You never said why you think you need "more CFM at idle", or how much more. Whether or not you get it from an e-fan depends entirely on what size e-fan you use. But the e-fan won't change CFM as the temperature increases, or the engine RPM increases, or the temperature DROPS... It'll always be the same, whenever the fan is on. So if you need more CFM at high RPM than you get at idle, you won't get it from that fan. And if the fan stays on after the engine temp falls, the engine will be running TOO cool for proper operation (fuel evaporation & PCV).
If the e-fan moves as much or more air than the mech. fan did, and you understand that the e-fan motor is LESS-efficient at converting engine torque to fan torque, then you have to know that you'll have LESS available torque at all times. The mechanical resistance at the WP pulley (what drove the mech. fan) will move to the alternator, but it will more-than-double (probably closer to quadruple). So you'll certainly have MORE mechanical resistance on the belt with an e-fan.
Just to be clear, you contend that the alternator resistance will necessarily go up by the addition of an electric fan. Not following the logic, but this may be me just hitting the limit of my understanding. The alternator's resistance is, to my understanding, relatively constant. Even adding a larger alternator (possibly more resistance, but also possibly more efficiency and amperage) this would still be a constant. Assuming I am close to correct, and ALSO subtracting any resistance that the original mechanical fan (variable resistance based on revs, fan style, shroud, balance, etc...), I just don't understand the loss the way you do. And, more importantly for my purposes, any loss in efficiency I may have, would now be constant, not variable. Right?
To my understanding the only purpose of the radiator and fan is to cool radiator fluid. The value of this, a by-product, is a cooler running engine, but its specific function is to cool radiator fluid only (unless it is tasked with cooling other fluids too). So, I come at this problem trying to understand when the radiator is taxed the most, and has the least available cool fresh air freely available. And, to my untrained mind, idle is when our radiators are at the greatest risk of heat accumulation. To improve this inherent vulnerability (weakness) in our system, I plan to experiment with an aluminum radiator (better material for heat transfer, and higher idling cfm of fresh air. Unfortunately, don't think anyone has solved exhausted heated air off the radiator out of the engine compartment in our Broncos yet.
As to you your question about my "why". Broncos run hot, I live in Florida, I ride on the street only. So, theoretically, my engine (and compartment) should be hottest in the summer, while idling. This is the exact, and only condition I am really trying to improve radiator cooling in. Once running (usually on back roads) I am up to speed and passive ventilation, I'm betting, takes on the bulk of the radiator and engine compartment cooling task, the fan itself doing comparably little. And, my guess is, like most vehicles, Broncos are bottom feeders anyway, when it comes to engine compartment breathing. So, again if I am right, at any speed above zero my fan is doing some, but little, and diminishing as vehicle speed increases) work to cool the radiator fluid, which really is it's only task, not engine compartment cooling anyway.
Actually, I bet a few side vents behind the front wheels might create some negative pressure in the engine compartment at speed (or at the very least improve/direct hot air flow out) would do more to cool the compartment than all the fans in the world. Well, okay that was an exaggeration... Anyone experimented with passive (or active) hot air exhaust ventilation?
DirtDonk, you are spot on in your assessment of my intentions. I am not doing this because I have a problem; I am doing this because the exploration is worth the cost, time, and negligible risks to my project. And, if in the end my motor stays cool enough, it was not a loss unless of course, I sacrificed noticeable power to achieve this mediocre result.
Steve83, I hope I explained my reasoning and limited physics justification well enough to keep the conversation going. I like learning about this stufff!
Thanks again all!