E-Fans with towing

03envoy

Original poster
Member
Dec 25, 2011
537
I was told in another thread, That the E Fans would not be good for towing as they would not keep the engine cool enough. I just emailed Pcmforless and they said towing is fine with them? Do you guys think it's not worth it when towing? I believe they add a decent amount of RWHP, So that could help when towing Also. Any thoughts? Thanks
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
How much HP do you think the existing mechanical fan can steal by way of the serpentine belt? 1-2 HP is my guess. Not enough to notice.

My experience with LS2 fans was not good, even going up a hill NOT towing, if it was above 90 degrees. Slow speed high RPM in 4LO range in desert offroading was even worse. I removed the unit, and wouldn't recommend it unless you have some other functional need. You would get a lot more bang for the buck in terms of saving money in the long run by adding a tranny cooler for less $.
 

03envoy

Original poster
Member
Dec 25, 2011
537
the roadie said:
How much HP do you think the existing mechanical fan can steal by way of the serpentine belt? 1-2 HP is my guess. Not enough to notice.

My experience with LS2 fans was not good, even going up a hill NOT towing, if it was above 90 degrees. Slow speed high RPM in 4LO range in desert offroading was even worse. I removed the unit, and wouldn't recommend it unless you have some other functional need. You would get a lot more bang for the buck in terms of saving money in the long run by adding a tranny cooler for less $.


I am for sure getting a trans cooler. I was a little skeptical as to that much increase in HP. Thanks Roadie. I am just going to get the cooler and intake.... The intake is for my pleasure not increase in HP lol:biggrin:
 

ScarabEpic22

Member
Nov 20, 2011
728
Actually Bill, the fan clutch has been proven to rob anywhere from 5-15rwhp on the SS, so maybe 3-10hp on an I6. 10hp is nothing to scoff at, especially on an I6!

Get one of PCM4Less' trans cooler kits, they are very good. Ive got the brake duct one with black braided lines on my SS, dropped my towing temps by 20-40F depending on conditions. I just finished up my efans install on my SS, Im skeptical how towing will be but I had all the parts already so I figured why the heck not use them. Ill monitor the temps closely with EFILive, if it overheats Ill swap back to the mechanical clutch for the summer.
 

03envoy

Original poster
Member
Dec 25, 2011
537
ScarabEpic22 said:
Actually Bill, the fan clutch has been proven to rob anywhere from 5-15rwhp on the SS, so maybe 3-10hp on an I6. 10hp is nothing to scoff at, especially on an I6!

Get one of PCM4Less' trans cooler kits, they are very good. Ive got the brake duct one with black braided lines on my SS, dropped my towing temps by 20-40F depending on conditions. I just finished up my efans install on my SS, Im skeptical how towing will be but I had all the parts already so I figured why the heck not use them. Ill monitor the temps closely with EFILive, if it overheats Ill swap back to the mechanical clutch for the summer.


Yea, I am not diggin these Efans..... But I was considering them. I am for sure getting the trans cooler, it should help alot!
 

n0kfb

Member
Dec 8, 2011
104
ScarabEpic22 said:
Actually Bill, the fan clutch has been proven to rob anywhere from 5-15rwhp on the SS, so maybe 3-10hp on an I6. 10hp is nothing to scoff at, especially on an I6!
<snip>

You've got to be kidding... 10 horsepower out of 300? That's barely out of the margin for error on a real dyno, let alone the seat-of-your pants.

I see no reason to go to an e-fan on our vehicles.

-- Dan Meyer :coffee:
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
ScarabEpic22 said:
Actually Bill, the fan clutch has been proven to rob anywhere from 5-15rwhp on the SS, so maybe 3-10hp on an I6. 10hp is nothing to scoff at, especially on an I6!
I keep trying to track down the source of this data. Do you have a link to any dyno tests where the only difference from run to run is the fan?

Basically, I remain skeptical for one simple reason. A mechanical fan has to be APPROXIMATELY equal in power to move about the same amount of air. A 1 HP electric motor consumes 746 Watts - by definition. 746 Watts at 14V = 53 Amps. Again - not by guessing - by the definition of electrical terms.

A mechanical fan that robs 1 HP from the engine would have to be replaced by an electrical fan that consumed about 50 Amps from the alternator and battery. At 3 HP and about 150A, the alternator's maxed out and you're stealing from the battery. And a 3 HP electric motor is about the size you see on 9000 pound winches. They weigh a bit more than the ones on electric fan assemblies.

An electric fan that has lets say two motors and draws 50A when both are running is about a HP, put another way.

A 10 HP electrical fan is out of the question. Therefore, I claim, a 10 HP mechanical fan is equally ludicrous.

Besides - any gains for the SS are for a few seconds of track use. Our electroviscous fan clutch might take a minute to disengage when you're cruising down the road and have adequate road speed airflow past the radiator (when the electric OR mechanical fan's airflow boost isn't needed), but on the road, even towing, unless you're over 220 coolant temp, the mechanical fan is only stealing a few Watts of residual energy from parasitic friction.
 

06Envoy

Member
Dec 4, 2011
419
the roadie said:
I keep trying to track down the source of this data. Do you have a link to any dyno tests where the only difference from run to run is the fan?

Basically, I remain skeptical for one simple reason. A mechanical fan has to be APPROXIMATELY equal in power to move about the same amount of air. A 1 HP electric motor consumes 746 Watts - by definition. 746 Watts at 14V = 53 Amps. Again - not by guessing - by the definition of electrical terms.

A mechanical fan that robs 1 HP from the engine would have to be replaced by an electrical fan that consumed about 50 Amps from the alternator and battery. At 3 HP and about 150A, the alternator's maxed out and you're stealing from the battery. And a 3 HP electric motor is about the size you see on 9000 pound winches. They weigh a bit more than the ones on electric fan assemblies.

An electric fan that has lets say two motors and draws 50A when both are running is about a HP, put another way.

A 10 HP electrical fan is out of the question. Therefore, I claim, a 10 HP mechanical fan is equally ludicrous.

Besides - any gains for the SS are for a few seconds of track use. Our electroviscous fan clutch might take a minute to disengage when you're cruising down the road and have adequate road speed airflow past the radiator (when the electric OR mechanical fan's airflow boost isn't needed), but on the road, even towing, unless you're over 220 coolant temp, the mechanical fan is only stealing a few Watts of residual energy from parasitic friction.

Well, you got my brain working overtime on this one!
Disclaimer: I have the PCM4L Dual Spal efan kit on my truck and my previous trucks have always had DIY efan kits. I swear by them. so when I read this post, I thought to myself, 'self, why do I swear by them???'
Roadie has an unquestionable background and is most certainly qualified to speak on this subject, so I found myself asking why do I find myself disagreeing with him then?

So the first thing I did a few days ago is look for a dyno chart that matches his request using search terms of 'trailblazer dyno chart efan'. I only found a reference to a post on the OS about the PCM4L original efan setup. Alvin actually ran on his dyno this exact request. Unfortunately that included pic of the dyno chart is no longer available. So, no I don't have a dyno chart. All I can offer is years of butt-o-meter results, which screams at me that I can feel it. Even recently with the new PCM4L kit I just got installed.

Next, I spent a day on self reflection. Did I imagine the effect or not? No. I can honestly say it feels faster on efans than mech fan. Great! I'm not going crazy, but Roadie isn't going to accept my self reflections as evidence!

So I still needed something to come up with to explain why efans work.
So this morning I offer these two points for Roadies dissection. So far these are the two best ideas that I can come up with.

1) A comparison of electrical motors vs Internal combustion motors:
- An Electric motor has peak torque at lower rpms and torque falls off at higher rpms
- An IC motor has peak torque at higher rpms
- The peak torque ranges are inverse between an electric and IC motors.
- Perhaps that's why the I-net is chock full of claims that a '10hp electric can do the work of a 25hp IC motor' without any scientific backing.


2) What if HP ratings only tell a part of the story?
- Consider the following;
Horsepower by itself doesn't really define much. You can have a 10 horse engine that runs at 7000 rpm that won't even cut the grass, or a 10 horse engine rated at 700 rpm that can pull a plow. Both are capable of producing 10 horsepower, but one has much more torque than the other. The leverage that the motor/engine puts on the crankshaft is the torque. To get a real idea of the performance of an engine/motor, you have to consider all three things - torque, horsepower, and rpm.
- I lack the math skills to compare torque, horsepower and rpm between the mechanical fan and the electric fan, but I'm hoping that Roadie does.


I know that Roadie has had a bad experience with efans in the past. But I'm hoping that he will take a closer look at why so many people all say the same thing. That they work and they actually do something that can be felt.
Sorry for my long winded ramblings. :smile:
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
06Envoy said:
So, no I don't have a dyno chart.
I've never seen or had a dyno test. Are they quick runs through the RPM range in a minute or less, or is the engine allowed to stabilize with the fan running at its final desired set point? Are data being taken by the OBDII connector in terms of ECT and commanded versus actual electroviscous fan RPM? Is the engine pushed into the RPM/Torque/groundspeed regime that would overheat it without added cooling from the Efan or the EV fan clutch?
All I can offer is years of butt-o-meter results, which screams at me that I can feel it. Even recently with the new PCM4L kit I just got installed.
A vehicle just plopped down on the starting line, having been idling, won't have high ECT. The EV fan clutch shouldn't be engaged, but there is residual friction of a hundred Watts equivalent max. You can stop the fan blade with a rag when it's not engaged. The Efan may not be running either if the engine is not calling for cooling. When you go WOT, the efans have no rotational inertia to overcome and the EV fan may. If the engine is already over 210 and calling for cooling and the EV fan clutch is partially engaged, then there will be a bit of rotational inertia to overcome. I just claim due to the mass of the fan and clutch, this isn't much. You have more rotational inertia in there when you go from idle to WOT RPM, in the alternator, PS pump, and possibly the AC compressor if that's not turned off. Actually the PCM will turn off the AC compressor if it notices a full WOT condition. I suspect most folks aren't going full WOT for launches. OK, so after the initial ramp up to WOT RPM, what's left for the EV fan clutch to steal for power is steady state airflow. Below a certain ground speed, the fan will be accelerating the air flowing past it, and that's what steals power. At WOT and low ground speed, that's the worst conditions. As your ground speed goes up, the air speed going into the fan goes up, and the fan has to less work to accelerate it. I should do the math on what the OUTPUT airflow speed is, and take an educated guess on the fan blade efficiency. No time for that right now.
Next, I spent a day on self reflection. Did I imagine the effect or not? No. I can honestly say it feels faster on efans than mech fan. Great! I'm not going crazy, but Roadie isn't going to accept my self reflections as evidence!
You got that right. :wink:
So I still needed something to come up with to explain why efans work.
So this morning I offer these two points for Roadies dissection. So far these are the two best ideas that I can come up with.

1) A comparison of electrical motors vs Internal combustion motors:
- An Electric motor has peak torque at lower rpms and torque falls off at higher rpms
- An IC motor has peak torque at higher rpms
- The peak torque ranges are inverse between an electric and IC motors.
- Perhaps that's why the I-net is chock full of claims that a '10hp electric can do the work of a 25hp IC motor' without any scientific backing.
True, but look at the SIZE of the motors on the efans. Compare it to the size of an electric 10 HP motor.
2) What if HP ratings only tell a part of the story?
- Consider the following;
- I lack the math skills to compare torque, horsepower and rpm between the mechanical fan and the electric fan, but I'm hoping that Roadie does.
I have mechanical engineering friends who can do that in their sleep, and IIRC, I asked them ten years ago and they said efans were bunk. I'll revisit that when I get some time.

I know that Roadie has had a bad experience with efans in the past.
My issue was my unit didn't offer enough airflow to adequately cool the radiator in my conditions of desert offroading. The controller was fine.
But I'm hoping that he will take a closer look at why so many people all say the same thing. That they work and they actually do something that can be felt.
Sorry for my long winded ramblings. :smile:
I can name a dozen things that so many people believe that I personally think is evidence of mass insanity, but that would be going WAY too deeply into politics or religion. On religion especially - just look around and ask: "They can't ALL be right, can they? Most every one denies the legitimacy of the others." I think many technical things are like that. Unless the kid says "Look, the emperor has no clothes!", everybody just plays along and believes blindly. I was raised to be skeptical. Trust, but verify. Bring data to the table to convince me.
 

Mooseman

Moderator
Dec 4, 2011
25,646
Ottawa, ON
Just having changed over to efans, I can give my personal experiences. No dyno tests, no charts or data except for butt-o-meter and MPG.

Before the switch, I knew my clutch was failing as it had play in the bearing allowing it to move back and forth at the fan edge a good 3/8 of an inch. Stopping and holding the fan with a rag, I could feel it really wanting to pull and that was at below engagement temps. I even unplugged it it make sure it wasn't the PCM commanding it for some reason and it made no difference. And while holding it, it wasn't an even pull neither, just an uneven jerking. Although I didn't, if the engine would have been revved, it probably would have ripped out of my hand. Other than that, my MPG sucked big time, getting about 10-12MPG combined. $100-120/week in gas really hurt. I was already planning going to efans having the fan errors taken out of my PCM during my tune.

Right away, I felt the difference. Engine was more responsive and quieter and MPG went to 16MPG combined and 19MPG highway. Cooling so far has been adequate but we haven't had any real heat here yet. I have also been experimenting with two different ways that the fans come on using my 2 fan controller. At first I was having fan1 come on at 100% and then fan2 at 100% when the temperature went 10F over the first set temperature. It rarely kicked in fan2. I then changed the wiring and relays to run both fans at 50%, wired in series, when the first temp was reached and then both at 100%. Theoretically, the CFM at low speed should be the same as one fan at 100% but at lower noise and across the whole radiator. Unfortunately, it's not working out that way as the full speed kicks in almost always not long after the first temp is reached. I will be going back to the first setup. Current draws are the same either way.

My setup is a bit different than most, using Montana/Venture van efans rather than the LS1's. Each fan is 14" wide instead of the 11" for the LS1/Camaro fans and they do pull a lot of air. I don't have any CFM numbers on these but the motors seem to be the same as the LS1's and 14" is bigger so they should pull more air. These vans are big with 3.9L V6 with front and rear A/C. In the TB, there is about 1" that go past each side of the radiator but the extra air these pull make up for it. I basically had this setup in my '00 Blazer and never had an issue, even when towing and running the A/C. In the TB, I may never know how it handles the A/C since it is currently non-functional (leaking evap).

So what can I interpret from this? Likely my failing fan was responsible for my poor MPG but I am not impressed by GM's failed experiment of the EV fans and I already had the efans, saving on costs. Until I can really try it out in hot weather, I won't be able know if this works but I'm pretty sure it will. If it does fail, I will replace the clutch with a 2008 old style viscous clutch. One thing though is I like the space to be able to do my weekly repairs :biggrin:
 

Sparky

Member
Dec 4, 2011
12,927
n0kfb said:
You've got to be kidding... 10 horsepower out of 300? That's barely out of the margin for error on a real dyno, let alone the seat-of-your pants.

I see no reason to go to an e-fan on our vehicles.

-- Dan Meyer :coffee:

I can see where the mpg side could come into play on the highway perhaps, if it indeed does rob 10hp. At cruising speeds the engine isn't putting out anywhere close to 300HP, so that 10HP is now a much higher percentage of the current output and therefore could impact mpg more.

I don't know what the actual differences may be however.

I'm planning an efan conversion on my 02 for a few reasons:

1) Easier to work on stuff in front of the engine (more room)
2) I have to take apart all that stuff to get to my stupid AC pulley bearing anyway
3) Because I want to :rotfl:
 

ScarabEpic22

Member
Nov 20, 2011
728
Ill look back through the posts on TBSSOwners and TrailVoy to find the posts for you Bill, they do exist and Chris at Vector MotorSports was in the exact same boat you are currently. Lots of SS guys were giving him crap for not offering an efans kit when Andy at ADM and Alvin were, so he set out to prove them wrong. Well, he was wrong and admitted it. He strapped a TBSS to his dyno, did a few baseline pulls, removed the fan clutch and installed efans, did more dyno pulls and got around 5-15rwhp (remember, multiple runs to try to take dyno error out).

Im not calling your math skills into question at all Bill, just stating that it seems to work on a repeatable basis on multiple trucks around the country. I was hoping to do a before/after dyno test, but I dont think thats an option right now. I could swap the efans out for the fan clutch in probably 30min now (not do the fan shroud as you have to pull the upper radiator hose), but at $100min/hr for dyno time, thats an expensive 30min!

I will say this, simply removing the mechanical fan clutch (08 so no PWM) and installing a set of LS1 efans, my engine revs faster. The only tune change was to take my existing tune and copy it over to an efans patched stock tune so the ECM could control the fans. No TCM changes, all ECM changes Ive been running a for a few weeks at this point. I was skeptical at first, then I went for a 30min drive to find some open area and open it up. Instant difference. Heck, even if it isnt good for the 5-15hp, Ill take a quicker revving engine any day of the week.
 

The_Roadie

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Nov 19, 2011
9,957
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ScarabEpic22 said:
...Ill take a quicker revving engine any day of the week.
Hmmmm, sounds like there are two effects going on, and you're describing one where you have less rotational mass to spin up during RPM increase. The other would be less power wasted by the mechanical fan when you're at a steady RPM.

Assuming you're not in neutral, anybody could do the experiment of removing the serpentine belt for a 60 second run and seeing if the EVEN LARGER reduction in rotating mass gave a noticeable difference. Even if you have efans, you could remove the belt for an experiment.

For me, this still fails the gedanken experiment (a scientific process of trying things out first by thinking about them) for the following reason. If the effect is caused by less rotating mass on the engine to spin up when you launch, and that weight is only 4-5 pounds, why is it so noticeable compared to the huge rotating mass of the tranny, drivetrain, and four wheels and tires - all of which have to be rotationally accelerated by the engine to get spinning?

The RPM versus time graph should be easy to plot.

I still think that at best it's a psychological effect of less noise coming from the engine compartment on a launch. But I'm certainly willing to be persuaded by dyno charts with some description of the root cause based in physics.
 

ScarabEpic22

Member
Nov 20, 2011
728
Eh hopefully you know me well enough by now Bill that Im pretty skeptical when it comes to these things too. Might not throw all the physics and math at it, but Im skeptical that simply removing a 10lb mass from the front of the engine makes this big of an improvement.

BTW, I never heard my fan go crazy on hard launches with the SS, the 02 would stick every now and again and take some revving to have it disengage. Id be willing to bet it is a little of both, city mileage improves because of less rotating mass to spin as the engine revs and hwy improves because if/when the clutch doesnt engage and rob the engine of power.

The effect might be more pronounced because the weight isnt located on the crank, but rather the water pump. Yes it weighs much less than the torque converter/trans/xfer case/driveline/rear end/wheels, but Im guessing the location of the weight plays more of a part in this. Removing the serp belt will let the engine rev faster and make more power, why do you think the SAE revised the HP/TQ rating standards in 07 or 08? The old tests used to test the engines sans accessory drive (not 100% positive, but pretty sure).

And all new fullsize trucks come with efans and have since 07, GM must have used them for a reason (Im thinking mileage). I believe the HD/Duramax trucks still have a clutch though.
 

ElAviator72

Member
Jan 11, 2012
118
kardain said:
^slightly OT, but I have two of these in my Sunfire.... Yay for lifetime warranty on the alternator! Fortunately, they don't kick on full speed that often due a two stage fan control.

Spal 12" High Output Fan: A1 Electric Online Store

Heh, they don't kick in at all on my Sunfire, unless the AC is on. I'm convinced that this is a defect in just about all front wheel drive GM vehicles with a Quad 4, as I've known several others that have the same issue :mad:

The only time you'll notice it is if you get stuck in stop-and-go traffic on the freeway or spend a long time waiting at a drive-thru. I turn my A/C on if I notice the temp going up. I'm wondering what the defect really is...seems like the computer would automatically cycle the fan on when needed based on coolant temp.
 

kardain

Member
Dec 16, 2011
557
ElAviator72 said:
Heh, they don't kick in at all on my Sunfire, unless the AC is on. I'm convinced that this is a defect in just about all front wheel drive GM vehicles with a Quad 4, as I've known several others that have the same issue :mad:

The only time you'll notice it is if you get stuck in stop-and-go traffic on the freeway or spend a long time waiting at a drive-thru. I turn my A/C on if I notice the temp going up. I'm wondering what the defect really is...seems like the computer would automatically cycle the fan on when needed based on coolant temp.

Using the stock tune in a cavalier/sunfire, the fan turn on temp is 223* (+/- a couple degrees), off at 200-ish unless the AC is turned on which overrides the turn on temp... Certain CEL's will also command the fan to turn on automatically. However, I don't have an I4 in mine, nor am I running a stock tune :tongue:
 

Mooseman

Moderator
Dec 4, 2011
25,646
Ottawa, ON
ScarabEpic22 said:
And all new fullsize trucks come with efans and have since 07, GM must have used them for a reason (Im thinking mileage). I believe the HD/Duramax trucks still have a clutch though.

I've seen them in 1994-1996 Caprice 5.7L V8 Police cars from the factory. They used to have clutch fans before. Why do you think they went to efans? If efans could cool those down, and the full size trucks, then the problem is with our setups. There's probably also a point where the heat generated cannot be cooled enough or that because of the torque produced by the engine (HD/Duramax diesel) that efans would not be of any benefit. Cost is also probably another factor.
 

ScarabEpic22

Member
Nov 20, 2011
728
The reason GM didnt include efans from the beginning with our trucks is rumored to be a small engine bay (mostly for the V8s) and smaller radiators. The fullsize trucks have MUCH bigger radiators and can dissipate heat much better than ours can. The I6 one is smaller than the V8 too! Eventually I want to get a Ron Davis radiator for my SS (fits all V8s) and PCM4Less' Spal fans setup, problem is I dont have the $$ to do that currently. Sometime soon yes, but I do want to see how towing with LS1 fans and the stock radiator goes.

P.S. GM had multiple test mules running around with efans on our trucks before they produced them, problem is they would overheat in extreme conditions (think Death Valley towing 6000lbs up a grade). So they stuck with the fan clutch, yay for us.
 

Jon A

Member
Dec 25, 2012
5
OK, how about we knock this one out once and for all?

Option A: Take the easy way out, skip to the final answer:

Fanhorsepower-1.jpg


That chart was posted by GMT360 on the other board. He was a cooling system Engineer at GM who worked extensively on the GMT360 (hence the username) and has given us all more info on the fan and fan clutch of these things than we likely know all put together. Chances are, that chart came from somewhere....

Option B: Let's assume GMT360 was full of beans and come up with our own number independently. I'll provide some data, if anybody else has some they'd like to add we'll throw it into the mix.

My electric fans, the Derale 16927, are rated to consume 50 amps at full speed at 12 volts. That equates to 50*12/746 = .804 HP. In reality, they've been measured consuming a bit more than that and a good alternator will keep the voltage higher than 12 volts so they probably operate closer to 1 HP, but to keep things as conservative as possible, we'll leave it at .804 HP. And let's even assume the alternator is 100% efficient at converting mechanical energy to electrical energy so we don't say it's consuming even more power from the engine....

How much air do they move compared with stock? I took some measurements....

PICT0334.JPG


Nothing too high tech, just a wind meter placed at various locations. While these measurements aren't accurate down to the last cfm, they're more than "in the ballpark," I'd say they're pretty darn close. If anybody would like to take more accurate measurements, I would welcome your data.

Averageariflow.jpg


That chart shows the results. It tells us that at full boogie, the stock fan can move air through the radiator at an average speed of 33.375 MPH, while my Derale fans can do the same at 13 MPH. This tells us the stock fan when fully activated moves 2.57 times as much air as my electric fans.

Now it's time to enter into evidence some "Universal Fan Laws"--available in pretty much any Engineering text that talks about fans at all:

FanLaws.jpg


OK, so let's say I want to increase the power of my electric fans to match the output of the stock fan. As you can see from above, given the best case scenario of my fan blades keeping the same efficiency, etc, I would need to increase their RPM by 2.57 times in order to increase the CFM 2.57 times as they are directly proportional.

So how much power would that require? Now you go to the equation that really smacks you in the face. The power required is proportional to the RPM CUBED!!!! Yes, CUBED!!!!

Ouch, that hurts.

This gives us, (2.57)^3*.804 = 13.65 HP required to spin my fans fast enough to match the output of the stock fan. And that's with many conservative assumptions that keep the number lower than it is in reality. Amazing how we validated GMT360's chart though, isn't it?

For those thinking the RPM Cubed equation seems off, you need to brush up on your fluid dynamics. It's pretty standard fair. Even forgetting fans, it's the same for a simple object moving through the air--the force of air resistance of an object with a given coefficient of drag and frontal area is proportional to the square if its velocity...and since power is force times velocity, the power required to apply that force is proportional to the cube of the velocity. Standard stuff.

So I do agree with the contention a 14 HP (or two 7 HP) electric motors would be quite impossible to package. The conclusion one should draw from these data are that once a certain level of airflow is required, one should simply cross electric fans off the list of possibilities as making electric motors that huge fit between the engine and the radiator becomes impossible. The more CFM you have, the harder each additional CFM is to get--it's exponential. But when you have a 270 HP "fan motor" and can tap into as much of that power as you want with your blade/clutch design of a mechanical fan, if you really need to move that much air mechanical is the only option.

Of course that wasn't the question posed in this thread. The answer to the question being posed--does the mechanical fan really take that much HP?--when you follow the math above, is an obvious "yes."
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
Jon! Fellow engineer! (40 year EE in semiconductor system design). I found your personal web site from your image hosting (I do the same thing) and am really interested in what you do for a day job.

In any event, I've been saying the same things about 746 Watts versus parasitic power loss. One second- or third-order effect to think about is the air velocity through the radiator caused by ground speed (or even headwind/tailwind). At some point, the velocity through the radiator EQUALS the velocity induced by the fan blades, so the net loss with the mechanical fan approaches zero. At some cruise RPMs and high enough ground speed, the fan force reverses and the fan steals energy from the airflow and drives energy into the rotating parts of the engine. Of course, this comes at the cost of increased drag, so there's no such thing as a free lunch.

If the vehicle never stops moving through the air (like a shark that can't cease moving), you can take the mechanical fan off. Induced airflow is sufficient.

Electrical fans can also act as a generator under those conditions. I think the entire claimed benefit for efans is justified in the 0-20 MPG regime and above that they generate electricity and are out of the equation.

But I suspect the HP versus fan RPM charts and the dyno charts are all done with zero ground speed, or at least an external fan-induced airflow that's not proportional to what the ground speed would have been if the vehicle was moving.

Any dyno enthusiasts know what kind of external airflow is being supplied during a run? None? Proportional velocity? Fixed velocity fan? Is the velocity being measured or compensated for in the calculations of HP-robbing?
 

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