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Electric fans are an often discussed modification on the various RX-7 forums. Unfortunately, it seems that there is also a lot of inaccurate information surrounding the "e-fan" and how it should be installed. This document will hopefully answer some of the most common questions with regards to e-fans and the 2nd gen RX-7 (though much of this applies to the 1st gen as well), and debunk some of the myths. This article makes no claims as to it's validity when applied to other vehicles. I get a lot of angry email from various automotive forums where this article is posted saying how wrong I am, yet every one of those people seem to miss the fact that this article is specific to the 2nd gen ('86-'92) RX-7.
Before we begin, let's just take a general overview of the cooling system on the RX-7. Forget for a moment that there is also an oil cooler. We will just concentrate on the water cooling system.
The cooling system consists of 5 basic parts: the engine, the water pump, the radiator, fan and the thermostat. Water passages inside the engine are filled with an ethylene glycol/water mixture (commonly just called "coolant"). These passages allow the heat that is generated within the engine to transfer into the coolant. Coolant is circulated through the engine via a water pump, which is driven from a belt off of the main eccentric shaft pulley. The stock fan and fan clutch assembly also mounts off the shaft of the water pump, and is therefore driven by the engine as well. The pump's job is to circulate water though the engine and the radiator. Air flowing over the radiator cools the coolant, and thus the engine. A thermostat is used to open or close flow of coolant through the rad to maintain the engine temperature at a constant (approximately 180 F). It is very important to realize that it is the thermostat that regulates the temperature of the engine, and not the fan. You can install the largest fan possibly and move as much air through the radiator as you want yet the temperature of the engine will remain the same as set by the thermostat.
Now that we know how the cooling system works, let's be clear about the function of the fan, and how the clutch assembly works. As mentioned, the stock clutch fan is attached to the water pump shaft. This shaft receives it's rotation from the engine itself. The fan is not directly attached to the shaft, it is actually bolted to a viscous clutch. The clutch then sits between the fan and the shaft. The purpose of the clutch is to keep the fan at a (fairly) constant speed, and to only engage the fan when it is needed. Think of the clutch as a very simple mini torque converter. Inside is an impeller that is connected directly to the input shaft. This impeller rides on a bearing so that it can rotate independently of the clutch housing. The housing has a disc that sits opposed to the impeller. This disc is controlled by a bimetal thermostat so that as the air flowing over the clutch heats up, the thermostat moves the disc closer to the impeller. Filling the entire housing is a thick silicon fluid (or sometimes a heavy gear oil). As the disc moves closer to the impeller, they become coupled by the swirling of this fluid. Because the disc is splined to the housing, the housing begins to rotate and thus the fan (which is bolted to the front of the housing) spins as well.
As cooling air flows over the clutch the thermostat relaxes and moves the disc away from the impeller. This breaks the coupling and allows the clutch to slip and finally freewheel. Similarly when you stab the throttle or when the engine is operating at high RPM there is not enough force coupling the impeller to the disc and the clutch slips to keep the fan operating at a constant speed and to avoid robbing power from the engine.
The (very crude) image below illustrates the assembly. A more detailed disassembly of a fan clutch can be found at Jaguar V12 Fan Clutch.
What's important to note is that the fan is only used when the engine is idling, or the car is creeping along at low speed (less then 20 MPH). If the car is stationary or creeping, there is not enough natural airflow through the rad to provide adequate cooling. Thus, the necessity of a fan.
Now that we know the basic operation of the cooling system and fan, let's look at the electric fan.
By far, the most common misconception is that the electric fan cools better then the stock. People see the high CFM numbers in the catalogues (for example, the Black Magic is advertised as pulling 2,800 CFM, and the Perma Cool "finger chopper" is advertised at 2,950 CFM) and are impressed. The truth is that no one actually knows the CFM rating of the stock fan. It has simply never been measured by anyone but Mazda. Some estimations have been made that it is approximately 2900 CFM at 2600 RPM using software modeling. But as these are estimates we can never know for certain how Mazda has rated this fan. All we do know is that it is rated at "enough". And to be honest, who cares how much air the stock fan moves? It is more then adequate as long as it's clutch and the rest of the cooling system is in good shape. The steeply raked blades of the stock fan move quite a lot of air, even if it turns slower then an aftermarket electric unit. It's also a bit irrelevant how much capacity the stock fan has as compared to aftermarket fans on a modified car because the fan is only used during idle and light-load situations. The fan to cool a 400HP engine is the same size fan as would be used to cool a 200HP engine since both engines are producing the same amount of heat during idle and light cruising.
Many people also seem to believe that an electric fan will cure their "at speed" overheating problems. Recall that the fan is not even used when at speed, as the natural flow of the of air through the radiator is more then enough (much more then a fan, in fact) to keep the car cool.
Of course, you're not necessarily after better cooling. You've seen those TV commercials or catalogue ads that promise "Up to 17 free HP!" by converting to an electric fan. Well, that's simply not true. There will be a horsepower gain for sure, on the order of 0.5 - 1HP. But remember that the electric power to run the fan must come from somewhere, and that somewhere is the alternator. Electric fans draw quite a lot of current. Most pull surges of 35A or more to start up, then run at 8A-10A. This puts extra load on the alternator. Ever pedaled a bicycle with a generator powered light? If so, then you will know that as the electrical load on a generator/alternator increases, the generator/alternator becomes harder to turn. Suddenly, all that "free" HP you just freed up is once again being used. This time, though, it is being used to turn the alternator. There is also an inefficiency in the whole system. We are converting mechanical energy, to electrical energy, to mechanical again to run the fan. This very inefficient, much less then just driving the fan directly via the mechanical energy of the rotating shaft.
Let's examine the "17 HP!" claim a little further to drive the point home. A little basic math is all that's needed.
One horsepower is 745.69 watts.
Therefore, 17HP = 12,676.89W.
Since W = Voltage * Current, 12,676.89W is 1056 Amps
So in order to do the same work as that stock "17HP" fan, an electric fan would have to continuously draw over 1000 A from the electrical system of the car. Clearly this does not happen. Now I'm sure that somewhere out there is a car or truck with a ridiculously inefficient and so poorly designed stock fan that it actually does rob 17HP from the engine as it turns. But since the RX-7 comes with a well designed freewheeling clutch fan as already explained, the RX-7 is certainly not that car.
There is a dyno chart floating around the RX-7 forums which supposedly show a car gain 10 HP across the board with no other change besides switching to an electric fan. I seriously question the validity of this claim as the 10 HP gain was shown throughout the entire RPM range, all the way to redline. As we all know, the fan clutch should have been disengaged after about 3000 RPM, so it appears that this car was dyno'ed with a stuck fan clutch.
We can do the same thing with regards to an electric fan and factor in the inefficiency of the alternator and the fan motor. Assuming it takes 1HP to turn the electric fan:
745W = 62A at 12V. This is obviously already incorrect since it's ridiculous to assume that an electric fan is drawing 62A continuously. But we'll continue anyway.
Assuming that the alternator and fan motor are both 70% efficient (an over estimate for sure considering the quality of automotive electrical parts...see Alternators) we need to take those losses into account.
So 62A + 30% = 62 + 18.6 = 80.6A
Then add another 30% for the inefficiency of the fan motor: 80.6A + 18.6A = 99.2A
If our e-fan really saved 1HP from the shaft of the engine, it would be drawing almost 100A from the electrical system of the car to do the same work! We can see that this is not the case, especially in a car with a 70A/90A alternator.
We can see that the electric fan is a large current draw. The '86-'88 alternator is rated at 70A, and the 89+ unit is rated at 90A. Real world tests have shown that actual output is somewhat less then the specifications would indicate. Add the load of the electric fan, and you are that much closer to overloading an already factory-stressed electrical system.
While on the subject of the electrical system, let's look at how the fan is actually connected. The proper way to control an electric fan is via a thermostat and relay combination (fused, of course). Unfortunately, very few people actually do this. Many installations simply connect the fan to run all the time, or to a switch inside the car. In the latter case, your brain actually becomes the thermostat. Needless to say, this is a very poor arrangement as it is easy forget to switch on the fan as the temperature climbs. Worse still, the 1989+ temperature gauge is horribly inaccurate. By the time you see it move, the car is already too hot. Leaving the fan on all the time presents a different set of problems. It becomes an unnecessary drain on the system, and is totally unnecessary at speed. Thus all that "advantage" the electric fan has over the stock fan disappears. This is not necessarily a flaw of the fan itself, but of the installer.
Lastly, there is the question of reliability. The stock clutch fan can have two failure modes: the clutch will fail, or the fan will physically break. The electric fan introduces many more failure points: fuse, all wiring connections, physical failure of the fan, failure of the motor, failure of the thermostat.
By now, you're probably wondering "Gee, why is this guy so against electric fans?". Well, I'm not really. But I recognize their disadvantages. There are also some useful advantages to the electric fan.
The subject of fan efficiency has been ignored until now as it can work both ways. The majority of a fans efficiency is dictated by the shape of the final few inches of the blade and whether it is ducted or not. Here, electric fans can have a significant advantage. It is hard to say what the efficiency of a fan blade is without aerodynamic testing. A stock fan may have very efficient blades while an electric fan has poor blades, or the stock blades might be horrible compared to the electric fan you are looking at. But a big gain in efficiency with an e-fan is in the fact that the blades are more easily ducted. Because the electric fan drive motor is mounted directly to the same frame which mounts the shroud, clearances between the fan and shroud can be much smaller then that of an engine mounted fan. The smaller the clearance between the tip of the fan blade and the shroud, the more efficiently the fan will move air. Some fans go a step further and have a ring around the outer circumference of the fan on the blade tips which rotates very closely to the shroud. This is a very efficient design.
Since you are no longer tied to the shaft of the water pump, the electric fan can be oriented in any way, and moved around to make space. Without the stock fan shroud in the way, there is much more room in the engine bay, as well as room to route things like intercooler piping, cold air intakes, etc. An electric fan may also be required when going to certain aftermarket radiators that do not have provision for mounting the stock fan shroud.
The electric fan can also be left on after the engine is shut off. This allows continuous cooling to minimize heat soak. A small turbo timer like device can be used to run the fan for a preset amount of time. The 3rd gen RX-7 actually does this from the factory, but that is about the only good point in the 3rd gen cooling system. I run an electric fan in my 2nd gen and find it very useful at the track between runs to keep the engine bay cool.
Perhaps the greatest advantage of the electric fan is cost. It provides an affordable alternative to replacing a failed stock clutch fan. The stock unit is very expensive when purchased new, so it is possible to save several hundred dollars by purchasing an electric fan instead. However, this advantage is not as great if you have a decent wrecking yard in your area. Used clutch fans are very cheap.
There you have it. For the most part, electric fans are not recommended as an upgrade unless you have a very specific reason to do so. They may provide inferior cooling to the stock fan, and may create other problems if improperly installed. If you want to install an electric fan, I have done a basic writeup of the generic process. Read How To Properly Install An Electric Fan.
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