If the idea is to get more power at high rpm, then the way to do it is to reduce the drag created by the working alternator rather than reducing engine cooling by (whatever means) disconnecting the drive source to the fan.

With the fan drive disconnected, there will be much reduction in airflow despite any perceived fan inertia as it is disconnected.

As for the main drag in this system, insert a switch into the charging system to prevent it from charging. This should save a few ponies, to be used in more preferred ways much like switching off the AC compressor.

Sherwood

how long would that fan spin thru the resistance of the air it is moving at say 5,000 rotations per minute? I just think that while it might continue to spin when it isn't really doing any "work", it ain't going to do that when its actually moving a large volume of air.

Would like to see a test where the fan speed is measured and engine is revved to 5k and then shut off. HOw quickly does the fan slow down.

Some folks might be thinking, 'better cooling for my air cooled engine without cooling air'. Hmmm.

Here's a DIY home test. Have a helper rev the engine with the car at rest. Spin it up to say, 3000 rpm, then observe the fan as the engine stops. Of course, with the drive belt connected, the fan will spin down as soon as the engine does.

Without drive, the fan should just coast to a stop and during that time, there might be a very short time when air is fed through the fan shroud. The "Theoretical gains include more efficient engine cooling and better engine response" will be moot at best. During decel, the engine still produces heat. How would the engine dissipate that heat if there's no force-fed air flow?

Possibly minimal repercussions on a water cooled engine. However, I'm sure we're all eager to see a valid demo of cause and effect. Might be possible in very low ambient conditions..... for awhile.

Thanks,
Sherwood

Yes. keep the fan spinning always... as mentioned you want to disengage the alternator...

BTDT ;)

Old track trick: use a switch so that at WOT (or anything but idle throttle) the alternator field wire is disabled (grounded? Open? - anyway, no juice being produced). So you lose the drag of the alternator during acceleration, but put some amps back in the battery under braking.

I could see how there might be some benefit to avoiding the fan belt from coming off with a one way clutch, cushioning the shock of the RPM drop when upshifting? Some racers have recurrent issues with this, while others don't.

But the alternator's drag is going to be there, and the drag from the air is going to be there when no drive is applied. A better description of what this does, and how it does it, would be required for me to open my wallet.

PS - the 3.6s have a dual drive, alternator spinning slower than fan. The 996s don't have a fan.

Would it be possible to have a thermostatically controlled electric fan? Maybe have the alternator sit off to one side similar to where the AC compressor sits and have the alternator controlled by a rev limiter off \ on switch?

Anything is possible. But check out the flow rates for electric fans, and compare those with the stock fans in CFM. When I have looked at this, using usual sources for readily available fans, the nu8mbers haven't looked good. Fans can only be a performance benefit when running your electrical stuff from a battery only. Otherwise, using the alternator to drive the fan can't be a net gain in performance. Though maybe at cruise slowing down the fan might gain something. But why?

A fine water mist introduced into the air stream can reduce air temps about 18ºF, but there must be air flow for adequate distribution.

Sherwood

The idea of an electric fan to cool a 911 is interesting, but not easy to do well. It has been done (one of the Norwood “Doom” cars had electric fans). The 911 fan puts out A LOT of air (thousands of CFM, I forget the exact numbers). Electric fans put out less for the size, and usually their published numbers are measured unrestricted. You’d be hard pressed to find one that would fit in the small space where the 911 fan goes, so you’d have to design a different shroud.

I think you’d have to work hard to design a setup where an electric fan would really free up hp. The 911 fan + alternator takes about 20hp to drive it at high RPM. An electric fan would theoretically free up some of this, but then you’d probably need a robust alternator to power an electric fan strong enough to cool the motor. With the additional losses from converting energy back and forth from electrical to mechanical through the alternator to power an electric fan. I’d think for sustained use it wouldn’t be the best option. For short term use like autoX or sprint racing you could have a dedicated battery that you’d charge between runs.

I wonder how much power an alternator takes to drive?

Bottom line, maybe just maybe in extreme track conditions you might gain a few HP with a complex setup. It will add weight.

Likely the best performance increase, lose 25 pounds yourself, and spend the money on professional instruction from a race instructor. If you happen to be a skinny guy, the weight loss is not really possible, but for most people is it.

In the end, unless you are a up and coming professional racer, is is all just money spent having fun. How many sets of race tires can the same money buy?

Where did you get this spec from? Just curious.

I inferred it from when Porsche used to rate their engines in both DIN and SAE hp. The 2.7 RS, for example, was rated at 210 DIN hp and 230 SAE hp. The difference between DIN and SAE ratings is that for SAE, you can drive ancillaries like alternator and fan off an external source. For an early 911, the fan and alternator are the only things that qualify, so the difference is hp ratings (20 hp) represents that taken to drive the fan + alternator.

Scott

~750 Watts = one HP. You'd think a 750 watt rated alternator would, if called upon to produce that, cost something over one HP (over because it can't be 100% efficient). It also has an MOI, though that probably is fairly small compared with the four tires/wheels/brakes.

Seems like a pretty sharp idea putting a sprag clutch on the fan, just got done looking at all the available oem pulleys out there. Let's hope the pricing will be reasonable.

Seems like a solution looking for a problem to me...

Why would you want a sprag (one way) clutch on the fan/alternator? When I lift off the gas, I want the car to slow down, but I don't think the stored energy in these parts impedes slowing down in any significant way. Both still present drag. Maybe a tiny bit less when off the gas, but not significant.

If their stored energy leads to fan belt issues when you abruptly get off the gas and onto the brakes, that would make sense to me. But overall?

Why wouldn't you want the fan to keep pumping air at say 3000 rpm and then dropping slowly instead of instantly dropping to 900 idle speed while you are sitting at a stop light, etc?
Would even be good for more cabin heat in the winter, no?

The way I see it the fan would be driven (locked) as normal on acceleration and would freewheel on the way down until it matched the lower rpm and then be driven (locked up) again

Useless and unnecessary. I’m sure they’ll sell millions.
Kinda like if a plane on a treadmill will develop enough lift to take off. Discuss.

fan loss
 

I run a Toyota alternator off the back of the cam on the 4,5,6 side to take the drag out of the fan.Only downside is there is no charging until 1800 RPM as the cam does not spin the same rate as the crank.Fan belt life is enhanced.Gut an alternator and just use the shaft and voila.Doing this since the 80,s.Wayne Baker ran his IMSA 914 with a pulley on CV joint flange.Ciao Fred

So when driving the only advantage is the engine RPM can drop faster during a shift but nowhere as much as a lightweight flywheel and clutch. So the rotational energy advantage is really minimal and that same advantage can be realized with more effect, less money, and less mechanical complexity by simply using lightweight clutch components or lightened crank shafts. And those more conventional solutions will offer the greater advantage of faster engine acceleration.

The reduced reliability from adding a possible failure point to something that doesn't have a failure mode added to having to pay for it makes the choice seem obvious.