Viscous-Torque limiting, just a fluid coupling that provides some drive all the time.

Viscous-Temperature-controlled (not thermostatic). Fan speed is increased as temperature increases.

Viscous-(Thermostatic) turns on and off at a predetermined temperature. The most sophisticated design, the fan "idles" and doesn't run at engine speed until needed.

The difficulty with using these is that temperature calibration is critical. According to Scott Leon, a former GM proving ground technician, production OEM clutch fans are normally calibrated to turn on when the coolant temperature reaches 220 degrees but calibrations vary. The average guy doesn't have the option of going into the parts store and asking for a 178-degree clutch, for a 16-inch fan. Even if he did, he would have to know what temperature cut-in he needed before he asked for it. Too low a cut-in and it would run all the time, too high and it will never turn on.

Electric Fans
Electric fans are especially effective at low vehicle speeds-and as they aren't powered directly by the engine, horsepower isn't lost to their operation. However, big electric or multiple electric fans can draw lots of current and that current has to come from somewhere. At some point the alternator has to provide the energy and that does require horsepower to produce. Nonetheless, as electric fans don't operate continually the total amount of energy needed to operate them is probably less than a mechanical fan.

Another advantage to an electric fan is packaging-in many cases they will fit where other fans won't. For the greatest effectiveness, electric fans should have a shroud, and they work better pulling the air through the core rather than pushing it.

Thermostats Simply stated, always use a thermostat. They ensure the engine reaches operating temperature quickly, provide the restriction necessary to build water pressure in the block, which eliminates steam pockets, and unless they malfunction, tossing them in the trash will not cure an overheating problem.

Two types of thermostats will be found, the poppet valve style and the sleeve style. The poppet style is either open or closed, while the sleeve style is a non-linear design and opens gradually. Sleeve style thermostats often eliminate the cycling often seen on the temperature gauge as a poppet thermostat opens and closes.

Radiator Caps
The radiator cap keeps the cooling system under pressure, generally somewhere between 12 and 15 psi. (Always follow the radiator manufacturer's recommendation when selecting a radiator cap). Pressure in the cooling system raises the boiling point of the coolant approximately 3 degress for every pound of pressure. Another benefit of a pressurized cooling system is the elimination of steam pockets in the water jackets.

Along with a spring that keeps a preset amount of pressure in the cooling system, radiator caps have a vacuum valve. When an engine is shut off, the coolant stops flowing through the radiator. However it continues to absorb heat from the heads and block. As a result the coolant temperature in the system increases as does the pressure created by the expanding liquid. The pressure often increases to the point where the radiator cap's pressure rating is exceeded and coolant is released through the radiator's overflow. When the system cools and the coolant contracts, air (or liquid) is drawn back into the radiator through the vacuum valve.

Overflow Tanks
Because cooling systems do, from time to time, burp some fluids, an overflow tank is a wise addition. Allowing the cooling system to dump coolant on the ground, then suck air back in means that eventually the radiator will be low on coolant. An overflow tank catches any coolant released; it's then sucked back in when the system cools keeping it cool.