1/9/2006 - Tech Articles

How to Choose the Right Fan

It’s the middle of summer, it’s 90-plus degrees, and you’re stuck in some construction zone. Your engine is showing its displeasure by pegging the temperature gauge way into the red. You know the rest of the scenario—clouds of steam, puddles of coolant, and a long wait for a wrecker.

Don’t let this happen to you—pay some attention to your vehicle’s cooling system now. Flush and refill it with a 50/50 mix of antifreeze and water. Check all belts and hoses and replace as necessary. Replace the thermostat. Add a cooling system treatment like Red Line’s Water Wetter. Have the radiator checked for buildup; if it is too far gone, replace it withnew one. If you've hopped up your engine, consider a larger capacity radiator, preferably aluminum.

Clutch Fans
The clutch fan was the factory's choice for many years. Usually controlled by a thermostat,it uses a clutch to engage or disengage the fan at a specified engine speed or temperature. The fan essentially operates only when engine temperature gets too high, or at slower speeds when more airflow isneeded to cool the engine.

The downside to the clutch fan is that the clutch never fully disengages--it keeps the fan spinning at about 30% of water pump speed. That drag costs horsepower and shortens water pump and belt life. The clutch also limits how fast the fan can spin. Most clutches can spin a fan up to about 2,800 rpm. Even below 2,800 rpm, the clutch only turns the fan at 60% to 90% of water pump speed, depending on if it's a standard or heavy-duty clutch.

The temperature at which the fan engages is fixed as well. The clutch reacts to air temperature in the engine compartment, not actual water temperature. For example, if the clutch engages at about 170° F, the fan will typically engage around 210° F of water temperature, which may or may not be when you want it to engage. And when the fluid within the clutch breaks down or leaks out, it will no longer engage the fan and has to be replaced.

Clutch fans are ideal for mildly modified street vehicles. Flex-a-lite makes fans and heavy-duty clutches. The fans have a blade design that flows more air than a stock fan. The clutches have a patented silicone fluid reservoir for longer life, and a factory seal to prevent leakage. Flex-a-lite offers clutches controlled by engine speed or temperature, plus a heavy-duty temperature-controlled clutch for RVs and tow vehicles.

Flex Fans
If you need more cooling capacity than a clutch fan can provide, the next step up the ladder is a flex fan. A flex fan is a belt-driven fan that replaces the stock fan. Because it doesn’t use a clutch, it can run at 100% of water pump speed for maximum cooling.

Flex fans offer other advantages. The blades flatten out at higher rpm to reduce horsepower-robbing drag. And because a flex fan is lighter than a clutch fan, water pump life is increased.

Electric Fans
Electric fans are probably the ultimate in fan selection. They provide constant cooling and maintain their rated airflow (measured in cfm) at all times. Electric fans are not engine-driven, so there is very little horsepower loss compared to belt-driven fans. They also take the entire load off of the water pump, vastly reducing wear.

Electric fans are versatile, too. They can be mounted in front of or behind the radiator, are available in sizes from up to a gigundeous 20 inch diamter, and can be had in dual fan arrangements with airflow ratings over 5,000 cfm. You use an electric fan as the primary cooling source, or as an auxiliary unit with a belt-driven flex or clutch fan.

Electric fans are easier to control, too. For example, Flex-a-lite’s Black Magic electric fans come with an adjustable (180° and 240° F ) thermostat that directly senses your engine’s water temperature. The fans can be equipped with an optional air conditioning relay or a manual override switch that lets you control fan operation from the driver’s seat.

Flex-a-lite rates their electric fans by horsepower and airflow levels—just choose the one that is rated the same as or higher than your vehicle’s power output or airflow requirements. When it comes to custom or high power (300 horsepower and up) applications, you’ll probably need to run a dual fan arrangement. Flex-a-lite has those too.

No matter what your cooling needs are, there is a Flex-a-lite fan that can help meet them. Your choice comes down to how much airflow you need, the amount of horsepower your engine generates, and what type of fan you require. It’ll go a long way to keeping your vehicle cool—and off the wrecker’s hook.