Cooling System Function and Operation part 5
Fans
We have discussed how the coolant carries heat from the engine to the radiator to be expelled into the air. To remove the heat at the same rate or higher rate than it is produced an air supply must be constantly available to the radiator core. The air supply is variable, the higher the engine r.p.m. the faster the water pump operates, and the faster the fan turns.
Why must we have a fan on our cars? They are not required in all circumstances, when the car is stopped, idling, or is moving slowly air flow must be present. At twenty m.p.h. and above the fan is no longer needed. Air in front of a moving car is pressurized and seeking an escape route through the grill and radiator core. This is the "Ram Air Effect", air forced against the frontal opening in adequate volume to cool the engine. The fans sole purpose is to pull air through the radiator core when the ram air effect of positive pressure in front of the grill is no longer present. When rotating the fan creates a negative pressure behind the radiator pulling air through the core. The heat it removes from the coolant exits the car by going out around the engine and under the car body.
Fans are only efficient at certain rpms (usually lower speeds) above that speed they become parasitic and rob engine horsepower. Manufacturers have been required to change fan configuration many times to accomplish evolving needs. The early Fairlane V-8 fan was a simple 4 blade cross, this changed quickly to 5 blade as horsepower (heat) rose and compression increased. Later versions had even more blades, becoming heavier and noisier requiring a change of design to reduce these effects. Fairlane V-8 fans range from 14" to 18" in diameter, most at or near 17".
Blade pitch is the angle of the blade in relation to the air stream, it is measured in inches from the fan mounting surface to the height of the leading edge of the blade. Increases in blade pitch are effective in improving airflow at a given rpm but excessive pitch angles create some concern about vibration.
If you have noticed the tips of the fan blade they are curled back, this causes the air to move in a rearward direction instead of flying off the ends. This increases pressure behind the fan, rather than being slung radial outward.
Improving on the fixed fan, Ford went to the thermal clutch fan. The clutch unlocks or slips at low air temperatures and high rpm.
We have several choices in fans to use on our cars, electric, fixed, clutch and flex type.
Electric fans are the choice of car builders where clearance is in short supply. An electric fan is designed to be mounted in front of or behind the radiator. Multiple electric fans can mount in various configurations such as stacked, diagonal, and side by side, your clearances determine the arrangement. The diameter of electric fan blades or the motor r.p.m. are not always an indicator of its ability to cool. The true indicator of fan performance is cubic feet per minute of air it can move. Be aware that a fan must be quite as well as efficient. An electric fan of poor quality can vibrate your car annoyingly each time it switches on. Some manufacturers will increase motor r.p.m. to achieve c.f.m (cubic feet per minute). In unibody cars with the fans bolted directly to the core support this can be a concern. The better brands of electric fans can move almost 3000 c.f.m. and have 10 or more blades.
Reversing polarity of the fan motors is how the electric fan can be placed in front of or to the rear of the radiator and still rotate in the correct direction. A very important fact about electric fans is that they are much more efficient as a pull through (on the rear of the radiator) than as a pusher. If you visualize the rotation of the blades with a front mounted fan you realize that the air moves off the blade tips in a spiral motion, turning at approximately a 45* angle to the fins of the radiator core. Much of this air will be disrupted and bounce off the fins as turbulence. When mounted on the rear of the radiator, the fan is creating a negative pressure area behind the radiator, actually sucking air straight through the core. Airflow volume is much greater straight through the core than bouncing off corners.
A great source for electric fan information is <flex-a-lite.com> web home of Flex-A-Lite cooling products.
Flex fans are light in weight and usually constructed with a steel center and either aluminum or stainless steel blades riveted on. The advantage of the flex fan is that it can have very steep pitch angles on the blades that flatten out at higher r.p.m. Again this will reduce blade loading, noise, vibration, and parasitic power lose.
Fan clutches offer an opportunity to make the fixed fan more efficient in many applications. There are two basic types of fan clutches.
The centrifugal clutch, relying on engine R.P.M. to release its coupling to the water pump shaft is somewhat engaged at all times. At low R.P.M. the clutch locks up fully engaging the fan, as engine speed increases the clutch will slightly release allowing the fan to slip. This is giving the engine back the power the fan would use and allowing the fan to reduce speed (noise and vibration) in high way usage. There is a problem about centrifugal fan clutches used in vehicles with low gearing (high numerical), the engine can reach speeds where the fan clutch disengages yet the vehicle does not have enough forward speed to get the ram air effect to take over the cooling job. One case that would illustrate this might be an off road 4x4 in low lock range moving at about 4 miles pre hour, the engine taching 3K and the fan clutch is mostly disengaged. Ram air not being sufficient it would not take but a few seconds for over heating to occur.
Thermal sensing clutches are by far the number one choice in most applications. This clutch has a bi-metal coil spring at the front that reacts to temperature, the hotter the air temperature off the radiator the more lock up is applied. Usually this clutch is only working at idle speed, as the vehicle moves through the air the bi-metal spring cools down releasing the clutch. Thermal clutches use silicon as an internal lubricant because of its constant viscosity under wide temperature variations.
If you suspect that a thermal clutch is not performing as it should these items need to be checked:
Inspecting a thermal clutches condition should be done with the cooling system at full operating temperature. To check clutch lock up there are two ways.
#1 With the engine idling looking at the fan, have another person shut off the engine. The fan must stop turning within 2-3 revolutions, if it does not but continues to free wheel, replace it.
#2 With the engine stopped grab the fan blade and try to spin it by hand, if it only moves about half a turn its OK, normally if it spins freely replace it.
Check for oil on the front near the spring or on the rear at the shaft. If oil is leaking out the clutch is about to lay down, replace it.
With the engine shut off shake the fan tip fore and aft, if looseness is detected, replace it as the bearings are failing. Failed bearings can sometimes be heard before they fail but not in all cases, fans go through the radiator when a clutch comes off.
If you plan on installing one of the later model 5.0 fuel injected engines there are a few thing that you will want to know. Starting in 1980 some Ford engines used the serpentine belt arrangement. Most of this type accessory drive used a reverse rotation water pump and fan. If you are not sure which rotation is correct, standing in front of the engine standard rotation is counterclockwise, serpentine is clockwise. If you are mixing and matching parts to get this swap accomplished you can see that problems can occur. Ford made the water pump supposedly fool proof having a different size pilot pin on the shaft as well as a different fan to water pump bolt pattern. If rotation is backward your fan will be blowing back through the radiator and the water pump will be trying to push coolant into the bottom of the radiator. The pump does not work well rotating backward as the passages are not designed for efficiency in reverse. Other than these differences the water pumps will interchange and if there is a way to goof up someone will find it.
Shrouds
Compared to increasing core area on a radiator a fan shroud provides the biggest cooling improvement.
The job of the fan shroud is to make the fan pull all its air through the radiator core. Mechanical fans without shrouds tend to pull almost as much air from around the fan blade tips as they do through the core. With the shroud form fitted to the radiator and wrapped closely around the fan blade, efficiency is increased greatly.
The fan ideally should fit one half of the pitch depth of the blade into the shroud opening. If it is too far rearward out of the shroud air will be pulled from around the blade tips. Being too deep into the shroud can cause turbulence affecting airflow.
The deficiencies of the Fairlane 2-row cored radiators leave them scarcely able to cool a small block in light duty use. When these cars were new they had problems with over heating in warmer climites. Magnified with added options like, A/C, higher horsepower engines, more displacement, trailer towing, and lower differential gears, they are totally unacceptable. By adding a fan shroud, you can gain much better cooling from the stock radiator.
Early Fairlane shrouds are very hard to find. They were only installed on cars with A/C. Stock shrouds were made of sheet metal in two pieces, bolted together at the sides and using small screws to fasten them to the radiator mounting brackets. Some fiberglass shrouds are starting to be produced in the after market most of these are for big block cars. Crites Restoration makes some shrouds.
If you have noticed the tips of the fan blade they are curled back, this causes the air to move in a rearward direction instead of flying off the ends. This increases pressure behind the fan, rather than being slung radial outward. There are some chrome shroud kits that are universal fit, that may work. Other kits are partial shrouds, mostly covering the top half of the fan. I would call that a "finger guard" rather than a fan shroud. There are some universal shroud kits that may be massaged to work, but this is trial and error method. When attempting to fabricate a shroud the height and width are critical for fan clearance. Fairlane radiators have a fan well formed into the back of the top tank, the fan blade extends above the core height, this well must be in the radiator. Lower hose connections, called castings, are another tight area on the Fairlane radiator. These are the main reasons that the early Mustang radiators will not interchange with our cars, let alone the fact that they have approximately a 17% smaller core area and not a good choice.
Shroud to fan clearance can be affected by engine mounts that are weak or broken, allowing the engine to rock. Custom engine installations with home built engine mounts can change the water pump centerline as it relates to the shroud. Core supports, having been in an accident may be off location, bent, or modified. If installing an after market fan blade you should be aware of the shroud clearance, some factory installations only have 1/4" between fan and shroud.
If you have noticed the tips of the fan blade they are curled back, this causes the air to move in a rearward direction instead of flying off the ends. This increases pressure behind the fan, rather than being slung radial outward.
A word of caution about the small block shroud, it must be for a "FAIRLANE" type car, even though new shrouds are available for the Mustang radiators they are 1" shorter than what is needed, they will not fit.
Pulleys
With the high volume of sales by companies like March Pulleys and Brackets <marchperformance.com>, it is obvious that pulley ratios are something we can gain advantages from. Most of the kits sold today are of the under drive veryity. Their goal is to improve low end torque by reducing the amount of power required to pull the water pump/fan, alternator/generator, and even air-conditioning. Usually the power steering is not a concern as it is not being worked in straight line driving. The water pump usually does not lose much of its capacity to move coolant when under driven, the stock pump being very efficient.
Ford did play with earlier engine pulleys on the 352-390-406 HiPo cars, their purpose was not to gain low end torque, because they already had gobs of that, but to slow down water pump speeds. These engines cranked out some very high r.p.m., pump cavitation occurred at maximum speed. The pulleys on the high performance engines were under driven by as much as 20%, slowing water pump speed. When the 289 HiPo was introduced in the middle of the 1963 model year, it's 6500 r.p.m. redline was the highest of any small block at that time. Slowing the water pump by pulley ratio was an easy fix for the problem. Ford had another problem with the quick winding HiPo then, it had a habit of throwing the accessory belts off during sprints through the gears. Ford came out with a special performance belt to correct this, it was easy to distinguish from the rest having a black/white checker stripe back on it.
Pulleys come in varying stock sizes, swapping around pulleys can gain an under drive or an over drive ratio for your engine. Between 62 and 69 Ford offered water pump pulleys in sizes from 5 1/2" to 6 1/2".
Fan Spacers
Doing an engine swap or changing to a larger radiator you may find that changes in radiator to fan clearance is needed. Ford used varying lengths of fan spacers to accomplish proper fan location. Fairlanes used lengths as short as 1 1/2" or as long as 3 5/8", other Ford car/engine combinations had even longer ones. Full size Galaxie had a spacer of 4 1/4" using the 289 engine. In most cases we are dealing with a lack of fan clearance and requiring a shorter spacer. Even with 1 1/2" being the shortest (Mustang & Falcon V-8) available it is possible to fabricate even shorter spacers. Using a lathe, spacers can be modified in length as needed.
Mike the Radiator Man