Quote:
Originally Posted by Marc Bixen
Porsche started installing an alternator clutch bearing on all manual trans cars starting with 996's. Tiptronic and PDK don't use it. The actual reason for the clutch release, and remember on air cooled cars, the alternator is behind the fan, is to prevent over-rev on high rpm shifts. Track guys will get this. What happens is that at high rpm when you depress the clutch the centrifugal spinning force of the alternator will actually cause the rpm to go up another 200-300 rpm after you push the clutch pedal and lift off the gas, thus creating an over-rev. Very common on 964/993 track cars. I've never heard of this causing over-rev type damage, but it is a little disconcerting, and does slightly slow down your shift, which equals increased lap times. Not what you're looking for. Cool innovation guys, thinking outside the box. I'm sure there are other bonuses from this also, maybe increased cooling, as the fan continues to spin at higher rpm than the engine on decel, as well as possibly slightly better fuel economy. I've thought about something like this ever since learning about the system on 996's. Thumbs up. Hope you can market it well.
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Just my rambling thoughts:
The inertia of the alternator will not cause an over rev. It will only slow the RPM decrease on deccel. The clutch is engaged on acceleration , so performs just like a standard pulley.
The clutch is there to preserve belt life and to keep the inertia of the alternator from causing the belt to go slack on the non-tensioned side of the belt run and jump off.
https://www.ntnglobal.com/en/products/review/pdf/NTN_TR75_en_P124.pdf
From looking at the video it appears that the alternator is still directly connected to the belt via the fixed pulley as the pulley nut is stationary when the fan is freewheeling with the engine stopped. I don't know the rotational moment of inertia of the fan vs the alternator, but I would guess the alternator is the greater of the two, steel rotor with heavy copper winding (though a smaller diameter), so I'm not sure how significantly the belt tension will be effected by the decoupled fan with the alter still connected to the pulley. There is also reduced risk of belt path run length change on an air-cooled than the 996+ due to the tensioning method. With the 996+ the tensioner is a spring-loaded unit so when the inertia of the alt is driving the system it is actually applying tension to the tensioner side and in theory could compress the tensioner and cause the belt to go slack on the normally tensioned side. On an air-cooled car the tensioning is accomplished via adjusting the diameter of the upper pulley, so regardless which it driving, the crank or the alternator the belt run is a fixed length.
There could be some slight benefit in additional cooling, but again I’m a little skeptical. I understand that the fan will continue to free wheel when the engine is shut off, but the speed the fan is freewheeling at may only see a small pressure differential across the fan. It would be more interesting to see how long it takes for the fan to drop from high RPM to idle RPM. One possible method, video using a strobe and with a make on the pulley and one on the fan, run the engine to high RPM then release the throttle and see how long it takes for the fan and pulley to start rotating at the same speed. If the fan actually has enough momentum to maintain an RPM split for a significant time, then cooling is possibly improved, but if they return to matched idle speed shortly after throttle release, then I would guess you won’t see any significant change in cooling performance.