In contrast to the '66 L72,...
In contrast to the '66 L72, check out this '67 L88. It's common knowledge that these cars came without fan shrouds. The idea here was to discourage use on the street (as if a lumpy cam, 12:1+ compression ratio and a decided lack of creature comforts wasn't enough). Yes, it's neat to look at, but in practice, operating a Corvette on the street without a tight fitting shroud is just asking for heating trouble.
In the hypothetical world, a fan wouldn't be required if a Corvette was constantly driven at high speeds (definitely an enticing concept, but not too practical). Airflow from the vehicle's velocity would be sufficient to allow for adequate airflow over the radiator surface, with the result being proper cooling. After all, that's pretty much how World War II aircraft with liquid cooled engines worked (good examples being the P-51 Mustang, Spitfire, Warhawk, and so on). While this would be an ideal situation, it's seldom possible-or realistic. Because of this, a fan of some sort becomes a necessary evil.
Given the fact that a fan is pretty much an essential commodity, it's probably a good idea to install one that actually works. There are a number of good quality fans on the market, and they range from OEM, factory produced models to stainless steel flex versions, and of course, electric jobs that are so common on late-model Corvettes and used with regularity on many Vette Rods. The typical "standard" fan assembly is fixed. It rotates constantly with the water pump shaft. A thermostatic fan is just that-a fan that slows down when cooling requirements have diminished. This type of fan has seldom been used in North American passenger car applications or Corvettes (cost, size, and complexities being factors). Flexible fans reduce their pitch as engine rpm increases (or more correctly, as pulley speed increases). Fluid coupling fans (sometimes referred to as viscous clutch fans) speed up or slow down-again dependent upon engine or pulley speed. Viscous or "clutch" fans were regular fixtures on Corvettes for years. Electric fans are simply remote units that depend upon the vehicle's electrical system for operation. Dependent upon the application, they can be manually switched or can operate via an integral thermostatic coupling. Of course, late-model Corvettes all came factory-equipped with electric fans.
Horsepower is Horsepower
Certain types of fans require more horsepower for operation than others. Leading the pack in terms of least power absorption is, of course, the electric fan. They rely upon battery power to operate, but keep in mind that the battery will be drained when the unit is in operation. That means the charging system has to keep up (basically, the engine has to power the alternator demands instead of turning the fan, and it still must power the water pump and other accessories). In terms of conventional fans, both the flex fan and the clutch fan offer considerable advantages as far as horsepower losses are concerned. Obviously, a flex fan is far less complicated than its clutch fan stable mate, but it too has some drawbacks. Some flex fan designs simply take a "set" at a given position following sustained use. The result, of course, is too little fan action and, ultimately, reduced cooling. Depending upon the Corvette, one of the safest bets in terms of fans is the original equipment style clutch or fluid coupling system. When the clutch mechanism is in competent operating condition, the fan works flawlessly, declutching as the engine speed or pulley speed increases. The final outcome is more available horsepower when you need it.
When dealing with conventional fans, one area that you should think about is a phenomenon called blade stall. Similar to an aircraft propeller, the fan attached to your Corvette engine can in fact be turned too fast. A massive amount of turbulence is created that effectively decreases the airflow through the radiator. Obviously, overheating will be the consequence, but fixing the problem might be more difficult. The only real solution is to reduce the speed of the fan, which can be handled easily with a pulley that's a different diameter.
This is a very common arrangement...
This is a very common arrangement found on many Corvettes up to the C4 models. It's a viscous or "clutch" fan. This is the fan type that, in most cases, should deliver the best performance in any vintage Corvette. Yes, it's an OEM factory clutch fan. And it's certainly not a trick, sophisticated piece. In operation, the clutch allows the fan to speed up or slow down-again dependent upon engine or pulley speed. Because of this, the fan decouples as engine speed increases, more or less freewheeling at high rpm. Given the freewheeling nature, there's little power loss when the car is accelerating. In contrast, the fan couples up when the engine speed is low. By coincidence (or perhaps more correctly, design) this gives the engine the maximum amount of cooling when it's needed the most.
Take a close look at the fan...
Take a close look at the fan and shroud in this '66 L72 Corvette. Two things are important: There's a shroud in use, and the fan blade tips are actually very close to the edge of the shroud. In effect, this leads all of the available air through the radiator.
Late-model C3 Vettes were...
Late-model C3 Vettes were jammed with hardware. The fan was still crank driven, and obviously, positioned behind the radiator. The trouble here is there is so much stuff inside the engine compartment, there's little room for airflow. This would eventually change as the Corvette matured.