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When 2,500 cfm isn’t equal to 2,500 cfm

2012 January 11
by admin

We’ve had quite a few people question our advice when we recommend a 2,500, 2,800 or even 3,000 cfm electric fan for their hot rod or 4×4. They often tell us that they have a 2,500 cfm fan now and their vehicle overheats in low speeds or when they are stuck in traffic. A little more investigative work usually uncovers that what they have now is a basket fan, which leads us to this question: Did you know that 2,500 cfm doesn’t always equal 2,500 cfm?

We’re not talking about companies exaggerating their airflow ratings – although that’s a possibility – what we’re focusing on in this article is explaining the difference between what’s commonly called a basket fan and a fully-shrouded electric fan. Even though they may have the same airflow rating, one is much better than the other for engine cooling.

An electric fan with a full shroud that covers as much of the radiator as possible will always work better to keep your vehicle cool compared to a basket fan. There are several reasons for this.

Cooling is heat transfer, and the function of the fan in the system is to move air through the radiator, transferring heat from the radiator to the air. A fully shrouded fan covers more radiator core surface than a simple basket fan. A basket fan will move air that looks like a doughnut; only moving air in a tight circle around the fan blades, and there will be a hole in the middle of the airflow blocked by the fan motor. A shrouded fan that has the motor mounted 2-3 inches away from the radiator surface will pull air through the entire area. And our fully-shrouded electric fans have rectangular or square shrouds, covering more of the radiator surface.

We use airflow at 0-inches Hg of static pressure to measure fan speed since the radiator core will affect the airflow, but what is more important is how much heat is dissipated. Even though an electric fan may have the same or only a slightly higher cfm rating, the ability of a fully shrouded electric fan to pull heat out of a radiator is much higher. We tested a Flex-a-lite 16-inch fan assembly (Part #396) and the same fan assembly in a fully shrouded assembly (Part #180) and found 40-percent more heat dissipation!

Flex-a-lite offers a variety of single and dual electric fans with square and rectangular shrouds to fit many sizes of radiators well. We also realize that space constraints can be an issue, so we offer a line of slim-profile electric fans that still offer good shrouding, although the fan motor is moved closer to the radiator surface. Visit our electric fan applications page at flex-a-lite.com for a complete listing of fan sizes and cfm ratings to find one that will work best in your application.

Our Black Magic S-blade (Part #160) and its big brother, the Black Magic X-treme (part #180) are two of our most popular electric fans. The #160 moves 3,000 cfm of air the #180 is rated at 3,300 cfm, and both use a 15-inch blade. The cooling advantage of the Black Magic design is a rectangular shroud that covers more of the radiator surface and having the electric motor mounted farther from the surface to reduce an air eddy affect.

Our LoBoy fan (Part #118 for a puller and #119 for a pusher) also has a 16-inch fan blade and moves 2,500 cfm of air, only about 17 percent less than the #160 fan shown above. And it is only 1.25 inches tall at the shroud. It’s a great fan for tight clearances, but it will not work well as the primary cooling fan for your 1939 Ford with a 500-horsepower engine.

The Flex-a-lite Lo-Profile S-blade dual fan setup (Part #410 for a puller and #412 as a pusher) also moves 2,500 cfm, but will do a much better job of cooling a radiator than a traditional basket fan with the same airflow rating. Even though the airflow rating is the same, it’s spread out over a much larger surface area of the radiator. It’s ideal to cover at least 70 percent of the radiator surface with the fan shroud.