Updated on 2/9/08
Introduction
Here you will find information about electric fans that you should be aware of before buying one.
There is dyno proof of up to 10+ rwhp gain from removing the clutch fan.
Electric Fan Related Links
About Electric Fans
The first thing to remember about electric fans is they are primarily used at low speeds, meaning that while you are cruising around at 35+ mph, the electric fan should not turn on at all. This will vary depending on outside temps. This is because at speeds high enough, air flow through the radiator will supersede the amount of CFM the electric fan could move. If your fan does come on, then you need to make sure that all your cooling system parts are performing exactly they way they should.
Please see [Cooling System Overview and Troubleshooting]
The specific airflow of the stock mechanical clutch fan is unknown though an estimate has been made (See
[S4 Fan CFM Estimation]), it is generally believed that the stock fan pulls more than enough CFMs for most applications. In fact, the stock fan is more advanced than most aftermarket electric fans because it ramps its power consumption up and down as engine heat necessitates. The viscous clutch uses a fluid that gets thicker as it gets hotter, gradually allowing less and less slip between the engine and the fan; it will never completely disengage, it always has some parasitic drag and does consume some power. This contrasts with most electric fans which are either fully on or fully off.
Furthermore, electric fans may be less efficient because their energy source went from mechanical to electrical and back to mechanical again; converting energy types is rarely efficient, and one alternator company suggests using 85% efficiency as a rule-of-thumb. Combine that 85% efficiency with the efficiency of an electric fan converting electricity into mechanical movement, and you get an overall efficiency of perhaps 70%
(see "Relating Current Draw to Horsepower Loss" below). The stock fan has no such energy conversion.
That said, there are some benefits to electric fans;
First: Since the stock fan attaches to the water pump pulley, it can cause belt slip on the pulley, especially when the air pump is removed.
Second: Electric fans tend to be far more compact than the stock fan, and make the engine easier to work around.
Third: The stock fan needs the stock shroud to work properly, and keeping the stock fan and shroud is sometimes impossible when switching radiators, piping intercooler piping, installing a V-mount, or doing any other modifications to the front end
Forth: The clutch fan will have to spin up when accelerating which will consume even more power. Remember you have the rotational inertia of the fan assembly plus the amount of drag the fan its self has.
With an electric fan, you will not have nearly as much HP consumption above 1500rpm especially when the fan is powered off since you now have eliminated the drag and rotational inertia of the clutch fan, plus you don't have the extra load on the alternator. Remember that while the fan is running you have the load on the alternator which will actually be more strenuous for an idle engine since the majority of the electric fans consume about .30-.50 hp. This is way more then the clutch fan at idle and below 1500rpm.
Electric fans can be far more efficient than a clutch fan when it comes down to CFM vs HP consumption.
Will an Electric Fan Solve my Cooling Issues?
Aside from slipping belts on the water pump, bad clutch or a broken fan, probably not but this depends on the modifications, if any, you have done to your engine.
The Good about electric fans are that when they turn on, they are moving as much air as they possibly can which if enough, can help greatly when you have cooling issue at low speeds at low rpms where the clutch fan is less effective. At idle or lower speed cruising, the amount of CFM the electric fan moves could easily be overkill considering the amount of HP the engine is producing at such times. This can also be debated further when you have multiple heat exchangers for the airflow to pass through, such as A/C condenser, FMIC, Oil Cooler, radiator and any other obstacles. The electric fan must be able to move enough air flow through the radiator and other heat exchangers to properly cool the engine. At higher rpms the clutch fan has a greater possibility of moving large amounts of air through all heat exchangers but will also take more HP to do so.
The Bad about electric fans is when they turn on, they spin up as fast as the possibly can. This creates a current spike which on some motors may cause the idle to dip down 100-300RPM for a few seconds and only come back up about 50-200rpm till the electric fan turns off. On some motors, this will cause the engine to stall out. The reason for this is the amount of current the alternator has to create which in-turn, consumes more HP. The engine makes very very little HP at idle so a 10-15amp may be enough, depending on how the engine is tuned at idle, to stall the engine if there is no electronic idle air valve being used.
Most of the time you don't need the electric fan to turn on fully while idling or low speed cruising. So a PWM Controller will work good in a case like this to control how much current is available to the fan at a given temp. Some PWM controllers allow the fan to spin up and then slow down to a desired set speed. In this case, you will still get the current spike. Some other PWM controllers don't allow the fan to spin up, so in that case , you will not have the large load put on the alternator at once and instead, the fan will spin up slower and stay at a preset speed till the desired 2nd HI Temp or Off Temp setting is reached. Keep in mind that the slower your run the fan, the less CFM it will move. Idle or slow cruise may be the only situations where you can slow the fan down and still have proper cooling.
Relating Current Draw to Horsepower Loss
When electrical power is needed from the alternator this means more mechanical power needs to be taken from the engine. Why? Well the alternator doesn't generate power, it converts mechanical energy to electrical energy.
Assuming the alternator converts mechanical power to electrical power with 85% efficiency (i.e. to get 1W of electricity you need ~18% more mechanical power, or 1.18W), you can approximate the horsepower requirement for an electric fan as follows:
Electrical Fan Power * 1.18 = Mechanical HP Requirement from Engine (HP loss)
(Amps x Volts / 745.7) * 1.18 = HP loss
For example, the popular Flex-a-Lite Black Magic 150 takes a rated 14 Amps at 13 Volts -
the electrical system's voltage when the alternator is running - resulting in roughly 0.3 HP loss. Thankfully, this isn't much, so HP loss due to an electric fan swap will be minimal.
Interestingly, there is one more efficiency to take into account when comparing
CFM (instead of current draw) to horsepower loss. Because the electric fan has to convert electrical power into mechanical power, there is another efficiency factor of perhaps 85%, giving an overall efficiency of mechanical-to-electrical-to-mechanical of 0.85 * 0.85 = 0.72.
Other Cooling Considerations
ShroudsShrouds ensure that the entire area of a radiator is used for cooling when the fan is on. To have maximum cooling efficiency, a shroud is recommended, be it on the stock fan or aftermarket electric fan.
Shrouds will allow the electric fan to run more efficient, since there is more surface area for the fan to utilize, the static pressure within the shroud will drop, increasing the amount of CFM the fan can move and could lower the amount of current the fan uses.
While driving at high speeds, the shroud makes zero positive effects on the cooling efficiency since it could actually cause more resistance with the air flowing through the radiator. The actual effect can go without notice.
The Belly PanThe belly pan is not just for keeping debris out of the engine compartment, it also helps the cooling system by increasing the airflow through the radiator by not allowing air to flow under the heat exchangers as long as there is a seal between the belly pan and heat exchangers.
Electric Fan vs Stock Clutch Fan Summarized
The electric fan and clutch fan both have their pros and cons. The electric fan will be over kill for most engines at idle or low speed cruising. An electric fan that pulls 15A will consume about .40 hp with a 50% efficient alternator so while the electric fan is powered, it will also require more mechanical power from the engine then the clutch fan during normal driving conditions. When the electric fan is off, you have zero power loss and only gained any amount of power that could have been taken by either the e-fan or clutch fan.
Mazda designed the stock clutch fan system to work with the rotary engine and it works very well and does not need to be replaced with an electric fan if there is nothing wrong with it or if the extra space is not needed.
The amount of HP or HEAT the engine creates at the times the clutch fan needs to be used is very small so you should not need MORE CFM then what the stock clutch fan is capable of.
If you have cooling issues, you probably should look into other areas of the cooling system.
The electric fan is capable of more cfm for less mechanical power. The stock clutch fan would require about 2hp to move 2500 CFM of air. An electric fan will only require about .30-.50 hp to move the same if not more air.
Stick with the stock clutch fan if you are not having issues, and if you are, check the rest of the cooling system. An electric fan may not solve your cooling issues. The only times I can see an electric fan being more beneficial is at low rpms where the clutch fan is much less effective, if you have a FMIC, or if you are looking for more RWHP.
See Also...
Aaron Cake's website is also informative on this subject.
[»http://www.aaroncake.net/rx-7/efaninstall.htm»]
[»http://www.aaroncake.net/rx-7/efanmyth.htm»]
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