My stock-engined Morris Minor has a mechanical fan. It's a puny little two-bladed thing, but it does the job. And the engine only makes 37bhp! Just throwing that in. To say that past hot rodders would have been all over new technologies had they been available misses the fact that there is a specific paradigmatic difference between the sort of technology we call "traditional" and the sort of technology which came after. It isn't just a matter of degree. Of course it didn't all happen at once, one day in 1964 or at any other time. Hence my contention, controversial as it turned out, that despite historic precedent sealed-beam headlights don't really sit happily in the "traditional" technological repertoire. Point is, it's not about the date in history, it's about the technological approach. The more I think about it, the clearer it becomes that a basic principle of the "traditional" technological approach is to regard electricity as the last-resort option for doing any job in a car, rather than the obvious solution for everything as it is today. Case in point is all the uses of manifold vacuum, some more successful than others, for everything from power brakes to wipers to concealed headlights. And anything vacuum does badly positive air pressure can do well, and if you've already got engine-driven OBA for your bags ... That's perhaps the best argument yet for the traditional bona-fides of air suspension, at least as regards technological approach. Air motors eat air, though; no good for driving a radiator fan. Personally, I like the idea of avoiding electrical systems like the plague if it is even half-way viable to do so.
Definitely some confused people here.... First, why do cars have fans? Answer, to pull air though the radiator for cooling when the vehicle speed is too low to supply that air. If you could maintain that minimum speed all the time, you wouldn't need a fan. Second, how much power does it take to move a given CFM? Answer, it depends... How efficient is your fan design, and what is its optimum operating RPM? How much resistance (or help) to flow do you have? That dyno test is interesting, but a few more data points would have been helpful. How much power was consumed at 800 rpm? 1600? And how many CFM were moved at 4500 rpm? And it's also interesting to note that when fan speed doubled from 800 rpm to 1600 rpm, CFM didn't. The average increase is only about 35%, so fan efficiency is definitely suspect; again, this is pointed out. And while this is only an excerpt, the full post went on to test an electric fan set-up. The two fans drew about 65 amps total. Watts is amps times volts, so that's 780 watts, or just a bit over 1 hp. The dyno showed about a 1.3 hp drop, so allowing for electrical/belt losses, that's right on the money. As for the guy trying to 'debunk' mechanical fan hp loss, well, all I can say is while his math is correct, his premise isn't...
Shouldn't the loss of power be measured in a percentage of the actual engine's power, instead of a fixed horsepower number? Different engines demands different amounts of air to keep at correct operating temperature, and fan/belt/radiator configuration differs between engines. If it takes X kW to move Y cfm of air, an electrical motor will be very close to X, since it's extremely efficient compared to an old gasoline engine. Moving the same amount of air with a mechanical fan on a gasoline engine would take more power due to larger energy losses? Add further losses at drivebelts, and less efficient air flow it seems that, HP-wise, an electrical fan should be better. That being said, I used a mechanical fan (with shroud) in my Cadillac 390-powered model A with hood and hood sides, and it seemed to work fine. Electrical would probably been better, but oh so ugly.
A point not mentioned. A mechanical fan robs most of hp from the wall it creates while spinning, while not needed at cruising speed.
The way I understand the article is: The 40hp loss is a measurement of the extra power it takes to accelerate the mass of the mechanical fan from idle to 4500 rpm, not how much power it takes at a stable speed, such as cruising. Having a mechanical fan is like having a heavy flywheel or heavy tires and rims. It takes more power to get that heavy mass spinning. Once there, it takes very little power to keep it turning. If you remove the weight of the mechanical fan, it's like installing a lighter flywheel. Since the electric fan is unchanged by engine RPM, it has no power robbing effect during acceleration. If you had two engines running side by side, one with an electric fan and one with a mechanical fan, there would only be a minor difference in horsepower loss between them. If the engine with the mechanical fan were accelerated slowly, such as when driving around town, you also probably wouldn't notice very much loss. Only when the engine is accelerating from idle to wide open could you measure a 40hp loss over the entire time it takes to get it from idle speed to maximum rpm. Sent from my iPhone using The H.A.M.B. mobile app
No, there's 2 different numbers in their tests. One is the horsepower loss at a steady 4500 rpm and the other is the time difference it takes to accelerate the engine from idle to 4500 with and without a fan. You're correct that it would take little power to keep the fan spinning at a constant speed if the flywheel effect was the only consideration. But the flywheel portion of the total power requirement is very small as compared to the air horsepower portion. Flywheel effect considers only the mass and the effective distance of the center of mass from the center of rotation. If you put a fan and an equivalent flat disc of steel in a vacuum chamber and spin both of them up to speed, you would be measuring pure flywheel effect and both would be expected to spin up at the same rate. If you then spin both up in air, the flat disc would accelerate at near the same rate that it did in a vacuum, with the only losses being the air friction across the surface of the disc. But the fan is going to accelerate much more slowly, not due to flywheel effect but rather due to the fact that it is acting as an air pump.
I understand what you mean, and I think the story is rather confusing, in how it presented the data, and the procedure...it's rather vague. But I think you have it wrong, simply because the work it takes to spin up the fan is just that, a quantity of work. The power it takes to spin the fan at a constant RPM is just that, a level of power. Power is a rate, that of work over time. They are two different things that you can't equate.
I originally posted on here, and deleted it immediately. Why? I figured it would turn into a typical HAMB "stupidfest" and figured I didn't want to get involved......
Danny, with all due respect, when there is no room, there is no room (as in my avatar), I've said this before, if we would've had them back in the day, we would've used them!
Agree with Squirrel that the first set of data is presented in a confusing manner. The second set of calculations, where the author tries to debunk the 17hp fan, only succeeds in showing the author has no clue about how fans work. The giveaway there is when he starts talking about how some inefficient fan might draw 17 hp but an efficient one will draw far less power. In reality, the opposite is true. A mechanical fan has to be designed to be very efficient at the worst operating conditions. Worst conditions being idling in traffic on a hot day at near zero speed. This same low speed efficiency causes the fan to be to be a power hog at higher speeds, but that's because the fan is efficient rather than inefficient. A flex fan is an example of a fan that becomes less efficient at higher speeds, and uses less power as compared to a rigid fan at the same speeds. To clarify, I'm just explaining how flex fans work rather than recommending them, as I'm no "fan" of them from a safety standpoint. The 40hp figure at 4500 rpm is misleading in that it might lead people to assume they're eating up 25 hp or so at normal cruising speeds. The reality is that the power required for a propeller fan is proportional to the cube of the rpm. So, if your fan really does eat 40hp at 4500, but your highway cruising rpm is half that, or 2250 rpm, then that same fan would be expected to eat only 5 hp at that speed. In a cubic relationship, doubling the speed increases the power by a factor of 8 to 1. By the same relationship, if you're puttering around at 1500 rpm, then the same fan would pull about 1.5 hp. So, unless you're racing or cruising the streets with a 5.13 gear, the power used by a mechanical fin is going to be fairly insignificant. There is no question a well designed electric fan uses less power since the blades can be designed for max efficiency at one speed that never changes, but that's the nature of the beast. If you could figure out how to run a mechanical fan at a constant speed rather than varying with engine speed, then the mechanical fan wouldn't use any more power than an electric one.
Another factor absent from the calculations is the effect of ambient air movement. According to my calculations the figures around 2000cfm mentioned in the studies translate to air moving through the radiator at around 16-17mph. How does this compare to what actually happens under the hood of a moving vehicle? Add to this the way the aerodynamic efficiency of a hot radiator can be slightly better than that of a cold one due to expansion of air as it passes through. There are a lot of unpredictable variables, but I think it's safe to say that it takes less power to move air that is already moving in the desired direction than air that is standing still. How much I can't say.
I'm with HRP. I don't care much for the electric fan myself. I like the original look. I guess if you need one, you need one and can't blame anyone for needing one.
There are some builds that simply cannot use a mechanical fan. Does that mean that they should not be built? Some of you seem to believe that the answer should be yes. I do not.
One of the car mags ran a test on flex fans back in the day (early 70s IIRC, and no, I don't remember which one) and their results were reasonably close to the dyno test posted. This was strictly a comparison between engine-driven fans, electrics weren't being seen yet. In addition to a dyno test, they also went to the strip, and the flex fan was worth 2-3 tenths in the quarter. It should be noted that the hard-core racers remove the mechanical fan, and either run with no fan or these days install an electric. If you look around on the 'net, you'll find reports from diesel pick-up owners gaining 2-3 mpg after switching to an electric fan. That would add up rather quickly... There's also a bunch of caveats connected to electric fans. One, puller fans work much better than pusher fans. Two, mounting the fan too close to the radiator reduces it's efficiency. Three, if you're running a brass/copper radiator, they work less well as the air resistance through these is higher than an aluminum radiator. If you're running a 3 or 4 row copper radiator, an electric isn't recommended at all except as a 'supplemental' fan (which is why you don't see them on big trucks, motor homes, etc) because of this resistance.
You can't build a traditional car and use a electric fan - self evident truth. The E fans lets us place stuff in a way they couldn't, move things around and get flatter floors, lower riding engines, big stuff under stick hoods and have foot room, ride heights ans the list goes on and on. All that said- here's my radiator, fan and shroud for my blown hemi.
Very true, but as I have said, you can build yesterday's car, but you cannot drive it on yesterday's roads, with yesterday's drivers, in yesterday's traffic, and run it on yesterday's fuel.
As a rule, my #1 choice is a clutch fan. My sb s10 has twin electric fans, they work very well, but are a lot of extra complication. Neither is an option for a period-correct car. I am putting a chromed 4 blade fan on the olds I am putting in my '39, but I don't plan to beat that car. Although I will still probably rev it past 4500 rpm.
Clutch fan doesn't bother me. Jaguars had the shortest ones and available from NAPA. If it's painted back you can't tell WTF it is at 4500 Rpms
I've got a mega blade fan that looks like a clutch fan but bolts directly to the hub of the water pump that will suck a cat though the radiator if you goose the engine but that sob is so noisy you can't stand to drive the rig for very far. Had it on my 71 GMC for a while and it does what it is intended to do and that is move air. I took it off because of the noise. I've had both stainless and fiberglass flex fans break over the years and my feeling is it isn't if they will break but when they will break. The main that bugged/bugs me about electric fans is the ones that are wired on street rods so they run after the engine is turned off and the car owner is half the fairgrounds away and you have to listen to the damned thing sit there and hum. Still it is what works on what you have to work with and what space you have. My 48 being a prime example. The space I have only allows for a crappy little flex fan that doesn't pull any air at all to speak of. Run 183 on the road in any temp and shoot up sky high as soon as it slows to the point that normal air flow though the grill isn't there. An electric fan is probably going to be a must have this time around.
The Jalopy Journal
