Ford Mustang Cooling System Basics
Understanding how your Mustang’s cooling system works
Moving the Air
Having the biggest radiator you can fit in your engine bay and a high-flow water pump won't mean much without adequate airflow. Essentially, you have two avenues: electric cooling fans and belt driven fans. Each category has its good and bad points. For most, the traditional belt driven fan is what we're used to in the vintage Mustang engine bay, though Ford did use a belt-driven fan up until 1994 when the Mustang finally went with an electric fan from the factory. The fan bolts to the water pump's hub and is driven by a pulley and belt combination. We've seen fans installed backwards, we've seen the wrong pulley ratio used (salvage yard pulley swap for example), and we've seen belt routings that do not put enough belt surface on the pulley, causing slippage. Of course, you can have too small a fan, or a fan with not enough blades to cool as well.
When using a belt driven fan, we recommend the “A/C“ style fan with deep curved blades to really pull the air through the radiator core instead of the stock four-blade fan most vintage Mustangs came with (again unless you have to run the stock parts for show use). If you're using a belt-driven fan, then use a shroud, no excuses. Air flow will take the path of least resistance, and that will mean around the radiator instead of through it. Ensure the fan blade pitch is half into the shroud and half out by changing or adding a fan spacer as required to ensure the air flow is drawn through the radiator instead of around it.
As we just stated, the Mustang has used an electric fan setup with multiple speeds since 1994 (that's nearly two decades now) so electric fans certainly aren't just for race cars or for added towing insurance anymore. Electric fans allow for more room in the engine bay and offer better control over your cooling system, as they are an “on demand' cooling device, only running when the thermostatic switch in the coolant stream deems the fan necessary (low speed driving, long periods of idling, etc.). In some instances, like modular V-8 swaps, they are mandatory since there's no way to mount a belt driven fan to the engine's water pump hub. There are essentially only two ways to mount an electric fan, either as a pusher or a puller; meaning on the engine side of the radiator pulling air through, or on the grille side of the radiator pushing air through. An electric fan is going to be more efficient as a puller, but if you must use a pusher type, ensure it has adequate core coverage and moves enough air for your application.
When it comes to fan coverage, you want as much of the radiator core covered by the fan as possible, but a minimum that should be considered is 70 percent. If a shroud is available for your electric fan(s) package, by all means use it. The shroud not only makes the fan more efficient, as it is pulling air through the entire core, but the shroud usually makes the installation of the fan easier since the shroud reaches the mounting edges of the radiator itself. While we've all done it, the last thing you should use for any sort of long term fan mounting solution are those plastic “through the core“ zip tie affairs. The weight of the fan, coupled with the vibration of it in use, can cause the fan to wear/cut through the radiator's cooling tubes when mounted in this manner. At the least use solid mounting ears/straps and when at all possible a shroud (1⁄4-inch deep at a minimum) is the best solution.
There's a fairly common misconception that S-shaped blades outflow straight blades on an electric fan. Truth of the matter is, more often than not the S-blade fan has a different motor on it which increases the airflow cfm, so we're not comparing apples to apples here. According to engineers we spoke with at SPAL, straight blade fans are usually the more efficient of the two styles if the motors are the same, however they do have a blade pitch that is slightly noisier than the S-blade style. No matter the size of the fan or the type of blade, it is going to make some noise. When you move air you create noise. When looking at electric fans, be aware of cheap models that cut corners. On large diameter fans, you'll find a support ring to stabilize the blades so they don't flex and cut into your radiator core. Also look for glass reinforced plastic for the fan body and blades. This increases the stiffness of the unit as a whole and prevents blade breakage.
Lastly, a quality fan will often have an IP68 rating for dust and water intrusion. Many low dollar fans aren't rated as such and driving in rain can severely shorten their lifespan to a matter of months. Many electric fans come without any wiring or controls, leaving it up to the installer to determine how to control the fan. We recommend controlling an electric fan via a thermostatic switch in the engine. Fan wiring should be sized properly for the amp draw of the fan motor, and due to the rather high amp draws of the typical fan, you should always use a relay to allow direct connection to the battery (properly fused) so that the thermostatic switch turns the relay off and on for fan control.