Should you get a heavy duty fan clutch?

Captain Phoenix said:
Why do you think driving style and RPM matter? A thermal fan clutch locks when the engine reaches a certain temp and when locked the 2791 moves more air. The 2771 moves less air when the engine is hot and more air when the engine is cool, if that's what you want then fine.
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Reading comprehension is key...

It doesn't unlock below 3200. I seldom turn the engine faster than that. Therefore, it moves more air - a LOT more air. The clutch doesn't only lock/unlock by temperature, it also locks/unlocks at certain RPMs. Some people here complain that it sounds like a cement mixer - and they're not wrong. I just don't care.
 
Zorba said:
Reading comprehension is key.
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True, try it sometime.

Zorba said:
It doesn't unlock below 3200.
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False. A thermal fan clutch locks and unlocks based on engine temperature not on RPM.

Zorba said:
The clutch doesn't only lock/unlock by temperature, it also locks/unlocks at certain RPMs.
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Source? Everything I can find says RPM is irrelevant. Are you confused by the fact that the fan still spins at idle when cold and think that means it's locked?
 
Captain Phoenix said:
True, try it sometime.



False. A thermal fan clutch locks and unlocks based on engine temperature not on RPM.



Source? Everything I can find says RPM is irrelevant. Are you confused by the fact that the fan still spins at idle when cold and think that means it's locked?
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WRONG. I'm not going to waste time arguing this - this clutch absolutely, positively does NOT unlock below 3200-ish. Howls like a banshee, no question that its locked. Yes, temperature is also a factor, its BOTH temp and RPMs.
 
In the data provided :
2771 70%-90% shaft speed clutch engaged
2791 80%-90% shaft speed clutch engaged
In theory if they were both fully engaged and reach the 90% , airflow would be the same with the clutch locked .

2771 25%-35% shaft speed clutch disengaged
2791 20%-30% shaft speed clutch disengaged
It would seem that the 2771 just might move more air through the engine compartment while disengaged therefore pulling more air over the condenser and helping to keep temperatures lower throughout the engine compartment while the clutch is disengaged .

As far as longevity , the 2791 may very well better . This we can suspect , but not easy to prove without trying them both.
 
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Harold&Cat said:
It would seem that the 2771 just might move more air through the engine compartment while disengaged therefore pulling more air over the condenser and helping to keep temperatures lower throughout the engine compartment while the clutch is disengaged .
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If that's something you want then go with the 2771. 🤷‍♂️
 
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Captain Phoenix said:
If that's something you want then go with the 2771. 🤷‍♂️
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Yes , that is what I am looking for . If I had an electric fan , I would want it running while trail riding to get as much fresh air over the condenser and under the hood as possible . I currently do not wish to covert to electric , so having a fan clutch that moves more air disengaged seems like a good alternative .
 
Zorba said:
Are you a complete idiot, or are you being purposely obtuse? Blocked.
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I'm trying to figure out why you assume the fan clutch is engaged at all times when below 3,200 rpm when it isn't supposed to be. The only thing I can think of is that you think the fact that the fan is spinning means the clutch is engaged. 🤷‍♂️
 
TheBoogieman said:
Does anybody have any actual proof/data for these claims or is this like using a butt dyno with your new CAI for HP increase readings? 🤔 :rolleyes:
p.s. Don't get me started on the gas savings and HP increase with my Tornado.

View attachment 669769
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Yes.

https://hvacknowitall.com/blog/the-3-fan-laws-and-fan-curve-charts



This is knowable. The equations and laws governing them are well known as it is important in industrial design for automobiles, buildings and all sorts of other equipment. Someone at Chrysler did all those calculations at one point to determine which clutch was needed.

If you wanted to collect the data, you can use a magnet on the clutch or inner arm of the fan and a sensor to get a signal for the actual rpm for the fan, plus a temperature probe to know when it locks and unlocks. And if you want to get really nerdy, place an anemometer in the engine bay to capture the cfm. Logged with the engine data from an ODBII live feed, you can run it on the dyno and actually have specific numbers. All it takes is curiosity, time and money.

In a worse case scenario, the severe duty unlocked at 20% shaft speed and the heavy duty at 35% would result in a 1000 rpm for the severe duty vs 1750 rpm heavy duty with the engine at 5000rpm. Which is taking it to the extreme. The heavy duty would be consuming 5.4x more power under this worse case scenario. If it is actually spinning at 25% shaft speed, it would be 1250/1000^3 or 1.9x more power consumed. I couldn't tell you what hp the stock fan requires at 1000rpm though.

Honestly, I'm not trying to stir shit up. Fans just take power to run and the formulas are known.
 
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Captain Phoenix said:
I've already provided proof of what I'm saying, what more do you want?
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How about .............
expert1.jpg
 
TheBoogieman said:
I have none of the above. (y)
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You could just jump to about 6:20 the video from Engine Masters above. In that test the unlocked clutch running a fan took 7 ft-lb of torque and 14 hp at peak off the motor. They also used the factory style fan and ran it without a clutch (or 100% locked) and it took 30hp at 5100rpm. But at 3000rpm where they start the tests it looks like only takes 2-5hp off the numbers of the comparison run without a fan.

Not the same fans or motor as ours, but it gives a pretty good example of the effect that increasing fan speed has on power consumption.

That said, the fans do seem to break out of the fan laws at extremely high rpms. You'd expect power consumption between 23.5x and 64x of the unlocked (25-35%) 14 hp. The engine is around 350hp so that would scale to 339hp to 896hp and obviously that's not happening. I would guess that they are just creating so much turbulence at those speeds that they break out of the expected behavior and the CFMs are way below expected.

And to take this back to the topic at hand - if @Zorba is always below 3200rpm and we're talking the difference between these clutches, we're talking 1-3 hp difference between the 2 clutches in the real world.
 
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Not my battle.

The fan consumes power as a function of fan rpm until the flow gets too turbulent and the equation breaks down. Automotive fans seem to closely obey the fan laws in these ranges of fan speeds under 1500 rpm but break down at higher speeds.

If the engine is turning at 3000 rpm cruising down the highway at part throttle. Going from a fan clutch at 20% shaft speed to one 25% shaft speed is still a 1.9x hp load difference, which is not intuitive. If one setup is wasting 3hp, which does seem realistic for our engine, the other will be wasting 5.7hp. If the engine needs to produce 40-50hp to maintain this cruising speed and that much hp is wasted, then yes, that extra 2.7hp wasted will have an effect on mpg that can be calculated and observed.
 
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