Cooling fan HP

[mpdevelopment]

I have been thinking about producing a slower speed fan drive pulley for the 911 racing engines. By doing the calculations one finds that the factory 225mm racing fan with 1.3 drive ratio has about the same air flow output at 7800rpm as the 245mm street fan if the drive ratio is reduced to 1.14 or so. The fan power of the stock 1.3 ratio 245mm fan at 7800rpm is 53% higher than the 225mm fan at the same speed. The fan power of the stock fan with 1.68 drive at 7800rpm is 330% greater. If we assume that the stock 1.3 ratio fan at 6500 draws 6hp (I will check) then that fan at 7800 draws 10 hp the 1.68 at 7800 draws 22hp. The 225mm fan at 7800 draws 6.5hp. As you can see there is power to gain. I doubt if the 1.68 drive fan even reaches that rpm due to belt slip. The factory seemed to be convinced that the volume of cooling air delivered by the small dia fan was sufficient for cooling a 300+ hp engine so we may be ok. The fan output as rpm increases is linier up to a point , then it drops off. I will research further.

[kach22i]

I don't know "jack" about the subject, so keep an open mind. Has anyone looked at using an electric fan like many other cars use?

[addictionMS]

SRP has one, not sure of the size or dimensions, I am sure they would tell you.

Jim

[RickM]

Here's a thread to help with your research: Poly belt pulley sources

[mpdevelopment]

I have made a prototype for testing. Will test on engine dyno with cylinder head temps etc. Seems that the high speed drives are for better cooling at the lower rpm run on the street by the larger/turbo engines (tall gears more torque) and to increase the alternator output at lower rpm (greater electrical loads in the later cars). If you look at the fan belt drive tables from Continental you will see that the high drive ratios overspeed and overload the belt.

[RickM]

Does RoninLB or Wil know about this? LOL

[Wil Ferch]

Yes.....reading with interest.....

My goal was improved cooling ( alternator output) at the expense of increased hp....this thread goes the other way to see if an underdrive arrangement can yield Hp reduction for racing applications.

I never did finish the research of the posted thread within this thread. What the fan curves do show ( as anticiapted) is that delta pressure ( "head") is proportional to RPM squared. This would allow us to ( basically) move the posted fan curves up and down the graph that "island" provided.

The fan curves posted by "island" are in German, and the rpm's listed are fan rpm's ( not engine rpm's) the "N gebl / N kw" is apparantly the ratio of fan speed relative to crank speed ( kw= kurbelwelle).

A little work here and we should be "there".......

Wil

[mpdevelopment]

Yes fan pressure is propotional to the speed squared, power to the speed cubed times diameter to the 5th. Wil Can you send me the fan curves you have? they will be a great help.

[Wil Ferch]

Do you have a fax?

Otherwise, the best way would be for Island ( if he's reading this) to send directly.

If you're unsure about posting a fax number directly on a public forum, either PM me or Email me at work ( Wil_Ferch at praxair.com), use underscore between my first and last name.

Wil

[Wil Ferch]

John Luetjen Emailed me with an offer to help...and I found Island's original jpg files that I then sent along to John.

John, if you're reading this, please advise if they came through in good order. Perhaps you can digitize and re-post. The quality of my faxes didn't look too good because of the small original size of the material I had.

Wil

[jluetjen]

Wil; I just sent you another email. I had sent the wrong number. Let's try again!

[jluetjen]

Wil ended up scanning them and emailing them to me. Since he hasn't posted them, I'll put them up for discussion.





There is actually a complete page of text to go with this, but I can't attach it since it appears that that .jpg is too big.

[randywebb]

Here's a tidbit...

On the 1963 911, the cooling fan took only 4.7% of the 901's gross engine output - p. 402 Ludvigson, Excellence Was Expected, 1st ed.

[mpdevelopment]

Great thanks for the post, now if you guys can tell me what the units of delta P are (can't read too fuzzy) I have some compressor power equations to compute HP. Thanks again, Bill

[kach22i]

I did a search and could not find a situation where someone converted a 911 to an electric motor cooling fan. I'm sure somebody has visited this question before..................can it be done?

How is the volume of air sucked in measured? Can't tell from the above chart.

http://www.speedstash.com/cooling_fans.html

Air Flow @ 0-inch static pressure: 1050 CFM

Would the alternator have to be upgraded to provided enough juice?

Fan RPM @ 13.5VDC: 2150.

I'm sure there would be a weight (increase) to the fan motor which is above and beyond a pully based system.

Any thoughts on this?

[mpdevelopment]

Fan output is in liters/sec 3.0-3.2 engine 1500liters/sec = 3178cfm this takes approx 10hp. Have you ever seen a 10hp 12volt 10000rpm motor?

[911pcars]

Since we're talking a little here and little there, what is the power gain from "disconnecting" the alternator (open the field circuit) at max. rpm?

Sherwood

[mpdevelopment]

If we have a 80% efficient 1000 watt alternator charging at max. output, the gain would be approx 1.68hp.

[RickM]

Originally posted by mpdevelopment
If we have a 80% efficient 1000 watt alternator charging at max. output, the gain would be approx 1.68hp.

So if we remove the fan and alternator we could have a nice power gain

[mpdevelopment]

The pully from SRP is 4.7" in diameter very close to stock 1.3/1 unit at 4.57" my prototype is 4.025" and requires quite a bit shorter belt.