Help Please--917 Engine Fan Info

#1
Sorry, gents, but I must ask you. I cannot find the answers myself for I have misplaced my Porsche 917 book somewhere in the excitement during the last mortar attack . . . :happy:


My questions for you concern the top-mounted, horizontally-placed fan that cooled the original Porsche 917 engine.



How did it serve to cool the engine? ("Well, newbie, it moved the hot air from here and the cool air into there . . . .") I know that. It's just that I don't know the specifics.
  • In what direction did fan move the air? relative to the car? relative to the engine?
  • How much air did the fan move?
  • Did the fan alway operate while the engine was on?
  • Did the fan system ever fail?
For any information you can provide I would be very grateful.

Bassanio et Portia :)
 
#2
Bassanio, I'm no 917 expert but I believe the fan pulled fresh air downward and routed it around the 12 finned cylinders with shrouds not unlike a contemporary (at the time) 911 engine except that obviously the fan on the 917 is horizontal compared to the vertical fan on a 911.

There's no clutch on it - it runs when the engine is turning. I seem to recall it's a bevel gear shaft drive rather than a belt but I'm not positive. CFM's - not sure.
 
#4
the air is moved from the top to the bottom.
the fan is actuated mecanically by the engine when the engine rpm increase, the fan engine increase.
I have to check my documentation to tell you the gear ratio and the volume displaced at top rpm.

the fan is 330mm diameter and 6 blades....

yes the fan did broke.....
not exactly the fan but the shaft...
on the early version the fan was solid mounted on the shaft. with high rpm changes the shaft broke and the fan lift off.... like an helicopter...

the problem was solved by installing between the fan and the shaft some "rubber" to run it smother....
 

Brian Hamilton

I'm on the verge of touching myself inappropriatel
#6
Only my observation, but wouldn't it work better if the fan were sucking air from the bottom of the engine/car and venting it through the top? Not only would this help the fact that warm air rises and would aide in the efficiency of the system, but it would also suck air from underneath the car and aide in downforce as it's extracting the air trapped under the vehicle.

Just my thoughts. Also, have you guys seen a Corvair engine fan setup? They're mounted on top of the engines like that. Maybe a solution for those looking to recreate the original look...


Laters,

Brian
 
#7
Only my observation, but wouldn't it work better if the fan were sucking air from the bottom of the engine/car and venting it through the top? Not only would this help the fact that warm air rises and would aide in the efficiency of the system, but it would also suck air from underneath the car and aide in downforce as it's extracting the air trapped under the vehicle.
you are right in some point....but

- in 1970 downforce doesn't exists.....it's the beginning... nobody knows any thing about ground effect....

- if you plan to extract the air with the fan, you are not cooling the engine you are warming it !!!! .....under the engine there are the exhausts pipes which are very hot....
 

Ian Anderson

Lifetime Premier Supporter
#8
Brian
I was thinking the same re "sucking" the car down to the track and blowing the air upwards.

But the LM Race is normally really hot and the temperature of the air just above the hot tarmac surface may have been too high to cool the engine enough - thus needing the fan to draw the air down instead.

Just what I summise.

Now with the fan drawing air downwards I wonder how much additinal lift this caused and potentially helped the originals being almost undrivable as they lifted at sopeed.

More question ?

Ian
 

Brian Hamilton

I'm on the verge of touching myself inappropriatel
#9
Hhmm...

Well, why can't we do it that way now? I doubt any of the 917 owners are going to be driving at insane speeds all the time, well, maybe... Anyway, why not duct the fan in this manner now. I mean we're not going to be using the 12 cylinder, but with the 6 cylinders, why not.

Worth a try...
 
#10
I read that drawing air from the bottom up had been tried and that the problem was the fan would suck up debris on the road and clog the cylinder fins.
 
#11
Brian,from an engineering standpoint,the easiest way to restrict a fan(or pump) where the intake is fed by atmospheric pressure(that is,not force fed) is by resricting the intake side.That's what happens when trying to draw air upward through the engine.The fan will be more efficient with a 'clean' intake. Intake from the exhaust side has more than just the heat factor.This air will be greatly expanded, further decreasing the fan efficiency.The Porsche engineers knew what they were doing(at least after puuting a shock absorbing coupling in the fan drive). A.J.
 

Garry

New Member
#12
Brian,
Not to hijack this thread, but the corvair fan had a major problem with eating belts when the engine revs were high and you lifted off the throttle. I am sure that some of the aftermarket guys figured this out, but unfortunately for me, when I had my '65 in the late 60's, I was constantly stopping by the roadside to replace the belt after it pulled off of the pully.
garry
 

Jack

Bronze Supporter
#13
Not to mention that the funny way of crossing the very long vee-belt. It would stretch enough to "slap" when lifting off. As far as I know there were no real aftermarket fixes.
 
#16
TO :Bassanio et Portia

here are some technical datas....

the cooling fan drive went through a reduction bevel gear from 17 to 19 teeth. thus at 8300 crankshaft RPM the fan is operated at 7426 RPM and drew in 2400 litres of air per second for a cost of only 17bhp.

(extract from one of my favourite book : "Porsche 917 the undercover" story by Gordon Wingrove)
 
#17
And the porsche engineers defenitely tried the reversed direction ( sucking Down to top) but after two overheated engines they went back to the original direction.

Only 17 bhp ? I think this is quite a lot. Don´t know how big mechanical losses are due to the drive ,but just think about how big even a 10 hp electrical motor would be to drive this fan . Would be interesting if we have an engineer on board who could calcualte the power need for this fan at the same volume.

Regarding the volume it is very difficult to believe the 2400 liters per second ( i read this too in the books) volume. Why ?
Do the simple math: 330mm outer diameter ; app 150 mm inner diameter( rather more if i look at the picture), app 120mm high( rather less) => gives Fan volume of max 7 - 8 liters. (rather like 6- 7 liters)

@7426 RPM this are 123 revs per second . Assuming the fan would be a positive pump this would be max 960 liters. The volumetric efficiency of a fan like this is max 65% ( if any) so max flow is 624 liters per second and never ever what the books tell. IMHO if it would have flown 500 liter/second then it is all it was able to do at max rpm.

What do i overlook ?

TOM
 
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#18
Tom, i think you have made your calculation with a single blade fan....

6 blades fan means approximatively 6 times more air flow.

with mecanical looses and air flow efficiency we are close to 2400 litres/sec
 
#20
Another problem with sucking the air from under the car and throwing it out of the top is the amount of rubbish that would be picked up (tyre rubber, grit, etc) and if it was very wet, a lot of water. Going from top to bottom you have a measurable amount of air and it is mostly clean.
 
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