[PKRWUD]

Bigger injectors or a big cam with stock heads.

Hissing Cobra-
I just wrote a lengthy explanation on the whole electric fan thing, and it's reported hp increase, over on F-150 online. Here:

Donate is correct about the source of the added power. The reason it isn't usually very noticeable is because of the way the stock fan clutch works. The stock fan clutch puts minimal drag on the engine most of the time. It's only at slow speeds when hot that it creates a drag. If the horsepower of an engine was measured on a stationary dyno, with a hot engine, there would be a substantial difference between a stock set-up and an electric fan. However, in most daily driving, the difference is minimal. It works something like this:

The stock fan is connected via a fan clutch. A fan clutch is kinda like a torque convertor, in that the two ends don't ever come into physical contact with each other, but have a liquid between them. This is why you can spin the fan by hand when the engine is not running. The oil inside the clutch "gels" when it reaches a certain, predetermined temperature. It reaches this temperature due to the temp of the air that hits it after passing through the radiator. When the temp rises to a certain point, the oil "gels", and the fan becomes temporarily "solid" with the pulley to which it's attached. This creates a vacuum inside the fan shroud which pulls more air through the radiator, which radiates the heat away from it, allowing the coolant to cool down, thus cooling the engine. In this condition, there is a substantial load placed on the engine in order to turn the fan. When the temp of the air coming through the radiator drops enough, the oil in the clutch becomes more fluid, and the fan just "free-spins" with the engine. In this condition, there is minimal load placed on the engine from turning the fan. Once you are moving more than 10 miles an hour, there should be sufficient air moving through the radiator to keep the temp down so that the oil in the clutch remains a liquid. However, once you come to a stop, or reduce your speed below 10 mph, there is no longer enough air being forced through the radiator, and the oil "gels" and the fan cools. Start moving again, and the clutch oil becomes liquid again. The bottom line is that the only time the fan is in use to the point where it robs power from you is in stop and go traffic, or when sitting still at idle. This isn't a time when peak horsepower is needed, so the loss isn't really noticed.

An electric fan is also thrmostatically controlled by the radiator temp. When it reaches a certain point, the fan kicks on, and sucks air through the radiator. Once the radiator cools, the fan is turned off. Because of this, an electric fan is almost never in use when driving more than 10 mph.

The advantages to an electric fan are many, however. They can have a manual switch added so that you can turn it on at will, although this is rarely necessary. They also can remain on after the engine is shut off, continuing to cool the engine and radiator while you are parked. They also help you when towing, or other times you need top power at very slow vehicle speeds. The down side to electric fans is their lifespan. Most aftermarket electric fans won't last more than 30 or 40k miles, and they often don't give any warning before quitting. Also, if not installed properly, they can drain your battery, or even cause a short in your electrical system. This usually happens when someone installs a manual override switch, and doesn't wire it through a relay. The switch quickly overheats and melts, and you've got a short on your hands.

A stock fan clutch is usually good for 100k miles or more before the oil inside starts to lose it's viscosity, and starts to fail. Also, this will be a gradual failure, so you won't be suddenly stranded somewhere with a fan failure. Most really enthusiastic stories I've heard about an increase in power after installing an electric fan(s) were cases in which the pullies were changed at the same time, and the owner has just decided to give the power increase credit to the fan(s) instead of the pullies. Remember, though, they do offer increased hp, but it's usually at a time when you don't really need it.

Also, however, a good, well installed electric fan system will yeild a noticeable improvement in cooling at idle. When those fans kick on, they work quickly. They also put a heavy load on your charging system at a time when the alternator is generating the least amount of energy, so if your charging system is marginal, forget it.

When my fan clutch starts to lose it's reliability, I will probably replace it with a pair of electric fans. Maybe sooner. I do alot of towing, and the extra power while still cooling well would be nice.

Take care,
-Chris