Griffin
If the car still has a properly operating mechanical clutch fan, it will pull more cfm than an electric fan, and consideration should be given to keeping it. If the mechanical fan clutch is not working properly, then it should be replaced or an electric fan can be used. Keep in mind that the only time a fan should be necessary is when the car is standing still. If an electric fan will keep the car cool while at idle, then an electric fan is fine. Today's electric fans are producing more cfm than ever, and 3,000 cfm is common.
Mattson's
The engine-driven fan is great for higher speeds. We recommend one if you have a vehicle that does a lot of towing and driving at highway speed. It's a good idea to use the engine fan. If a vehicle does a lot of idling, cruising, or driving slow, the electric fan on a shroud is the way to go.
U.S. Radiator
Why rely on another operating system (that being electrical) if you don't have to? Mechanical fans turn when the motor turns. However, we highly recommend a shroud properly fitted to the fan and radiator. The only time you really depend on a fan is at idle or low speed where there is little or no air flowing through the grille. Shrouds are necessary to maximize the amount of ambient air being pulled through the grille and radiator. Proper fan and shroud alignment should have the leading edge of the fan 1/3 in, and trailing edge 2/3 out. The airflow off the back of the fan deflects at about a 45-degree angle when set this way. When the blade extends farther into the shroud, the air off the back of the blade flows straight back into the block and decreases the airflow efficiency by about 15 percent.
If an electrical fan is the only way to go, place it on a shroud that covers the entire core. We often see an electric fan attached directly to the core and the only thing this does is waste the rest of the core surface when you need it the most. A 16-inch electric fan attached to a core only cools a 16-inch circular section of that core.
Walker
Mechanical fans with fan shrouds work well. The main problem with a mechanical fan is that you should never use less than 17 inches in diameter and never without a fan shroud. We have found the engine will take longer to recover with a mechanical fan rather than with an electric fan when you have to make a sudden slow-down or stop in extreme temperatures at interstate speeds.
Electric fans work well if they are properly sized. As a rule, you should never use less than a 16-inch-diameter blade. Hook design blades are more efficient and are quieter than a paint stir stick design.
8. How much pressure should the system be under?
Be Cool
It depends on the application. Under normal street conditions, the cooling-system operating pressure will be under the rating of the radiator cap. All Be Cool radiators are designed to function with a 13-psi radiator cap. If the cap allows the radiator to vent frequently, there is another problem with the cooling system. The radiator may be too small, the electric fan may not have the correct flow rate, or the water pump may be incorrectly sized.
Flex-a-lite
In older vehicles without a coolant-recovery system, the pressure can be as low as 7 lbs. The system on these older cars would expand, forcing the coolant out and then leave a cavity in the top tank after they cooled. In vehicles with a coolant-recovery system, the pressure can be about 16 lbs.
Griffin
Up to a 17lb cap can be used for street use. Circle-track racers run between 22-29 lbs. For higher pressures, such as Nextel Cup or Busch Series cars, Griffin manufactures the Survivor Series, which will handle 50-60 lbs.
Mattson's
It's always best to use the factory recommendation; however, there are sometimes no recommendations available to the customer for the modifications to old cars with late-model engines. The radiator cap has a spring that will lift up and allow the fluid to escape through to the overflow. The different-pound caps will lift up at different temperatures. A 13lb cap is most common, with a 16lb cap raising the boiling point even higher. A 7lb cap is common in the older vehicles. If a higher-pressure cap is used on the wrong system, it may make the radiator tank come apart.
U.S. Radiator
By increasing the pressure by 1 lb, we increase the boiling point by 3 degrees; so, by running a 12lb cap, our water won't boil until it gets to 248 degrees. An engine that wants to run at 248 degrees will open that cap up long before it gets that hot.