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."