Most stock radiator fans that are turned by the motor are made of metal with sturdy blades that are always blowing if the engine is turning. The aluminum or thin stainless steel blades on a flex fan act very differently. At low RPM, when the vehicle's engine needs cooling the most, a flex fan will blow sufficient air over the radiator. However, as the RPMs increase and the car goes faster, the fan speed increases as well on a stock vehicle, robbing valuable power and mileage. The blades of a flex fan will actually flatten out at a certain RPM (depending on the application), and therefore encounter almost zero resistance. Once the engine slows down again, the blades return to their natural shape and move air again. In this manner, power is freed up for the automobile, mileage is increased, and the fan is not unnecessarily blowing air over a radiator that is getting wind flow anyway.
A large number of vehicles with a fan attached to the motor will also have a fan clutch. The job of the clutch is to disengage the fan at a certain engine speed, because at that time there is sufficient air flowing over the radiator simply from the vehicle moving forward. A flex style fan is not as useful on a system like this, because the fan already disengages and stops spinning. The trouble with clutch fans is that over time, the clutch wears out, and the fan might stop spinning altogether. If the vehicle has a fan that works directly off the pulley, with no clutch pack, then a flex fan can be of great benefit.
Flex fans, like any other fan, have a maximum RPM range they can operate at, so be aware that if the fan will be installed in a race car application, you need to purchase a fan with a maximum RPM range that will match your engine speeds. If your engine came with a fan shroud, do not remove it when the flex fan is installed. Shrouds are an extremely important component of the cooling system. Also, not every flex fan will work with every size engine, so make sure that if you have a V8, a flex fan intended for a V8 is purchased, and the same for four-cylinder and six-cylinder applications. Some vehicles may require an after-market fan spacer so that things line up as they did when the stock fan was used.
