Fans: Mechanical vs. Electric
With most hot rods, especially on pre-'49 cars, there's a clearance problem between the radiator and the modern V-8 water pump system; later model cars, like a Tri-Five or '60s-era cars that have ample clearance between modern water pump system and the radiator. Knowing this rodders are faced with using thin (2-5/8 to 3 inches thick) electric fans in place of mechanical fans (block driven). In our '34 Ford sedan that was experiencing cooling woes we opted for an electrical fan as there wasn't sufficient room to run a mechanical. Since we are running a Vintage Air A/C unit (condenser and other related components) we opted to stick with one of their electric fans; the LoPro electric fan and shroud assembly that measure 2.62 inches in thickness. Another Vintage Air item is the Adjustable Fan Thermostat that allows you to set the engine temperature (180 to 240 degrees) that the electric fan will engage. There is a probe that installs into the radiator drain via a fitting. It can be used separately or with a Vintage Air A/C trinary function safety switch.
However, according to Armstrong wherever possible he likes to use the mechanical fan. He prefers mechanical fans with flex curved blades (3 inches deep from leading edge to trailing edge) with six or seven blades dependent on radiator core size and in one of three popular diameters of 15, 16, or 17 inches. Fan shrouds are always a must for mechanical or electric. Now, electric fans generally come with at least a minimum of a shroud but you can also make a more elaborate shroud that will yield these fans even more efficiently.
Your cooling system is dependent upon a mechanical fan at idle or low speed where there's little or no airflow. Shrouds are necessary to maximize the amount of ambient air being pulled through the grille and radiator. For the record, proper fan and shroud alignment should be leading edge one-third in and trailing edge of the fan two-thirds out. The airflow off the back of the fan deflects at about a 45-degree angle when set this way. When the blade extends further into the shroud the air off the back of the blade flows straight back into the block and decreases the airflow efficiency by about 15 percent.
That being said, if an electrical fan is the only way to go then by all means place it on a shroud that covers the entire core. We often see an electric fan attached directly to the core and the only thing this does is waste the rest of the core surface when you need it the most. A 16-inch electric fan attached to a core only cools a 16-inch circular section of that core. Where space is an issue U.S. Radiator offers a 3/4-inch-deep aluminum shroud that enables adequate airflow over the entire core and when combined with a thin-line electric fan it results in a total depth of 2-3/4 inches.
According to Love (like Armstrong) they used to always recommend engine-driven (mechanical) fans because the electric fans available usually just weren't up to the task of a primary cooling fan. However, Vintage Air offers several electric fans (their own model LoPro, SPAL, and Maradyne) that they feel will work well as primary cooling fans. Electric fans have several advantages. They are not dependent on engine rpm to provide maximum airflow, they do not rob engine horsepower (although with today's engines this is probably of little practical consequence), and if properly wired, they only run when additional airflow is required, which leads to a quieter ride down the road.
Love also recommends a fan shroud with either fan. Shrouds dramatically improve the efficiency of the fan, and when properly designed they ensure that air is pulled through the entire core of the radiator, not just in front of the fan blades. Vintage Air also always recommends "puller" fans mounted to the backside of the radiator and condenser. History has shown that a puller fan is more efficient than a pusher mounted to the front of the radiator.
Electric Fan Engagement
Something of particular importance when operating an electric fan in conjunction with A/C is a compressor safety switch. Love enlightened us on the importance of a binary safety switch. (The use of "bi" meaning two functions.) The binary switch typically is installed in the high-pressure liquid line (No. 6) hose of the system, and provides low pressure and high-pressure protection for the compressor. The switch is connected in a series with the compressor clutch feed from the A/C thermostat (or compressor relay in our Gen IV systems). The binary switch will also open and disengage the compressor clutch if the A/C system pressure builds to 405 psi (usually caused by lack of sufficient airflow through the condenser to remove heat from the refrigerant) to prevent system damage.
If the vehicle is equipped with an electric fan as its primary cooling fan, they recommend a trinary safety switch. (The use of "tri" meaning three functions.) The trinary switch provides the same low- and high-pressure protection as a binary switch, but includes the added feature of an electric fan engagement signal at 254 psi. The correct way to control a primary electric fan would be to have the temperature sensor (set point or adjustable) and trinary switch trigger to the same electric fan relay. Connecting the switch in this manner allows the electric fan to be engaged by either engine temperature or high A/C head pressure.
According to Love, "We never recommend engaging the fan every time the A/C system is on, but prefer to have the system pressure or coolant temperature dictate when the fan is engaged. There is no reason to run the fan when the vehicle is driving at highway speeds where the ram air should provide sufficient airflow through the condenser and radiator."