For those of you who actually have tried a flex fan on your car, did it really do anything for your cooling or your horsepower? thanks!

I don't like flex fans. Most designs are NOISY! The extra noise comes from oscillation/vibration in the flexible blades as they straddle the flex point. D'you know how an electric buzzer or old-fashioned "BRRRRRING!" doorbell works? A set of contacts is set up such that one is fixed and the other is on a movable arm. Further along the movable arm, on the opposite side of the arm as the fixed contact, is an electromagnet. The arm is spring-loaded so as to close the contacts. When the contacts are closed, the electromagnet is energised. It pulls the arm towards it, which breaks the contact, which switches off the electromagnet, which stops pulling the arm, which snaps back via spring tension, making the contact, powering the magnet, pulling the arm, breaking the contact, etc. etc. The buzzing noise is made by the rapid back-and-forth action of the arm.

The same thing happens with the blades of a flex fan. The blades are made of a flexible material (generally aluminum or spring steel or plastic) such that a given level of force against the blade will tend to flatten it out. This force comes from plain old air resistance as the fan swings through the air. Once a particular level of air resistance is met by the blades, they flatten out. The fan's overall build (blade contour and spacing, etc.) is calculated such that the fan will tend to flatten out at a given RPM.

Here's the thing: this flattening doesn't occur with a sharp transition. The air resistance threshhold is approached, which causes a lot air turbulence at the blade edges (which makes noise), and the blade then begins to flatten, which reduces air resistance, which causes the blade to curl, which increases air resistance, which causes the blade to flatten, which reduces air resistance, etc. Remind you of that electric buzzer? It should!

I don't think you'll find nearly as large a gain in performance or mileage as you would with a thermal-clutch fan (if you can fit one) or, better, a properly sized and installed electric fan. Also, flex fans are definitely not the safest things around; they have been known to fling blades through hoods and hapless bystanders.

I've used 'em in the past and generally found 'em not really worth messing with, but I haven't tried all the ones available, either.

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Too many people who were born on third base actually believe they've hit a triple.

SlantSixDan: thanks for taking the time for your reply! that all makes sense to me. as you can see, i live in Arizona and the temperature has been between 100*-106* lately. my car has been actually running pretty cool. so cooling is not a problem, but what i have heard about the flex fans is that they will have less resistance at top end and so will use less horsepower. it doesn't sound like the best way to go though. also, i think i remember reading on this forum about a concern with electric fans having quite a draw on the battery, is that true? i would think that i would get more horsepower through out with an electric fan, not just at top end. thanks, again!

I glad you asked that question. I been tumbling the flex fan option around in my mind for some time.

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2

Well, the amount of work required to do any given job (e.g. move a specified amount of air across the radiator in a specified amount of time) is going to be equal no matter whether the power comes directly from the crankshaft (belt drives the fan itself) or via the alternator (belt drives the alternator which supplies the power to run the electric fan). Thing is, the electric fan can turn faster than the engine-driven type at low engine speeds when it can be difficult to get enough air past the radiator with a belt-driven fan of any type, and an electric fan can likewise be cut completely out when it's not needed at road speeds sufficient to push enough air past the radiator without fan assistance. Also, there are electric fans available with very efficient curved blade designs. That said, many of the earlier Mopar charging systems are relatively weak at idle, which is something to keep in mind if you go to an electric fan -- you just need to make sure you have adequate charging capacity at low RPMs. That's not too hard; there are lots of options for how to make sure this is the case.

Oh, and another thing about electric fans: they can be set up to run for a few minutes after engine shutdown to cool the engine bay down. This can really help gasoline-evaporation-related hot start problems.

All that said, none of my RWD Mopars has an electric fan right now. I can't imagine finding one slim enough to fit my '62 Lancer (which still has the original 4-blade item, not adequate in high-demand situations with the A/C on), and I just haven't got around to it yet on the others.

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Too many people who were born on third base actually believe they've hit a triple.

I know it sounds stupid, but can you bend the blades of the fan to increase pitch and airflow?

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2

Sure, but you'll never get 'em all bent the same amount, and the result will be imbalance and noise. Best not to go this route.

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Too many people who were born on third base actually believe they've hit a triple.

I just bolted a 17" 6 blade flex fan on my 72 scamp. I don't notice the buzzing noise mentioned above, but it definitely doesn't keep the engine as cool as my good old stock 4 blade did. Usually I don't worry too much about high temps up here in the northeast but we do have our warm days, and have had a few lately.

I MIGHT feel a little more throttle response, and I MIGHT be getting a tad better milage, but I'm talking hardly noticeable stuff here. I'm going to go back to my 4 blade and see how she goes for a few weeks just to be sure. I would have rather gone the viscous clutch route, and made several attempts using junkyard parts but never had enenoughlearance to fit.

Of course there could be other cooling issues as well, since my entire cooling system is original. I'm also thinking that I might need a shroud to make the flex work better. I'll post after I go back to stock and let you know my results.

Like most people here, I'm always looking for smart, inexpensive ways to make the slant run better, faster, more efficiently so I'm hoping this fan actually ends up working out.
Todd

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72 Scamp 225 /6, auto, radio delete, alum super six, cold air duct, pertronix ignitor II and flame thrower, 3.23 rear, 2.25" free flow exhaust

Mopar used a 6 blade flex on the later Slant A-bodies with air. I have a real cheap aluminum flex on my '66 that has been there for years, even when it was my only car. Does it do anything? Maybe, maybe not, but it doesn't hurt anything and it doesn't make any noise.

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Official Cookie and Mater Tormentor.

Two years ago I drove my '89 Grand Marquis across the US in July, pulling a 3000 lb popup camper. I couldn't keep it cool enough to run the AC while traveling through the midwest. One blistering day I pulled off at an AutoZone and replaced the factory 5 blade (which included a thermal clutch) with a 6 blade flexfan, discarding the clutch.

It is definitely noisier (sounds like a B-29 at high RPMs), but cools wonderfully. The running temp dropped from 215 (with the AC off) to 180 with the AC on. Never a bit of problem after that, including driving across AZ in 115 degree heat.

BTW- The radiator in this car was fine, it was simply overwhelmed by the task. Moving more air solved the problem.

Whoops, I didn't make myself quite clear: The noise a flex fan makes as its blades oscillate isn't a buzzing, it's more of a roaring.

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Too many people who were born on third base actually believe they've hit a triple.

Quite likely your fan clutch was weak.

My 1979 Dodge D150 truck with 318 V8 came factory equipped with a 20" 5 blade flex fan. It makes a lot of noise and on hot days the temperature gauge would climb up to max. Plus the belt slips when reving the engine. One skinny belt will not pull that fan at speed no matter how much I tightened the belt.

I swaped on a 20" 7 blade clutch fan from a 1979 Cordoba 360 and that keeps the temp gauge down when the fan clutch works. I've been trying various used fan clutches and most are weak. A new fan clutch I installed worked great for a month or so but now does not drive the fan as hard as it did. Need better quality heavy duty fan clutch. This clutch fan moves a lot more air at low engine speed than that flex fan ever did.

Those big honkin' factory-issue early 70s 7-blade clutch fans with aggressively pitched blades look like they're JUST the ticket for heavy-duty cooling, don't they? But they aren't, because they take so much power that they overwhelm most fan clutches. Fan clutches are designed to DIS-engage whenever the speed differential between the fan and the shaft gets too great, and that's exactly what happens with the 7-blade fans with aggressive pitch. The fan winds up stalled out most of the time, unless you run one of those 3" thick monster heavy-duty fan clutches, and then the thing draws 20 horsepower and sounds like God's own vacuum cleaner.

I've found two fan/clutch combinations that provide very good cooling without too much noise or power draw. The best is the Chrysler FIVE-bladed clutch fan paired with a standard-duty fan clutch. This is what I run this setup on my '69 Coronet R/T with its 440 Magnum and air conditioning, and it does a great job keeping the temp in control even in traffic with the A/C on in Austin summer heat. This is the fan that Mopar Performance sells as its "MP Viscous fan package," but it was also the standard factory fan on 80s cars and trucks with (you guessed it!) the Slant Six engine. The v8s from that era also got a 5-blade fan, but its bigger in diameter and won't fit older cars.

The other combination that works OK is a standard-duty fan clutch with a Chrylser or aftermarket 7-blade fan with less pitch on the fan blades than the monsters Chrysler was using in the late '60s and most of the 70s. My '66 Dodge Polara (383 2-barrel A/C car) came with a fan like this from the factory, and I've also bought Hayden aftermarket fans that were almost identical. I run one of these on my '73 Satellite 318 car, which originally had a flex fan.

As for flex fans- no way, no how. I can't stand the things.

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That's definitely true of the centrifugal fan clutches (the kind without the coiled bimetallic thermostat spring on the front), but I'm not sure it's true of the thermostatic clutches. As far as I am aware, the thermostatic clutches are not designed to respond to speed differential, but rather either transmit or withhold fan drive depending on the position of the fluid control valve, which in turn depends on the thermostatic spring, which senses the temperature of the air coming off the back of the radiator.

I'm sure that even the thermostatic clutches have a slip point (all fluid couplings do, pretty much), but I'm not sure it's designed in as a part of the way the clutch is meant to work.

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Too many people who were born on third base actually believe they've hit a triple.

Based on observation, I think that thermostatic clutches DO have the same overriding slip feature under extreme load that non-thermostatic ones do. When I had the mongo 7-blade fan and standard type clutch on the 440, you could bring it to full operating temp with the AC on and then set it to a high fast-idle (~2500-3000 RPM) and watch the fan. It would roar like an F-16 for a minute or so, then almost completely disengage... and the temp guage would start climbing like crazy. As long as the engine speed was kept high, the fan would NEVER re-engage. That correlated well with what I was seeing on the road, too- sustained highway driving would tend to heat the car up, but slowing down would cool it off- and yet the radiator, water pump, and block were flowing well (high-speed overheating is USUALLY coolant-flow related, not fan related, which is why it took me so long to figure out what was going on!) Putting the same clutch behind the 5-bladed fan or the flatter-pitched 7-bladed fan does not produce the same behavior at all- at a constant 2500+/- RPM, the clutch will "cycle," alternately tightening and loosening, speeding and slowing the fan, and the coolant temp holds constant. Both in the driveway and on the highway.

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