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Thu Nov 05, 2009 2:28 am

I think I saw some place (in these great pages of wisdom - TheSamba) a rating of how much air (CFM) the stock (doghouse) fan moves at cruising RPM (say 4000 RPM).

If you can find this tid bit of information, you can easily compare it to the rating of an electric fan (or maybe a pair of them). If you can find a fan that moves at least as much air as the stock fan, then you've found your system.

Then check how many volts/amps it needs to run. If it is close to the max output of the alternator (55A?) then you may have to try again.

Not trying to encourage/discourage you, just explaining you can do some research so you don't have to spend money to find your answer.

Good hunting !

EDIT: Found this very interesting link...
http://www.offroadvw.net/tech/wes/fan.html

It basically shows that the stock doghouse fan generates between 1300-1500cfm of air at 4000 (engine) rpm.
If I were goona replace my stock fan I would probably target max airflow in that range (maybe a little less since I don't hit 4000rpm often).

Enjoy !

Thu Nov 05, 2009 5:47 am

ashman40 wrote: I think I saw some place (in these great pages of wisdom - TheSamba) a rating of how much air (CFM) the stock (doghouse) fan moves at cruising RPM (say 4000 RPM).

If you can find this tid bit of information, you can easily compare it to the rating of an electric fan (or maybe a pair of them). If you can find a fan that moves at least as much air as the stock fan, then you've found your system.

Then check how many volts/amps it needs to run. If it is close to the max output of the alternator (55A?) then you may have to try again.

Not trying to encourage/discourage you, just explaining you can do some research so you don't have to spend money to find your answer.

Good hunting !

EDIT: Found this very interesting link...
http://www.offroadvw.net/tech/wes/fan.html

It basically shows that the stock doghouse fan generates between 1300-1500cfm of air at 4000 (engine) rpm.
If I were goona replace my stock fan I would probably target max airflow in that range (maybe a little less since I don't hit 4000rpm often).

Enjoy !

EXCEPT that if you remove the doghouse, how do you know if that quantity of air is sufficient and that it is being directed properly? Without knowing (since I think it is a bad idea to try this), I imagine you would have to increase the airflow significantly to offset the lack of the doghouse.

Thu Nov 05, 2009 6:07 am

Just a thought but why couldnt you just remove the fan from the gen/alt that is in the car, relocate a newer style alternator (since you have to make custom brackets anyways) and find a way to have an electric motor run the fan that is already there? I agree with what has been said before that when you convert from one form of energy to another and then back you lose some efficiency, but if you had the fan thermostatically controlled maybe you can actually gain by not running the fan as much as it normally does and only as much as its needed?!
It sounds like a fun experiment though. Good luck, and like the others have said, research a ton first so you dont waste any money you dont have!

Thu Nov 05, 2009 7:49 pm

I agree with the notion that this may fail numerous times before it works, but I think it's a great idea, and I'm looking forward to seeing what happens.

I dig your willingness to try something outside the box.

Thu Nov 05, 2009 9:38 pm

MoparFreak69 wrote: Just a thought but why couldnt you just remove the fan from the gen/alt that is in the car, relocate a newer style alternator (since you have to make custom brackets anyways) and find a way to have an electric motor run the fan that is already there? I agree with what has been said before that when you convert from one form of energy to another and then back you lose some efficiency, but if you had the fan thermostatically controlled maybe you can actually gain by not running the fan as much as it normally does and only as much as its needed?!
It sounds like a fun experiment though. Good luck, and like the others have said, research a ton first so you dont waste any money you dont have!
this is exactly what I was thinking!

Fri Nov 06, 2009 4:31 pm

Ok. Simply put there is a certain amount of power that is required to turn a fan at a certain amount of cfm based upon the pressure it needs to push it. Every fan has a set fan curve that states its horsepower requirements. Fact is that once you get enough rpms on the vw fan the belt isn't even strong enough to keep pushing the fan and it begins to slip.

Power is power wether it is electric or mechanical. Somewhere that power needs to be generated. If you do it electrically then you will need an alternator powerful enough to generate it which will rob the power mechanically from the engine just as the fan does. A 12v motor that would be able to push enough cfm of air over the motor when producing its max heat would kill/fry your electric system unless you had it wired like the super sound system guys with multiple alternators.

Fri Nov 06, 2009 4:46 pm

Umm, just a thought...

...If VW couldn't make it work in 69 years, with billions (or at least a whole lotta millions) of dollars of r&d spent, it probably isn't very feasble.

Just sayin'.

Fri Nov 06, 2009 7:52 pm

I wouldnt say it isnt "feasable" because with enough bored time and money burning a hole in your pocket you can pretty much do anything. Is it worthwhile? Not to me.

Fri Nov 06, 2009 8:00 pm

Yellowbeard wrote: Umm, just a thought...

...If VW couldn't make it work in 69 years, with billions (or at least a whole lotta millions) of dollars of r&d spent, it probably isn't very feasble.

Just sayin'.
VW didn't foresee Hybrids, Hydrogen fuel cells and

multi valve heads until everyone did them for 5 years
turbos untill everyone else did them for 5 years
SUVs........
and more....

Fri Nov 06, 2009 8:11 pm

Glenn wrote: Yellowbeard wrote: Umm, just a thought...

...If VW couldn't make it work in 69 years, with billions (or at least a whole lotta millions) of dollars of r&d spent, it probably isn't very feasble.

Just sayin'.
VW didn't foresee Hybrids, Hydrogen fuel cells and

multi valve heads until everyone did them for 5 years
turbos untill everyone else did them for 5 years
SUVs........
and more....

Apple & oranges, Glenn.

Fri Nov 06, 2009 8:12 pm

I sort of agree... but hey, lets see what they come up with.

Fri Nov 06, 2009 8:17 pm

They are making some pretty compact and powerful electric motors as of late. The cooling fan I picked up for my boss's Nomad was rated at 750CFM at only 13 amp current draw. Thats really not too bad on the electrical system. It is rated for 100% duty cycle as well. Just a thought.

Electric cooling on watercooled frees up a lot of power, but the fan only runs part time, not all the time like an aircooled needs.
I personally would trust a fan belt over a fuse, wire, motor, etc to keep my engine from overheating.

Fri Nov 06, 2009 9:38 pm

mulewright wrote: Ok. Simply put there is a certain amount of power that is required to turn a fan at a certain amount of cfm based upon the pressure it needs to push it. Every fan has a set fan curve that states its horsepower requirements. Fact is that once you get enough rpms on the vw fan the belt isn't even strong enough to keep pushing the fan and it begins to slip.

Power is power wether it is electric or mechanical. Somewhere that power needs to be generated. If you do it electrically then you will need an alternator powerful enough to generate it which will rob the power mechanically from the engine just as the fan does. A 12v motor that would be able to push enough cfm of air over the motor when producing its max heat would kill/fry your electric system unless you had it wired like the super sound system guys with multiple alternators.
This hits the nail on the head. The laws of physics say electric cooling doesn't work.

Dyno tests have shown that it takes 12-15hp to spin the stock fan at 6k rpm. If you assumed 6-7hp at half that rpm, your fan would require an electric motor capable of 6-7hp. Ever seen one that big? Here's some math for you:

6hp x 746W/hp = 4.4kW.

4.4kW ÷ 14V = 320A.

So you see, it will take FOUR of those 90A Chevy alternators.

While you're at it, why not try to heat your car with an electric heater?

Sat Nov 07, 2009 12:04 am

Bruce wrote: ... Dyno tests have shown that it takes 12-15hp to spin the stock fan at 6k rpm. If you assumed 6-7hp at half that rpm, your fan would require an electric motor capable of 6-7hp.

I see two flaws in your math....

The article shows some of the math that is used to calculate hp needed to spin the fan. It mentions that the power needed is a cube (^3) of the rpm differences. This means the hp needed to drive the fan is not linear but is the cube of the ratio of the rpms. So reducing the rpm by half doesn't reduce hp by half, but by much more.

The article mentions that above 4000rpm (engine) the belt slips because the fan belt fails to transfer the needed hp. This makes me wonder if the 12-15hp @6000rpm is still 12-15hp @4000rpm and any increase in engine rpm above this does not result in increased fan rpm because the hp needed to rotate the fan faster cannot pass thru the fan belt.

Based on this and your 15hp @6000rpm...

E-RPM = Engine RPM
E-HP = Engine HP
F-RPM = Fan RPM

E-RPM E-HP F-RPM
6000 15.00 10667
5000 8.68 8889
4000 4.44 7111
3000 1.88 5333
2000 0.56 3556
1000 0.07 1778

Here are the same stats starting with 15hp @4000rpm...

E-RPM E-HP F-RPM
4000 15.00 7111
3000 6.33 5333
2000 1.88 3556
1000 0.23 1778

You can see that the amount of engine hp needed to drive the fan is very small at lower rpms. Also, all of this only applies to the stock fan & gen/alt system. If you use a non-stock fan you might get better performance (same cfm for lower rpm/hp).

FYI - I based the fan rpm (F-RPM) on the different radius I measured for the crank pulley and fan pulley:
crank pulley radius = 8mm
fan pulley radius = 4.5mm

This means the fan pulley spins approx 1.78x the engine rpm.

Sat Nov 07, 2009 12:31 am

ashman40 wrote: Bruce wrote: ... Dyno tests have shown that it takes 12-15hp to spin the stock fan at 6k rpm. If you assumed 6-7hp at half that rpm, your fan would require an electric motor capable of 6-7hp.

I see two flaws in your math....

3000 6.33 5333

Seems to agree with the number I came up with.

Where is this magical modern fan you found that takes almost no hp to deliver the same air as a VW fan?

Sat Nov 07, 2009 12:49 am

One other point is how much hp is needed to drive just the gen/alt function minus the fan? This should be subtracted to judge just the drag caused by the fan. Is the gen/alt drag constant thru all rpms?

Any motor mounted just to run the cooling fan would only need to overcome the resistance produced by the fan.

All hp/drag measurements of the fan/gen/alt I've seen are as a whole unit. Even the article above.

My thoughts are the target values we are using for a fan motor are too large. And as you can see, a small hp change at the high rpms can make a big difference as the rpms drop.

Sat Nov 07, 2009 8:59 am

ashman40 wrote: You can see that the amount of engine hp needed to drive the fan is very small at lower rpms. Also, all of this only applies to the stock fan & gen/alt system. If you use a non-stock fan you might get better performance (same cfm for lower rpm/hp).

this conclusion, no matter how much folks agree with how you got there, is the same conclusion that folks over the years that have tried to make it work, have come up with... since the OP's desire was better mileage seemingly based on more available HP without the fan in the equation, it's safe to assume that he won't be running high RPM's looking for power, and the required power in the lower RPM's is pretty small... back to the conclusion that it's not worth it, and may be more detrimental trying other ways than what you already have... That said, nothing wrong with trying for ones self if they wish.

Sat Nov 07, 2009 10:07 am

Yellowbeard wrote: Umm, just a thought...

...If VW couldn't make it work in 69 years, with billions (or at least a whole lotta millions) of dollars of r&d spent, it probably isn't very feasble.

Just sayin'.

Heh, yeah. Like overhead cams or water cooling.
Oh wait.

Sat Nov 07, 2009 11:13 am

Inane Cathode wrote: Yellowbeard wrote: Umm, just a thought...

...If VW couldn't make it work in 69 years, with billions (or at least a whole lotta millions) of dollars of r&d spent, it probably isn't very feasble.

Just sayin'.

Heh, yeah. Like overhead cams or water cooling.
Oh wait.

VW used overhead cams and water cooling in other cars of the period that many of the air-cooled cars were produced. Point being, given the annual r&d budget VW sunk into the air-cooled program, if there were a better way to cool the motor, which was reasonably cost effective, which resulted in radically improved fuel economy, you can bet your ass they'd have done it.

Sat Nov 07, 2009 11:21 am

Yellowbeard wrote: Umm, just a thought...

...If VW couldn't make it work in 69 years, with billions (or at least a whole lotta millions) of dollars of r&d spent, it probably isn't very feasble.

Just sayin'.

Just a funny thought, I bet not a single dollar was spent on R&D. I bet it was all Reichsmark/Deutschmark money spent.

P.s. Lighthearted joke!