You've built a great engine with lots of power. Now you need to keep it cool. All too often we see guys build fantastic cars, but neglect to pay attention to a critical component that keeps everything running smoothly: the cooling system. Tough to believe it gets overlooked, but think about how many times you've been at a show, cruise, autocross event, etc., and seen a really nice car with overheating issues.
Assuming you don't have any tuning issues causing your car to overheat (too much ignition advance, excessive lean condition, plugged exhaust), there are five basic factors that affect cooling system function and efficiency.
Heat Production (BTU/HP)
BTU (British thermal units) measure how much heat the engine produces. One horsepower is equal to about 42.44 BTU. About one third of the heat generated by the engine goes into the coolant/water mixture and must be dissipated by the radiator. When you're trying to calculate the amount of BTU your engine produces, you only need to consider the engine's horsepower that's continuously being used, not its peak power output. A car that cruises a lot and runs in the meat of its power band continuously for long periods of time will need more cooling capacity than a trailered show car or one that sees light driving duty.
Jim Walker, owner and founder of AutoRad, told us this: "Basically how much heat an engine displaces though the water system will determine how much radiator is needed to cool it. The horsepower is just one of many factors. You can pretty much cool a 650hp LS motor with the same-sized radiator as you would use with a 65hp flathead motor. The flathead motors are usually very hard to cool because so much heat is transferred to the water jackets, whereas the new LS motors are very well designed."
Radiator Capacity (Heat Dissipation)
The radiator's capacity is the amount of heat it can dissipate, not the amount of coolant it holds. Radiators can't be judged on physical size alone these days because of the different materials they're being made from. In the past, most radiators were made from copper because of its superior heat dissipation properties. The drawback was the solder used to assemble radiators would inhibit the copper's ability to dissipate heat. The advent of aluminum radiators has allowed the switch from 1⁄2- to 3⁄4-inch-wide tubes to 1- to 1.5-inch-wide tubes and the use of double-pass tanks. Wider tubes have more surface area, which allows for increased heat dissipation.
Dual-pass radiators force the water to travel the length of the radiator twice, increasing the amount of temperature drop capable for a given-size radiator. The downside to dual-pass designs is the coolant flow restriction is more than doubled. Surface area is the most important factor with radiators. Doubling the square-inch surface of your radiator will double the heat dissipation capacity, whereas doubling the thickness is less effective and restricts airflow.
Another factor is whether your car is running air conditioning and/or an automatic transmission or engine oil cooler. A typical A/C condenser is right in front of the radiator and exchanges heat with the air just like what it sits in front of. If you don't have enough radiator capacity, then every time you hit the A/C button, your car's bound to overheat.
Other factors playing a role in radiator design and function are fin count per inch and configuration, i.e. downflow (top-tank) or crossflow (side-tank) radiator designs. Inlet and outlet size also play a major role.
On radiators, Jim says, "Usually, the size of the radiator is defined by the size of available area. If you build the "biggest" radiator you can get into the area, it`s pretty hard to go wrong. This is the reason we [AutoRad] build our own core supports. We are usually able to provide a much larger radiator than you would be able to mount in a stock core support.
"Pretty much the available space will determine whether a radiator will be a downflow or a crossflow. Water doesn`t care if it flows up and down or side to side. You just have to be careful to keep the tubes covered with water. Getting air pockets in the water system can do a lot of damage. You need a recovery system with a crossflow radiator.
"People will always talk about aluminum versus copper/brass. The fact that the OEMs have not used copper/brass radiators in new cars for the past 30 years should really tell you something. The main fault with copper/brass radiators is the use of solder to hold them together. Over time, the solder breaks down between the brass and the copper and hinders the heat transfer from the tubes to the fins. Aluminum cores, on the other hand, are brazed in an inert gas oven and the flux bonds everything together."
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.