Do Dual Fans Increase Radiator Efficiency More Than a Single Larger Fan?

  • #1
TL;DR Summary
Are Two Fans Better Than One?
A while back, you guys helped me out with a project on my '69 Camaro. It was all about the Bernoulli principle and airflow taking the path of least resistance. I'm happy to say that the lower closeout panel I built (see pix below) seems to have done the trick as the car I no longer having any sort of overheating issues. I did install a more powerful single 19" fan to replace the two 12" ones and that put it over the top. And that's the basis for my new post... CFM.

Lets have some fun OK? Say you have a radiator core of roughly 23 X 17 which means 391 square inches of surface with a core depth of around 2.75". Using my Cold Case BBC radiator as the general example (actual core is 23" X 16.85"). Hard to calculate the actual cooling surface because its not only there size, plus the 1.250" flattened internal tubes, but then you have 16 fins per inch on the tubes and you'd have to measure the fin sizes as well.

But generally speaking a single 17" OD fan that's rated at 1750 CFM at zero static pressure (no shroud or radiator) is probably going to pull a greater percentage of air within its diameter when mounted to a shroud/radiator... right? That 17" OD equates out to 227 square inches. That means that about 2/3rd of the core/shroud is getting the most pull. I'd guess that the other 1/3rd is progressively getting less pull the further away from the fan blades it is.

So if we switch to a dual 12" setup with the same size radiator core and shroud, the total square inches of the two fans is almost exactly what one 17" fan is... 226 square inches. So while the square inches are similar, the circumference of the dual 12" fan setup is about 76 inches while the single 17" is only about 53 inches. Maybe that means something?

In the close quarters of the radiator/shroud/fan assembly I still have this nagging thought that two 1750 CFM fans will pull more air through the radiator than the single 1750 CFM fan because they would have shorter distances from the fan blades to the edges of the core and therefore promote better cooling. But everything is relative, and if the 17" fan had a more powerful motor and was rated at 4000 CFM it would obviously be the winner. There's so many variables in this theoretical discussion that its hard to conclude anything LOL. So here's a crude diagrams showing the comparison.

As my enlightened gear-head friends here in the Physics Forum explained to me a while back, air will follow the path of least resistance, and if helped along by one or two puller fans in the "controlled airspace", a greater volume of air will move through that controlled airspace. The controlled air space in this case being the finned openings in the radiator core followed immediately by the fan shroud, followed immediately by the puller fan(s), followed by the void of the open (underneath) engine bay.

Another question was raised about stacking one puller on top of another which may not be practical (space) or productive. My layman's understanding about fans is that air doesn't always flow exactly straight back from an angled fan blade, so turbulence might show its ugly face if one tried stacking two fans. There are helicopters and conventional aircraft with counter-rotating blades but I don't know if the distance between blades is fractions of an inch or several inches and whether this would work in an automotive environment.

This thread has my head spinning LOL. Which setup (single 17" OD 1750 CFM vs dual 12" OD 1750 CFM) will actually move a greater volume of air through the radiator core? Thanks in advance for your thoughts!

(Retired Industrial Designer)
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  • #3
Ranger Mike said:
That was interesting reading and I've seen similar thoughts over in the PC Overclocker's groups over the years when trying to figure out how to cool the CPU. I have an old Shuttle PC laying around with a sort of radiator + fan as a cooling solution. One post in that thread did get my attention:

"A fan that is not designed for it will not work efficiently in series. You are much better off arranging the fans in parallel. Your airflow will not have a higher velocity, but it will have a higher volume."

So does "higher volume" mean more CFM? When the poster suggests "Parallel" I take that to mean side by side, but on a PC (if that's what he was thinking) I haven't seen any side by side cooling solutions. And in the case of automotive eFans, I have seen plenty of single and double eFan setups. Some doubles even have two different size fans, and I'm guessing they are CPU-controlled to possibly run on one fan until temps rise above a certain threshold and then the 2nd fan would be powered on.

The question still remains: Are two Fans Better Than One?
  • #4
MikeMass said:
The question still remains: Are two Fans Better Than One?
Are two water pumps better than one if both setups produce the same GPM?

Unless one setup creates some vastly different turbulences in the inlet or outlet ducts compared to the other, I would expect them to produce very similar flows.
  • #5
Pardon me for being an ignorant hillbilly but the reason you even have an electric fan is to cool the engine in traffic, during idle when stalled in traffic.

The old muscle cars had clutch fans that cut off when the car was moving. Ifin you got enuff air flow to cool an engine idling, good job. One or two fans? Big deal. Problem resolved once you started moving. If it is not broke, don't fix it. Granted, electric fans save parasitic drag by 2 to 5 HP, plus weight savings of the whole heavy mass, and rotational drag required to drive them. Pulleys, belts etc.

To your question of two vs one electric fans, one major factor is the result of two fans generating turbulence, or specifically air streams in a non clean environment. The engine room! You are blasting in airflow that hits the engine block, water hoses, water pump, etc. not a stream lined environment to say the least. Now you have a total mess. If you have a belly pan under the car the air exit is really messes up. Add to this, two vortex
- a whirling mass or rotary motion in a liquid, AIR, gas, flame, etc, such as the spiraling movement of water around a whirlpool. Fighting each other for the exit point. Not cool.
Ifin the temperature gauge is ok in traffic, don't monkey with it!
my opinion.
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  • #6
I've run different cars/trucks with single and dual eFans, and many factory setups run both singles and duals. On my own 1969 Camaro right now, I run a single 19" brushless fan that is controlled by a custom PWM controller that's triggered by temperature near the exit point of the radiator. Works fine. The engine is a '72 461 cubic inch mildly modified BBC that's making about 525-530 HP at the shaft.

So nobody is messing with something that's broke, its just a discussion we are having in a performance car-specific forum. There are two schools of thought there, one thinks that a single 17" fan factory rated at 1750 CFM at zero static pressure will move less volume of air than two (2) 12" fans also factory rated at 1750 CFM. Either setup would be installed in a car without a belly pan, but there is a wide open old school engine bay with plenty of space around the V8 engine... unlike todays cars which in most cases you can't see much of the engine because there's tons of shrouding all over the place. Hell, I can stick my arm down from either side and touch the oil pan, headers, starter, oil filter or clutch bell housing. Try doing that on a new Mercedes or Mustang LOL.

What we're looking for is a technical answer in layman's terms... Will one 17" electric fan rated at 1750 CFM move more/less volume of air than two (2) side by side 12" electric fans each of which is also rated as being capable of moving 1750 CFM? Mechanical/clutch fans are not in the equation, it's all about electric fans. In either setup, as shown in the illustration above, the fans are in the same general vicinity relative to the radiator and the engine block.

Keep the answers coming.
  • #7
Lets have some fun OK?

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