12 blade cooling fan on 76 911

[Gulfblue]

Can I replace the 5 blade coolingfan on my 76 911S with a "regular" 12 bladefan ?

[tab tanner]

11 blade yes, 12 blade?? I don't think so

[Gulfblue]

11 blade Sure, will anyone fit? Would love to know more about your experience with the webers on the 76 911s engine. What else have you done? Maybe you can send me as pm and some pics?

[Bill Verburg]

there are 2 sizes of 11 blade fan
226mm used '78-79
245mm used all other years

A 5 blade 245mm '76 fan can be replaced by an 11 blade 245mm fan.

There were also dofferences in pulley drive ratio.

When I replaced the fan on my '76 C3 I used one from an '84 Carrera 3.2, in addition to having a pulley ratio that runs the fan a litttle faster(you do want better cooling?) it also has cooling slots for the alternator.

[Turbo_pro]

In most cases a faster fan is beneficial, especially at idle and low RPMs, stop & go traffic, that sort of thing.
It is important to remember that fans will stop functioning properly when the fan tips approach the speed of sound.
A supersonic fan tip will cause the fan to cavitate and the fan will cease to function efficiently not to mention become extremely loud.
That is why all race engines (high RPM) with vertical fans ran small fans (226 mm) and reduced pulley speeds.
Reducing the fan speed will also reduce horse power loss from overdriving the fan.

[cmonref]

"A supersonic fan tip ..." Lessee here ...

Speed of sound at seal level: 1100 fps

Say 7,000 rpm, 245 mm fan driven at 1.81 gives about 300 fps fan tip speed. At 10,000 rpm, about 425 fps.

Supersonic tips won't be a problem.

Fans are optimized for their design operating speed. Overdriving them will cost HP and efficiency.

[Henry Schmidt]

Solowerks has this issue nailed.
A fan does not have to go supersonic to be inefficient.
Generally a ducted fan will function it's best ( most flow/hp) at around 560 ft/s and still be relatively quiet.
Anything over 650 ft/s will tend to be very noisy.
Strangely enough when I calculate the tip speed of a 245mm fan at 7000 RPM I get 541.33 ft/s.
Later model Porsches generally redline between 6400-6800.
Racing Porsches used small fan turning @ a reduced ratio 1.3:1 because they ran at higher RPMs. Sometimes as high as 8000 RPM.

[cmonref]

Henry,

You are right, of course. I dropped the fan pulley ratio at some point in the process. Looks like Porsche did their fan homework, keeping the fan speed in a good operating range.

Brian

[Henry Schmidt]

Originally posted by tab tanner
11 blade yes, 12 blade?? I don't think so

Not to beat this subject to death but the most efficient fan (vertical) that Porsche has developed is the curved, speed reduced, 12 bladed fan as used on 89-98 3.6 engine.

By running a duel pulley Porsche was able to run the fan at a reduced speed and still turn the 90 amp alternator at a higher speed. The curved blades help reduce drag which means less hp loss and less noise.

One more thought: 12 blades seems strange. I thought an odd number of blades helped reduce harmonic vibrations.

[Keith Perry]

I've got an extra 11 blade fan and housing, no coil studs, says 901.106.101. 03 and then an H in a circle. What year is this originally for?

[BReyes]

Two questions. Unrelated.

Can I use the 87 fan alternator housing assembly into an early car? What about ge the early alternator into the 87 housing and bolt the 87 fan to it after I have a new CA finsih on the pulley.

What are the parts required for a horizontal fan. Please dont tell me the 4k OEM option there has to be a much better way these days.

Thanks.

[304065]

Keith, sent you a pm.

[Bill Verburg]

	         diameter mm	rpm    pulley ratio     fan tip speed ft/sec    volume l/s

early 911	    245	        6100	    1.3	               334                  1360

76-77 5 blade	    245	        6100   	    1.8	               462                  1265

SC	            226	        6100	    1.8	               426                  1380

930	            245	        6100	    1.68	       431                  1500

				

964/993	            279.4	6100	     1.6	       468                  1010

993tt	            279.4       6100	     1.8	       526                  1210

[BReyes]

hmmn ok. You have me thinking.

[T77911S]

gulfblue,
is your car running hot?
i put the 11 blade in my 77, didnt help, the oil thermostat was bad. still glad i have the 11 blade though. my temp would alwys climb, it never went down. most of the time i never drove the car long enough without turning it off to see how high it would get, so i thought it was just the typical hot running 911. now it stays at 190*

[BReyes]

Good point

[Bill Verburg]

The 11 blade fan won't make a huge difference in oil temps, it will make the heads happier