I agree and understand what you’re stating concerning inrush, you can hear the motor when fans turn on and that last for a split second but the RPMs remain lower until the fans are turned off and then the RPMs come back up 200.

I don’t have a controller, I just have each fan on a relay and each relay connected to a intake mounted temp switch. Both fans come on at the same time and at 100%.

Well..............the electric fan (s) don't run all the time.......most don't even run MOST of the time!  When that's figured into the equation I think the electric fans may draw less power.
Minimal HP savings over mechanical fan.  Like the difference between long tubes and mid-length headers. Sorta
6sal6

With dual fans you should be using some kind of controller.  Turning both fans on full at the same time is almost certainly your issue.  Two fans starting could easily draw 60 amps or more, and depending on which 3G you have you may be using 2/3 of the alternator's output to start the fans. 

As far as power draw goes, a regular fan takes 20-25HP to run.  A clutch fan drops that down when the clutch isn't locked, but it still probably draws 5-10HP.  A flex fan is supposed to flatten out and eat less power at higher speeds, but the debate over electric/mechanical aside I'm betting no one is going to recommend one of those. 

An electric by contrast draws about zip.  Another thing I'd consider is how that 3G is wired.  If its set up as a "one wire" alternator that's also an issue.  Those setups always have issues with high loads at idle because they lack a proper feedback circuit, so they don't adjust well to instantaneous changes in load. 

https://www.cjponyparts.com/pa-performance-alternator-conversion-kit-allows-use-of-3g-130-amp-alternator-1965-1985/p/ALTCK/?year=1980&gclid=EAIaIQobChMIhMn76p3M6AIVR_7jBx1efwRLEAQYAiABEgL2cPD_BwE

I used this back when I did the 3G swap , using a Taurus Alternator.
 

I am running contour fans no problem with bolt spike but I have it set up on a 3 relay system with 2 adjustable thermostats one for low, one for high. Both fans are wired in series on the low, then when the high both fans get full power.  I will post up the schematic of anyone is interested

I agree with those that said you should have some kind of controller that brings the fans on separate. Other than a second or two when the fan first kicks on, I believe the electric fan eats less HP.  Probably why new cars use them, eat lees power save gas.You may even try unplugging one of those fans and see how that works. Put an emergency over ride manual switch on it.

Pray all is well The good part of the fan controller failsafe part of cooling concepts fan controller is that it will stay on high. Even when the key is off Lol 1 fan two speeds when both of the power leads on the fan see power it goes on high real quiet fan pulls a lot of air lasted only 8 years before the bearing started making a little noise only need 2 5/8 clearance it comes on at 180 low speed 190 kicks on high or when the A//C request signal but it can control 2 fans as well

Last edited by Coupedaddy (4/04/2020 9:47 AM)

To answer your original question, "how much HP does a cooling fan draw?"  The formula for Power is P=IE, where P is power in watts, I is the current draw in amps, and E is the applied voltage.  One HP is equal to 740 Watts.  So if, as mentioned, your fans pull 60 amps at start and your charging system is regulating to around 14 volts...(Ford used to reg. the  generator systems to 14.6 V but I'm not sure what the newer systems regulate to.  My VM in the Heap reads about 13.8 at normal cruise)...so you can multiply 14 times 60 for 840 Watts, then divide by 740 and end up with 1.135 HP at fan start. 
If they pull 30 amps running then that works out to .568 HP.

BB1

TKOPerformance wrote:

Yes, always have an override switch.  That's a must.  If a sender or something else fails that ensures you can still run the fans.  In a dual system you may want to have two switches, once for each fan to avoid the large initial draw.  That way you can flick one on, then the other if needed. 

It doesn't look like you have AC, but its worth mentioning, if you plan to add it in the future, that you need at least one fan to come on with the compressor.  Otherwise you risk blowing something up when the car is sitting with no air moving through the condenser. 

Just wanted to share with my Vintage Air AC system the heater fan switch is not used, so that is my cooling over ride switch. im also using Autocoolguy 
 

Rudi wrote:

20-25 hp to run a mechanical fan
Show me the numbers!

Car Craft did a fan shootout dyno test in May 2000 comparing an electric Black Magic, and a couple different OE setups.  The electric made 494 HP, the best OE clutch system made 287, and an OE 4 blade (no clutch) made 473.  A six blade fan without a clutch only made 449.

https://wasteadvantagemag.com/studies-prove-electric-trash-mechanical-fans-for-new-engine-cooling-tech/

Here's an interesting real world study.  Its not done from a power standpoint per se, but it does show a marked improvement in fuel economy in large trucks by switching from a single mechanical to multiple electric cooling fans.  Improved fuel economy means the engine is using less fuel and thus more efficient, and whether we choose to view this as an "increase" in power or a "reduction" in parasitic load the end result is the same. 

 Mack (Volvo now days) and that model is a poor example. It uses a very small rad to fit under it's low hood profile. It uses a very VERY over driven fan speed to move much needed air for the difference. A garbage truck is a stop and go truck. When that air lock fan clutch grabs at full revs it will throw the belts off some times. I know from experience. Over the road trucks on the other hand don't need as much fan speed. Cool weather air at road speed is enough that the air clutch is rarely locked