Radiator Basics
Let's start with radiator materials. Our automotive forefathers were pretty sharp guys and used copper/brass radiators for a reason. Copper has an excellent thermal-conductivity rating. A copper-fin's thermal-conductivity rating is more than 50 percent higher than an aluminum fin. Brass, which is an alloy of copper, is not as good a conductor as aluminum but is used for the tubes because of its strength. One difficulty with copper is that the lead solder used in older copper/brass radiators has a terrible thermal-conductivity rating, which limits the efficiency of lead-soldered radiators. So companies such as U.S. Radiator have instituted a newer process that improves efficiency by changing the flux and solder and its contact with the fins.
If you've ever wondered why some copper/brass radiators are cheaper than others, it's all in the construction. The original radiators built in the musclecar era used 11/42-inch tubes 91/416-inch apart that are generally the least expensive. More modern radiator construction moved those centers closer together, with the 11/42-inch tubes 31/48-inch apart. This creates room for more tubes in the same-size radiator core. There are even copper-brass radiators now with 11/42-inch tubes on 51/416-inch centers. Each of these versions can be obtained in two-, three-, or four-row applications. As the radiators become denser, they become more expensive.
Tubes in all radiators are...
Tubes in all radiators are flattened to increase surface area that contacts the fins. Aluminum radiators use 1-inch-diameter tubes like this cutaway from Afco. American-made radiators such as this one also use thicker wall tubes that are less likely to fail under high pressure.
Then why have aluminum radiators become so popular? One big reason is that the OEMs saw the potential for a significant weight reduction and lower material costs. Racers are also big on aluminum radiators for that reason, with a weight difference of around 10 to 15 pounds. Plus, aluminum radiators start with 1-inch cooling tubes roughly 31/48 inch apart. Fin counts are also a critical radiator-design component, but a higher fin density (measured in fins per inch) may make airflow more difficult and not necessarily work well for street applications.
The two major designs for radiators are vertical flow and horizontal flow. As far as efficiency is concerned, there is no advantage to horizontal-flow radiators other than that they tend to allow a larger core to fit into a given engine compartment. Virtually all production-based radiators are built with a single-pass design, where coolant enters from the engine into the top of the radiator and travels across the core to the outlet on the opposite side. While dual-pass radiators have been around for a long time, they are now beginning to show up in high-performance and racing applications. A dual-pass horizontal-flow radiator moves coolant across the top half of the radiator on the first pass, then directs the coolant across the lower portion of the radiator face for a second pass. One reason this works is because the velocity of the coolant roughly doubles when the coolant is forced to travel across half as many tubes per pass. This creates turbulence in the tubes, exposing more coolant to the radiator tube walls and improving heat transfer. This also presents an increased load to the water pump, which means using a dual-pass radiator demands a better water pump if the system is to take advantage of the dual-pass concept.
 A single-pass radiator should...  A single-pass radiator should always place the inlet on the opposite side of the outlet. A dual-pass radiator such as this one positions the inlet and outlet on the same side since the coolant flows across the radiator core twice. Also look for the telltale weld on the same side as the inlet/outlet as on this Afco radiator. |  |  Flex-a-lite now makes an aluminum...  Flex-a-lite now makes an aluminum radiator with an interesting twist. The Flex-a-fit uses ridges extruded as part of the radiator tank that function as an additional heat sink while also creating convenient mounting places for electric fans, shrouds, and auxiliary-cooler mounts. |