Another critical aspect of airflow management at low speed or idle are "air dams" that prevent hot engine compartment air from flowing around to the front of the radiator. Any other placement of the fan in the shroud will result in the air flowing out of the shroud straight back toward the block and reducing airflow efficiency by approximately 20 percent in most vehicles. Ideal placement of a mechanical fan within a shroud is a third of the leading edge of the fan in the opening of the shroud and two-thirds of the trailing edge of the fan out of the shroud, which results in approximately a 45 percent angle of airflow out of the shroud. The same holds true for a mechanical fan but the placement of this fan within the shroud becomes more critical. Without a shroud the fan is only pulling air through an area equal to the circumference of the fan, utilizing only 60 percent of the radiator's cooling capacity. The shroud is only 3/4-inch thick and will fit in most tight engine-to-radiator applications and promotes airflow through all four corners of the radiator. Radiator we recommend using a shallow box shroud that covers the entire face of the radiator core and becomes the platform for mounting the fan. According to Armstrong, "With electric fans here at U.S. Fans are circular in design and radiator cores are rectangular so in order to utilize every square inch of the radiator core a shroud is paramount whether the fan is mechanical or electric.
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The most important aspect of airflow management is a shroud and well-placed fan in order to utilize the full area of the radiator core. Airflow management is crucial in low speed or idle temperature drop. The function of either type of fan is to keep airflow through the radiator at such a rate as to maintain temperature drop (approximately 30 to 40 degrees from radiator inlet to outlet). At low speed or idle you no longer have the luxury of ramming air through the grille and have to rely on fans, either mechanical or electric.
When the car slows down or gets stuck in traffic and overheats the only thing that has changed is the airflow and coolant flow-airflow being the more important of the two. If the car runs within the 10-25 degrees of thermostat while making horsepower you can pretty much rule out a radiator problem because it's handling the heat at the engine's peak output.
Think about this: Should your hot rod be overheating? At highway speeds, your engine is making horsepower and horsepower is heat. (Sometimes it helps to drill an 1/8-inch hole in the thermostat to allow air to bubble to escape from under the thermostat on the initial fill with a new motor.) A radiator full of coolant is most efficient. An overflow canister (tank or bottle) and appropriate cap keep the radiator full and "air out" the radiator. (As retired GM hot weather engineer Scot Leon told us, "It's amazing what industrious little critters can store in some unusual places in your hot rod especially between the radiator and condenser." Make sure your fan(s) are rotating in the right direction and, remember, a high pressure system can run hotter than a low pressure system without boiling. On cars with A/C check for debris in between the condenser and radiator, especially after winter storage.
A problem that typically arises with longer hoses. Don't overlook the obvious, like a collapsed lower radiator hose. Debris can obstruct airflow but be difficult to see due to the tight quarters inherent with most hot rods. Sometimes an overheating vehicle just requires a good visual inspection.