Electric Radiator Fans - Help or Hurt Airflow

I've always thought that electric fans (behind the radiator) would just be a hinderance after a certain speed - that speed being however fast the fans can spin and flow air. 
 

but I had an interesting conversation this morning with a guy I sold some parts to today.  He said that electric fans (at least the dc kind in most/all cars) are torque limited, not speed limited.  
 

the faster the car goes and the more air goes through the radiator, the faster the fan's motor can turn the blades. 
 

he sounds like he works with stuff like this as a profession and it is something I'd never heard  before.  It makes sense and is very interesting to have a decades-long belief upended in 3 minutes. 
 

sound true?

It's true - you need the torque to pull through multiple heat exchangers. Most "slimline" fans are basically useless and their flow is rated in clean air. You want a big fat power-hungry motor, especially if you've got AC and an intercooler along with the rad. Spal has charts for their fans showing the effects of relative pressure. I thought they were on the public website but if not I've got some at work.

The actual airspeed through a heat exchanger stack is a lot lower than you think unless you have a dedicated airflow path including a good exit. For production cars without big holes in the hood, it's pretty slow. Measuring actual airflow across the cores at speed would be a very interesting experiment, I remember doing some of it way back. 18 mph is a number that sticks in my head but it's the only one :)

Verus has some charts showing flow vs torque.

https://www.verus-engineering.com/blog/informative-8/radiator-fans-in-depth-explanation-and-information-32 

Here's one showing three of our kits. The white band is the range of static pressures where you expect to operate in a Miata. The Stage 1 is a pair of slim (but thicker than most) Spal fans. Stage 2 is a single brushless, Stage 3 is a pair of brushless monsters. All of them have bypass flaps on the shroud for when natural airflow overtakes the fan.

Check out the current draw for each in the upper right. Note that the brushless fans have a much wider range of operation which is why it specifies full speed, but all of these are with the fans running at max output.

If we had a set of Brand M slimline fans on this chart, they'd basically be a line going vertically down on the Y axis.

This is one of those questions where "it depends" definitely applies.  OEMs have multi-million dollar R&D budgets, wind tunnels, and extensive ASTM tests they can perform.  Aftermarket "fanufacturers" have a motor and some blades and they make them in different sizes for universal applications.  Little to no R&D other than putting it together and testing CFM in free air.

Many aftermarket electric fans are designed for trailer-queen T-buckets with a tiny radiator.  Their job is to provide the entirety of the airflow during a parade which means lots of blade real estate.  That same blade real estate can cause trouble on the highway if it can't spin fast enough.  Even if it spins as fast as the airflow lets it, it is still a restriction to some flow.  For that reason you might find that some electrics are great on the highway but suck in slower traffic because they're good at free flowing ambient air, but they aren't moving enough air at idle to keep things cool.  Other fans might be the opposite.  As Keith pointed out, it's not just the flow they advertise, it's the pressure delta the blades can generate.

Even if you find a great junkyard fan that fits (wasn't it the Ford Taurus fans everyone loved so much?), all bets are kind of off if you're not installing it in a Taurus.

I can offer this, though.  I do know that the fans on my former 96 Impala SS would parasitically spin at highway speeds, but if things got warm or I kicked on the A/C, the electric flowing to the fans spun them faster than the ambient airflow through the radiator.  I only know this because you could hear when they kicked on if you were paying attention.  This was likely due to the design of the blades... better at scooping than being pushed, if that makes sense.  So, in the case of my SS, I'm sure they were a modest hindrance to natural flow, but not enough to cause trouble.  The Flex-a-Lite Black Magic fan that we put in a model T with a chevy 350 and wee little mustang radiator did just fine at idle and slow speeds, but you only got about 15 minutes on the highway before it started boiling.

In reply to Curtis73 (Forum Supporter) :

Last year I happened to be running my F500 with the cowling off. Video from another competitiors car shows the fan spinning rather slowly, clearly being driven by the airflow. The fan is also clearly spinning much slower than when it's powered by the motor.

As Curtis pointed out, it definitely varies.  How powerful the fan is, the blade design on the fan, the shroud design and mounting, as well as how good natural airflow is through the radiator all determine how it'll cool at higher speeds and whether the fan being present will make it better or worse.  Fans with efficiently shaped blades and less total blade area will present less of a flow restriction when not running.  But as you reduce blade area, you need better blade shape and higher fan RPM when running to move enough air when the fan is on. 

this conversation started when I mentioned I had considered pulling the fans off when I was on track to help the car stay a bit cooler by getting more air through the radiator.   He said that running the fans would instead help cooling
 

What my new friend was saying is that the motor will ALWAYS spin faster than the airflow and will always pull more air through the radiator.  

He did say that at some point there is a limit that the fan can spin but it would never see that on a car, even a race car. 
 

so if you turn the fan off when driving, yes it will definitely hinder airflow.   But when turned on, it will always improve airflow, even at higher speeds   
 

and what Keith said about the airspeed through the exchanger stack makes it make even more sense because  the airflow the fan sees is far, far less than the speed of the car.  it is easier to see a fan exceeding that speed  (and thus helping pull more air through) than picturing a fan spinning at 90 mph and then increasing when you put power to it   (Obviously ground speed isnt how you rate fans, but using that for simplicity's sake)

jfryjfry said:

 

What my new friend was saying is that the motor will ALWAYS spin faster than the airflow and will always pull more air through the radiator.  
 

so if you turn the fan off when driving, yes it will definitely hinder airflow.   But when turned on, it will always improve airflow, even at higher speeds   
 

Your friend is partially correct.  The fan will most definitely spin faster under power and pull more air than when not powered.  Their assertion that it will always draw more than natural flow is not correct.  Just using round numbers, let's say you get 5000 cfm at highway speed with no fan at all, as in... naked radiator.  If you install an electric fan and don't turn it on, you might now have 3000 cfm at highway speed.  Turning on the fan you might have 4000cfm.  Again, this is highly dependent on a few hundred factors, just giving you one possible example.  But saying that an electric fan will always move more than passive air is not correct.  It will move more than if the fan is not running, but not necessarily more than if you had no fan at all.

If your friend were correct, the internet wouldn't be chock full of hot rodders whining about how their electric-fan 62 Cadillac overheats on the highway.  There have been countless times that I suggested to those folks that they remove the electric fan and get back on the highway and suddenly their problem disappears, indicating that even a running fan was restricting flow compared to what it could flow with no fan in place.

So yes, the fan will draw more air than the passive flow around the fan, but that is not to say that it will move MORE air than the passive flow would if the fan were not in place.  If you think about it this way, your car might require 50hp to maintain highway speed against the aero resistance it sees.  Even if we forget about the way the front of the car channels airflow, you have 50hp worth of airflow from the movement of the car.  It is unlikely that a 1/3hp electric motor can beat that potential.

jfryjfry said:

this conversation started when I mentioned I had considered pulling the fans off when I was on track to help the car stay a bit cooler by getting more air through the radiator.   He said that running the fans would instead help cooling

What kind of race?  A 500-lap race where you never drop below 60mph?  Autocross where you spend 60 seconds at 30 mph?  Drag race where you spend 5 minutes idling to staging and race for 13 seconds?

I had fans on my race cars but only for use when sitting idling in the pits prior to going out on the track - a delay there can result in either a cooked engine or an engine that you shut off and then find that it is reluctant to start after some heat soak.

I had a telltale light on the dash so I'd know at a glance when it was on. I was surprised the first time I was out on the track with the fan turned off and saw the light on - airflow presumably turned the fan into a generator.....

As everyone has said here, "it depends". With a very well designed front bumper and proper heat extraction from the engine compartment (the hood), it's possible that at some very high vehicle speed you'll be better off with no fans installed than with them running full blast... but that's a pile of 'ifs'. Especially in a car which has known front-end airflow issues (such as a C6 corvette). I mean shoot, the front end is so low that they had to tilt the radiator back to fit it. 

In general, electric motor torque is linear with current through its windings. If a fan setup is rated at 3000CFM at full speed with the vehicle stopped, then the same setup will see higher than 3000CFM if the vehicle is moving, i.e. an extra 1000CFM of passive airflow might get you up to 3500CFM or so. Yes, there will be diminishing returns... the fan blades will end up spinning faster, increasing the back-emf of the motor (higher impedance), therefore reducing the current through the windings (torque) given the same 12V input. At some very high rate of speed, your fans will eventually add zero, and beyond that will actually act as generators. 

Bottom line, for all practical purposes in a street car at anything but very high speed, turning your fans on will increase flow through the radiator compared to if they are off. Our endurance raced firebird (with a dual fan LS1 setup) *always* runs cooler if the fans are on low speed than if they're off. High speed only ever  happens when the car comes off the track into the pits. We've got dozens of 10+ hours races of data to show this.

As far as the discussion of full-speed fans vs. no fans? I think it's a moot point as that is really not a viable solution. What happens when you're in dirty air? What happens when you're running around under yellow? What about staging? What if you have some aero damage in the front end? 

I needed the 16 inch parts store fan to be switched on to keep temperatures in check when I used to track an RX-7.

After I stopped doing that and started drag racing instead, I realized that the carb was jetted way too lean 

But yes, even at 80-110mph it is still useful to have.

I think the short answer is "they help until they don't." When that transition happens is dependent on the engine type, radiator type/size, ambient temperature, thermostat opening temperature, fan size, vehicle speed, etc. I run some generic 16" fan from Summit on my track RX7. It's supposedly rated for 3000 cfm but that's probably optimistic. I have it set to come on at 205F and turn off at 185F. Why? Because those are just above the "full opening" and just below the "just opening" temperatures of the thermostat. I don't want the fan fighting the thermostat. On track, even in 100F ambient temperatures, my coolant temperature pegs itself at 200F so the fan is never running. Would turning on the fan help? Maybe. Would I like the engine to run a bit cooler? Sure. But before knocking down the fan activation temperature I want to play with some other options like shroud bypass flaps and post radiator ducting.

This certainly sheds some light on to my recent experience.

I put a dual fan system on to my 91' Firebird from a LS1 TransAm. Should be plenty of fan, that car has 300+ Hp compared to my 200 right? Car ran much warmer than normal, fans came on all the time. Went back to my big single with no shroud, temps are normal again. The duals did work better in heavy traffic and brought down the temp faster when running, but ran more often.

My V8 S10 would run hot with a similar dual set up.

This is not directly on topic, but close.  First is an opinion, second, an observation.

My trucks are all mechanical fans, and there is no way on earth an electric fan can pull as much air as a seven blade fan with the viscous clutch locked, and the rest of the time it's just idling along, not really costing mileage.  And when that great loud fan spools up, I can watch the temp gauge drop.  I know that doesn't apply.on race cars and fwd stuff.

The observation, which does have to do with velocity through the rad, is how much junk will make it through the condenser, but stop on the rad.  Huge pillows of grass and bugs, that you generally can't see until moving the rad back.

In reply to Streetwiseguy :

To this point, I just upgraded the radiator in my LS2 corvette from a stock system to a massive Ron Davis setup. I could not believe the amount of dirt and crap caught in the condenser and OEM radiator. 

PSA: if you're having cooling problems, go clean your radiator before doing an upgrade. All that dirt on the floor came from the condenser only! The radiator was just as bad if not worse.