed, be it a street car or racecar, is the point most considered when figuring out airflow necessary for proper cooling. Maintaining adequate airflow at a car's various operating speeds is crucial and complex. First, the radiator must be supplied with fresh air. The grille opening or air inlet can make all the difference here. Ideally it should be facing squarely into the wind. With older cars, frontal/grille area usually isn't an issue, except for Corvettes. The size of the grille opening should always be proportional to the vehicle's operating speed(s). Big-block-powered C2 and C3 Corvettes are notorious for cooling issues because of the smaller front surface area they have, along with their tighter engine compartments.
A radiator transfers the heat in the coolant to the cooler air passing through the fins and over the coolant tubes, or more simply put, the radiator's core. For the radiator to work properly, the flow of air must be under high pressure at the front side of the radiator and lower pressure behind. This pressure differential pushes the air past the fins. If air pressure builds up in the fan shroud or the engine compartment and the pressure differential is decreased, the airflow across the radiator can slow down and "stall" much like the airflow over the wing of an airplane. When planning out your car's cooling system, you have to consider both idle and cruise conditions and how fresh air can be presented to the radiator effectively in both situations.
"Electric fans versus mechanical/clutch-type fans is really a no-brainer. There are normally two types of overheating situations. If you are running hot at highway speed, you probably don't have enough radiator capacity. If you are overheating at idle/slow speed, you probably don't have enough airflow. This is where the electric fan works and the "engine" fan does not. The type and quality of electric fan is very important. Accurate airflow cfm numbers are critical. The more air you can move through the radiator, the more heat you can dissipate."
Coolant flow is usually the last aspect of the cooling system to be addressed. Ironically, it's also the usual cause for overheating problems. A typical stock water pump has excessive clearance and straight impeller blades, usually open front and back. With the engine running at low rpm, this produces little coolant flow and is typically responsible for cars overheating in traffic at idle speed. At high rpm, this design will cause cavitation and aeration, which can also cause the coolant flow to be reduced to the point of engine overheat. A common Band-Aid fix for this problem is to run underdrive pulleys, which slows down the revolutions of the water pump/impeller. While the high-rpm cavitation problem is solved, this solution usually contributes to a low-rpm overheat problem because the water pump isn't turning fast enough. With an engine-driven water pump, the only remedy is an aftermarket race-style pump with tight clearances and a swept-blade, closed-impeller design
Electric water pumps are a highly effective solution to these problems with multiple benefits. The constant speed of an electric pump eliminates high-rpm cavitation problems and low-rpm insufficient flow issues. An added bonus is being able to run the pump when the engine is shut off, especially useful racing applications.
The third benefit is the elimination of parasitic horsepower loss from the engine having to turn the water pump off the crankshaft.
Pump & System Pressure
For every pound of pressure in a closed cooling system, the boiling point is increased 3 degrees. For example, a properly functioning 16lb radiator cap can increase your boil-over point to 260° F [(16 x 3) + 212 = 260]. We mention properly working because an old or faulty radiator cap can prevent your cooling system from building enough pressure to work properly.
Even though your temp gauge might never go beyond 192 degrees, you can have hot spots around the combustion chamber that will be in excess of the coolant's boiling point. A lack of pressure in the cooling system allows boiling to start prematurely. Gasses produced by the coolant boiling push water out and simultaneously aerate the coolant, making the cooling inefficiency worse.
Water is diverted around these steam pockets leading to more serious problems, such as surface distortion, metal fatigue, and cracks. Once this premature boiling begins, it won't stop while the engine is under load. Coolant flow, temperature, and pressure all work to minimize boiling at hot spots, which can produce steam pockets that insulate the engine's metal surfaces from the coolant.
The more pressure the water pump produces, the less chance there is of steam pockets forming. The same boiling point law mentioned earlier works here too. Racing-style water pumps can generate pressure in the water jacket in excess of 30 psi to minimize hot spots and reduce detonation/pre-ignition.
According to Walker, the importance of using the correct type of coolant for your radiator cannot be overstated.
"Only use the correct antifreeze for aluminum radiators. Electrolysis decay is also common when stray voltage runs through the radiator," says Walker.