911 engine cooling fan: power consumption?

[911hillclimber]

Anyone know any facts on the power needed to drive the cooling fan on a 911 6 pot?

I've read recently, 30+ bhp at 6K rpm...

[Nine One One]

Estimates vary between 7 bhp and 16 bhp
https://forums.pelicanparts.com/porsche ... -cost.html

Then another article saying the 3.2 takes 10 bhp...........

https://forums.pelicanparts.com/porsche ... an-hp.html

Lightbulb Cooling fan HP
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.

[911hillclimber]

The passage I read recently was more about the racer flat fans.
If all it takes is 10bhp, not worth looking at an electric solution for the Lola.
Thanks for the reply!

[Nine One One]

Just run on aviation fuel!

[Lightweight_911]

.

I definitely remember reading many years ago - but can't recall where it was at the moment - about the power consumed by the Factory flat fan set-ups used on the 917's - somewhere in the region of 20-30bhp ...

.

[Nine One One]

Grahams is belt driven, the 917 was gear driven so maybe took more BHP?

About mid way down, shows the cut out of the get driven fan.........
https://www.flickr.com/photos/47153142@ ... 4722487690

[Nine One One]

https://forums.autosport.com/topic/1987 ... 7-cooling/


TC3000, on 18 Jun 2015 - 10:18, said:
for what it's worth, here some informations, which I have read somewhere, but I can't vouch for them, so use at your own risk:

the cooling fan was powered via a shaft drive from the center of the engine ( the engine also had a center power take-off, to combat torsional oscillations in the crankshaft)

shaft OD & material:

N/A engine: 22mm / steel
turbo: 24mm / titan

max. airflow rate:

N/A: 2400 l/min
turbo: 3100 l/min

power consumption of the fan:

N/A: 17 hp
turbo: 31 hp

[neilbardsley]

I still don't understand for a 40 sec effort why air flow can't be directed over the heads? Formula Vee cars race without fans. Admittedly they have less than 1/2 your hp

Sent from my 22011119UY using Tapatalk

[911hillclimber]

I suspect the fan on the 917 is huge flow to cover all the cylinders?
The Lola has a belt drive to run the alternator and the fan.
The drive to the fan is via very industrial looking 90 deg drive gearbox so losses all along the way.

I had toyed with the idea of replacing the angle drive with an electric 12V motor with 2 speeds, fast and very fast.
Have to keep the alternator of course.

Probably more bother than it is worth for 10 bhp, 30 may be another idea....

[chris68]

10hp is about 7.5kW, hence 625A at 12V. Going to struggle to find a motor, battery/alternator to deal with that. Cables will be pretty thick too.

[chris68]

For a fan like this, I think power is loosely proportional to speed cubed. So if you can run slower there might be a chance of going electric at 12V, otherwise you really need higher voltage to keep the current sensible. Just my thoughts ..
Cheers

[Sam]

I still don't understand for a 40 sec effort why air flow can't be directed over the heads? Formula Vee cars race without fans. Admittedly they have less than 1/2 your hp

Sent from my 22011119UY using Tapatalk

This feels like a good point to me.

If you’ve got oil temp and pressure gauges (or better still datalogs) you could try to find a suitable airfield day to do repeated warm ups and runs of the type you do in competition with the fan belt on and off.

Might be fun to build a system of some kind that disconnects the drive to the fan on a throttle switch or at high rpm.

[911hillclimber]

Thank you Chris, it was my thinking that a motor would be so big that the incoming air could not pass to the engine(!).

In hillclimbing, the procedure I've found for this engine is:

About 15 mins before you are called to the start line start the cold engine.
60 secs on fast tick-over is needed to get it to tick over at a constant 2000 rpm, no choke on this engine.

Warm the engine (and to some degree the transmission) to about 60C.
Kill the engine and get into the car and strap-up, takes a few mins (not that simple to do)
Await the call to the line which is actually to join the 20 car queue which you could be at the front or the rear.

Run the engine and get noise tested.
Keep a keen eye on the temp gauge. I have a small fan on the oil rad to help on hot days.
The plan is to get to the line with the engine at around 80C, not always possible for various reasons outside of my control.

Hit the start with WOT and about 30 to 50 seconds later finish the run and a few seconds after kill the engine in the parc firme.

Engine will be 100 C by then.

I am not at all sure under these conditions the fan delivers enough air to cool the engine for longer than 2 mins with that time mainly (about 80%) at WOT.

Previous owner of this engine ran endurance races in Australia on the circuits, but engine was a stock 3.2.

30 BHP is around 10% of the engine power, it delivers around 320 bhp with this fan at 7500 rpm.

Bottom line is the car sits still for about 10 mins and then the flat out run for 50 secs or less.

Just a reminder of the pace of this little engine:

Turn your sound up...

https://youtu.be/Btr1ATBrrLc?t=7

[chris68]

Hi Graham, Kind of thinking along the same line as others - how much extra heat does a 40s run put into the engine? There's quite a bit of thermal mass to absorb it. Also the engine isn't hot before the run anyway. Might be more important to keep some air moving over the cylinders after killing the motor.
Cheers

[Nine One One]

The only way you are going to find your power loss for the fan, is do a dyno run with the fan belt disconnected. I take it when it was set up after your twin spark install, the 320 bhp at 7,500 revs was with the fan connected up on the rollers.
For another short run with it disconnected, you will then be aware of your power consumption of the fan.
FWIW the wrong tyre pressures, binding brakes, changing gear too late or early is going to cost you more than another 10-30 bhp on a 50 second run.