Unlike airflow where more is always better, coolant flow at low speed or idle affects temperature drops in the radiator differently. While increased airflow has no negative effect on cooling, a slight increase in coolant flow at idle most often produces a positive effect on temperature drop but too much flow can push the coolant through the radiator too quickly and back to the engine without adequate cooling, resulting in an even greater amount of overheating. It helps to understand that coolant flow created by the water pump is regulated by the thermostat first by remaining closed until desired minimum coolant temperature is reached and then by restricting flow to approximately 3-1/2 to 4-1/2 gallons per minute at an 800-rpm idle in a small-block Chevy. These flow rates are designed by OE engineers and extensive testing for maximum efficiencies throughout the rpm range and getting too far afield from original factory specs can work against a low speed or idle overheating problem. The restriction in the small opening in the thermostat as opposed to the 1-1/2 inches opening in the housing or hose is designed to slow and regulate the coolant flow through the radiator to give ample time to drop temperature throughout the rpm range from idle to highway speeds and resulting water pump flow rates. According to Armstrong, "In our testing at U.S. Radiator we've found that increasing flow rates slightly at idle helps to minimize temperature gain. The most efficient ways to do this are by using a "super-stat" with a slightly larger opening that allows about 10-15 percent increase of flow at idle without creating a negative effect at high speeds or by increasing the flow rate of the water pump by about the same percentile." (Armstrong recommends a 160-degree super stat on most engines and 180-degree for fuel injection. Since many hot rodders are concerned with fuel efficiency he recommends engine-operating temps in the 175- to 195-degree range. Higher operating temps will burn fuel more efficiently but the increase in operating pressure and metal distortion can easily create problems over time.)
For cars that tend to run close to thermostat at low speed or idle but gradually climb up the temperature scale while driving, check out two functions. First would be water pump performance just because of the cost of the pump versus that of a new radiator. If the pump is in good condition with little or no wear to the impellers and designed for street use as opposed to track or racing where flow rates are extreme then it's time to look at the radiator, which is the most critical and expensive part in the cooling system.
In looking at U.S. Radiators' several different models of radiators Armstrong explained to us about current radiator design. It is referred to as "high-efficiency" core design, which describes a 3/8-inch fin and tube spacing with a maximum core thickness of 2-5/8-inch for adequate airflow. For aluminum radiators the most common performance core would be two rows of 1-inch tubes (two linear inches of fin bond) with 3/8-inch fin. For copper and brass radiators, four rows of 1/2-inch tubes (two linear inches of fin bond) with 3/8-inch fin and tube spacing has become the standard. Both aluminum and copper/brass in high efficiency design are more than adequate for most street rods and performance restorations with the advantage going to copper/brass because of copper's greater efficiency of heat transfer.
Armstrong recommends the following radiator styles based on engine horsepower: Four-row high-efficiency copper/brass radiator up to 450 hp. Above that horsepower and for blower motors he recommends using their Optima core, which is 1/4-inch fin and tube spacing and "triple flow" the radiator with baffles, which is as good as it can get when it comes to heat transfer and pure temperature drop inlet to outlet.
The Triple Flow Option
Something relatively new in radiator design from U.S. Radiator is the triple pass core. As the name implies, coolant flows through the core more than once. In a standard single-pass radiator the coolant enters one tank, travels through the core, and drops in temperature based on that particular core's ability to reduce temperature (usually about 30 degrees).
Once the coolant reaches the other tank it goes back to the engine at whatever temperature the core was able to reduce it to. In a triple pass design the coolant actually travels three times as far in the airflow without changing the speed of the flow, which creates an even greater temperature drop (usually an additional 20 degrees), depending on the design of the core or heat-transfer points.
Brass/Copper vs. Aluminum
The thermal conductivity or heat transfer rate of copper is 92 percent versus aluminum at 49 percent. However the copper fin is bonded to the tubes or water passages using lead solder, which is very inefficient and slows the heat transfer rate to just slightly better than that of aluminum. This can be a disadvantage if the bonding process does not allow the copper fin to touch the brass tube.
Copper/brass radiators, because of their weight and durability, have been around a long time and are easily disassembled and reassembled for cleaning purposes. Not the case with aluminum unless speaking of the OE version that comes with crimp-mounted plastic tanks. As a result, the life expectancy of the aftermarket aluminum radiators will be far less than that of copper/brass.