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Fri Sep 05, 2025 4:57 pm

anyone know how much HP it takes to spin the cooling fan ? I know itll vary but even just ballpark or max?
I tried to search it but i maybe the wording wasnt giving me any answers.

Fri Sep 05, 2025 5:23 pm

BFB wrote: anyone know how much HP it takes to spin the cooling fan ? I know itll vary but even just ballpark or max?
I tried to search it but i maybe the wording wasnt giving me any answers.

Man....there was a lot about this with data at one point in time over on the STF.

Have to do some searches. Vague memory says between 5 and 7 hp on type 4 depending on rpm. At some point it will cavitate.

Ray

Fri Sep 05, 2025 5:31 pm

Scroll down in this thread. A member name Karl posted a quote from Jake Raby back in 2008 with a link to some test data.

The quote states teh fan losses found by Jake Raby on numerous engines and cooling systems.

https://www.thesamba.com/vw/forum/viewtopic.php?t=291600&highlight=cooling+fan+horsepower+loss

Ray

Fri Sep 05, 2025 5:44 pm

Instead of poking 1" holes in the shroud how about slowing down the fan?

Fri Sep 05, 2025 5:46 pm

Another piece of discussion

https://www.thesamba.com/vw/forum/viewtopic.php?t=561660

Ray

Fri Sep 05, 2025 6:35 pm

Don’t forget the best trick .slightly off topic but hey make a bit more jam under load.
Good quality used solid gates belt .
Then adjust so it barley grips . Basically you should be able to spin the generator with just a light load applied with a wrench on the generator pulley nut .

Sat Sep 06, 2025 1:58 am

I think this covers all the myths
http://www.offroadvw.net/tech/wes/fan.html

The recent youtube dyno series "Fun with Dubs" did they showed the results belt on and belt off . it depends on RPM and belt type .
If you have a loose 11mm belt like Jim mentioned then at a stock 5k rpm the loose will be a lot lower than an engine at 7500rpm with a tight multi rib belt .

Sat Sep 06, 2025 5:08 am

This subject in my opinion is way overthought. The big concern is what happens at 6000+ RPM. Who cares? Nobody drives all day at 6000 RPM and if you feel the need to then buy a crotch rocket. Those that hit those RPMs in a VW only do it for a second or two so let the belt slip.

Everyone that has tested power loss seems to agree at the 3 to 4 HP at cruising speeds. That is the number that really counts in my mind. Can that be improved upon? Maybe. We have a thread I have been following about the fans Andigs has been developing and the big hype is how much more airflow does it produce? No one asks how much HP does it draw?! We know it makes more noise. It takes power to create noise.

It's my view that in the chase for more cooling (is more cooling really needed?) people are working on the wrong end of things. Only so much air can get past the cooling fins. There is a finite area through which that air can pass. There will come a point as you try to push more air past the cooling fins when you reach the "wall", pressure goes up but very little more air is being moved. At what pressure does that happen? Does anyone have test results for that? Is there any point in pushing that boundary? Is that when these 10 to 15 HP losses happen?

Also I look at pictures of aftermarket heads compared to stock heads and the holes for cooling air to flow through are tiny to nonexistent in some places. Yes there are threads about opening those up. How many people do that? What about the cylinders? The bigger the bore the smaller the cylinder cooling fins are. How about enlarging those fins and producing tins with a bigger size/volume to accommodate them? At one time someone was producing cylinders with aluminum fins. That has to be better.

I think all the focus on the fan is a distraction for progress.

...in cases where more cooling is ACTUALLY needed that is... I am not totally convinced that need is real.

Sat Sep 06, 2025 7:26 am

https://youtu.be/DV6xwzTfDww?t=2075

Sat Sep 06, 2025 9:43 am

on multiple engines ive dynoed (at jaycee in HB) both with and without the belt, the doghouse fan took 18-20 hp

one time i tested every muffler i could get my hands on.
the bugpack merge tuck-away took 11hp from a 2110cc
the dual QP took 6hp
the mondo took 5hp
my home made super turbo that i made w/ an $11 generic blank took 3hp

the absolute worst i ever tested (just because i saw it sitting at my neighbors mc shop and bolted on) was a 3 chamber straight through motorcycle muffler that bolted up to a big flange 2" header. it took 49hp away from a 236hp 2332cc.
we sawzalled each chamber off 1 at a time and ended up the 1st chamber only took 6hp

Sat Sep 06, 2025 10:06 am

Back to back pulls on Geoff Hart's dyno, the fan took 19hp @ 7000 rpm. Full size pulley, Gates belt.

Sat Sep 06, 2025 10:34 am

Bill Schwimmer wrote: Back to back pulls on Geoff Hart's dyno, the fan took 19hp @ 7000 rpm. Full size pulley, Gates belt.
Which indicates in my mind that the pressure differential across that fan is huge. It's trying to move far more air than the rest of the system can deal with. Complete waste of energy.

Sat Sep 06, 2025 10:39 am

oprn wrote: This subject in my opinion is way overthought. The big concern is what happens at 6000+ RPM. Who cares? Nobody drives all day at 6000 RPM and if you feel the need to then buy a crotch rocket. Those that hit those RPMs in a VW only do it for a second or two so let the belt slip.

Everyone that has tested power loss seems to agree at the 3 to 4 HP at cruising speeds. That is the number that really counts in my mind.

I agree wit this ^^^^

I am not sure exactly what BFB was searching for when he asked the question...whether it was just a point of interest or whether he had modifications in mind that he is chasing.

The way I took the question....was at face value. How much HP does the fan require?

I am not even looking at more or less cooling capacity in my mind. Just an interesting question.

From various long ago discussions (and I mainly speak from type 4 perspective)....I "thought" I had seen claims of 5-7 hp at continous running/cruising.
As you note, unless I have an engine that will be racing (and many people might)....I do not care about peak rpm loss. As you note, its momentary.

Not to throw too many wrenches into it....but one might also think of the "fan" as an assembly along with the belt and alternator/generator.

If you dig around (and its actually more complex than this crude number), most sources say a rule of thumb of about 1hp per 25 amps of output. More things come into play like heat generation, rpm vs total output at that rpm etc. Good link with the calculations below

https://electronics.stackexchange.com/questions/40...ent-speeds

But...in general, a type 4 with 55 amp alternator at cruising speed is going to pull about 2hp minimum and 3hp maximum.

Ray

Sat Sep 06, 2025 6:07 pm

raygreenwood wrote: oprn wrote: This subject in my opinion is way overthought. The big concern is what happens at 6000+ RPM. Who cares? Nobody drives all day at 6000 RPM and if you feel the need to then buy a crotch rocket. Those that hit those RPMs in a VW only do it for a second or two so let the belt slip.

Everyone that has tested power loss seems to agree at the 3 to 4 HP at cruising speeds. That is the number that really counts in my mind.

I agree wit this ^^^^

I am not sure exactly what BFB was searching for when he asked the question...whether it was just a point of interest or whether he had modifications in mind that he is chasing.

The way I took the question....was at face value. How much HP does the fan require?

I am not even looking at more or less cooling capacity in my mind. Just an interesting question.

From various long ago discussions (and I mainly speak from type 4 perspective)....I "thought" I had seen claims of 5-7 hp at continous running/cruising.
As you note, unless I have an engine that will be racing (and many people might)....I do not care about peak rpm loss. As you note, its momentary.

Not to throw too many wrenches into it....but one might also think of the "fan" as an assembly along with the belt and alternator/generator.

If you dig around (and its actually more complex than this crude number), most sources say a rule of thumb of about 1hp per 25 amps of output. More things come into play like heat generation, rpm vs total output at that rpm etc. Good link with the calculations below

https://electronics.stackexchange.com/questions/40...ent-speeds

But...in general, a type 4 with 55 amp alternator at cruising speed is going to pull about 2hp minimum and 3hp maximum.

Ray

Heh heh, it’s like you know me or something and know id be searching even if i dont post it.
Most of the time i try to post a simple question and not elaborate on what im searching or doing as it just overcomplicates things and I’ll likely not get the answer im looking for but instead get ideas for something that i dont want or may never come to fruition.
In this case, 1 i was just curious., 2 thought itd might be a good topic. And 3 im thinking of making my own fan, and to test rpm limits wanted to get an idea of how much hp itd take to spin it. Just kicking ideas around is all

Sun Sep 07, 2025 4:40 am

You are correct! Now that I know what the basis is for your question I want to throw some fan blade design ideas at you!

Sun Sep 07, 2025 8:41 am

oprn wrote: You are correct! Now that I know what the basis is for your question I want to throw some fan blade design ideas at you!
haha! well lets hear them.
maybe I should start a new thread...