From what I read of fluid fan clutches they don't let the fan spin any faster than about 2300 rpm. So that's going to limit the maximum HP draw. The 40 hp does seem like a lot but remember, this is at 4500 rpm so that mechanical fan is really spinning. And if as you say, things go up cubed instead of squared, a direct coupled fan of 5 HP at 1500 is going to be pulling a heck of a lot of HP at 4500.
Just as a reference point, I read from a semi-authoritative source that the old heavy duty GM Turbo-Hydromatic 400 could take as much as
40 hp off the engine output on a dyno. So imagine the drain on power of an older car with the TH400 and a hd mechanical fan when it's at full throttle.
Trader, keep in mind that the 40 hp was for a super hd mechanical direct connect fan. Almost NO automakers used such a fan on typical cars. These guys were using it to see what the most hp drain might be. Back in the day almost all non ac cars just had a crappy stamped steel almost flat 4 blade fan if a 6 and a crappy 5 blade fan if a v8. If they had AC they usually had either a fluid clutch (GM and Chrysler) or a flex fan (Ford). And except for when you were "gunning it" to pass or the stop light grand prix these engines were only running 1800 to 2500 rpm unless you got a performance axle ratio. So they might only be pulling 5 - 8 hp most of the time.
That was kind of one of my points, that the "test" numbers some are using to justify these high fan HP are not representative of the type of fan under discussion, ie one in 2000 - 2007 BMW X5 or similar. It sounds like some of the companies selling electric fans and the proponents of them may have glommed on to those numbers. The problem is, I haven't seen any test numbers for say a 90s era car or current SUV, light truck that use modern hydraulic clutch fans.
The fan in my PT cruiser went out. It caused the head pressure in the A/C to go sky high at idle and the AC would cut out. When it cut back in after the pressure dropped back to perhaps 300 psi it would almost stall the engine. That was the only reason I noticed the fan had stopped working until the weather got real hot. Then the gauge would climb more then normal but it never actually overheated. That's completely unlike the old cars I have with V8. No fan in them and it's a boil over.
The only great value I see to electric fans over mechanical with clutch is that they can run at MAX when the car is idling. For some cars that's important, for others it's not. On my 69 firebird I have an electric fan off a big Lincoln. It just runs hot and this works better overall then either the original style fan and clutch or a flex fan.
One of the things I think some people are failing to recognize in their comparisons of electric industrial fans versus these car fans is the industrial fans are not pulling the air thru a honeycomb of a radiator. There is a fair amount of resistance to pulling the air thru that and just as the hp required for the fan being cubed as speed increases, I'm pretty sure the resistance of drawing air thru a honeycomb increases faster then a linear rate as the CFM's increase as the fan speed increases.
A mechanical fan with clutch is always turning - always. There is no free-wheeling. The electrics in my car hardly ever go on in the winter. The only reason I can think of for using a mechanical fan is the auto manufacturer didn't want to change engine controls, or electrics couldn't keep it cool enough. I don't know how much HP the fan uses. The dyno results I've run across are all over the place. But there's a HP difference in favor of electrics. I never had either type fail.
Well, you can call that luck. I've had customers' clutches sieze up - so they run full tilt all the tome. I've had then fail that they don't drive and overheat. I;ve had them get so sloppy the fan came off and went through the rad.. I've had flex fan blades crack and come apart. I've had rivetted fixed pitch solid fans loose blades (and come through the hood). And I've had electric fans get so stiff they've burned out the protection fusible link, I've had them totally sieze, and I've haf them get so sloppy the rotor ground away the magnets, as well as having them just plain burn out. I've also seen them get so hot they melted the plastic blade right off at the hub.
That said, Istill prefer the electric fan. (after 46 years experience as a mechanic)
I put a new fan clutch in my Aerostar a few months ago when I replaced the radiator. Worked fine for a few weeks, then started overheating when idling on a hot day - not every time, just when I'm not watching the gauge. Sure enough, it was failing already. It's such a bear to swap it out, I wouldn't mind putting an electric fan in there this time. I can see wiring fun though. But I wouldn't mind if it ran all the time.
Just about everything still leaves a lot: BMW, MB, Volkswagen, Audi cars and the huge number of SUVs, vans, pickups, from all manufacturers. They mostly used mechanical fans well into the 2000's. Some of those only went to electric fans less than a decade ago. BMW with the very popular X5 for example, only went electric in 2006,
No problem wiring it at all. power wire direct from battery positive, through fuse to relay to motor, and to ground. a temp switch on the rop rad hose if you can't find a place to thread one in (they make them for this purpose) and run power from switched ignition to the relay coil, and from the relay coil to the switch and to ground. Less than 10 feet of wire in total will do the job. You really do NOT want it running all the time. You can connect a second feed to the relayelay, to run the fan when the AC is on (or add a second fan controlled by the AC)
I agree with Vic and I think you do too. An electric fan can be controlled so that it only runs when necessary, which is only a fraction of the time. I've seen people claiming they have taken out mechanical fans and not replaced them with anything and the vehicle doesn't overheat. I for sure wouldn't do that, but when the car is moving at road speed, winter, etc, the fan isn't needed and the electric can stay off. I also agree that another advantage is that the elec fan can run at max speed when the car is idling. I never doubted that an electric fan could have some advantage, only questioning, like you, how many HP it really adds up to.
Now for an example of how you can go too far, here's what BMW does with their existing auxiliary fan on the E53 series X5. Instead of making it a simple on/off fan, they made it variable speed, with the variable speed electronics in the fan motor. Great idea. Put electronics in a fan that's in one of the worst possible spots, ie heat, road spray, etc. And then they send it a PWM control signal. So now, if the fan isn't working, there is no easy way to diagnose it. You can't just put 12V on it, see if it spins. Neither can you put a VOM on the incoming wires to see if the system is telling it to spin. And for a final good measure, they supplied it with 12V all the time. One of the failure modes is for the miracle fan to create a parasitic draw that drains the battery overnight. It's great for the BMW dealership though, those fans are $500 and a couple hours labor to put in.
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