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I think the dynamics and stability statements have made it very clear that it can be straight dangerous to be regenerating while driving, so then how to keep the smaller battery pack charged?

Isaac97's idea of using the now unused output shaft of the AWD transmission is a good one if a generator can be appropriately sized for the RPM range. I'm not sure on the SW controls the cars have regarding torque split between the rear diff and front diff... i assume some custom ECU work would be required to determine the right amount of torque split to the generator based on the state of charge of the battery.

a lot of work but could make quite the performance hybrid!
 

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For a more efficient drivetrain, how about the Tesla SDU up front and then a smaller generator connected to the transmission's front wheel output shaft? Well that's more weight and more complexity... Hybrids are hard.
That would work. Of course, with an AWD transaxle just using the stock AWD is much lighter and cheaper, and the stock AWD system plus a single motor-generator would provide a temporary power boost and regenerative braking, using all four wheels. Yes, hybrid vehicle design is inherently complex and difficult.

But it would still offer better generation capabilities, although the ideal is connection directly to the flywheel (allows generation while idling/at any speed/in neutral).
At the engine output is a good place for a motor-generator because it turns at a suitable speed without additional gearing, and stays in a suitable shaft speed range even with changes in road speed. On the other hand, generation while not driving isn't typically an important feature. Most hybrid vehicles use the hybrid system to avoid running the engine at all when stationary, in neutral, or driving at very low speed; the rare exception is someone using the vehicle as portable generator set for a work site or emergency power backup.
 

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Isaac97's idea of using the now unused output shaft of the AWD transmission is a good one if a generator can be appropriately sized for the RPM range. I'm not sure on the SW controls the cars have regarding torque split between the rear diff and front diff... i assume some custom ECU work would be required to determine the right amount of torque split to the generator based on the state of charge of the battery.
My understanding is that in the 996 generation of 911 this is not an issue, because the output shaft to the front is the same output as to the rear differential: there is no centre differential. The required speed difference between front and rear axles is accommodated by a viscous coupling between that shaft and the front differential, mounted at the differential end of the shaft. That means that a generator (or motor-generator; see my previous post) can be mounted at either end of this shaft (in front of the transaxle or behind the front unit) and will act like it is on the transmission output. The generator can be controlled to produce whatever power is desired, regardless of the AWD system operation. Both earlier and later 911 AWD systems are different - this is the easiest generation to work with.
Hagerty: Behavior modification: All-wheel drive tames the 911
 

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Isaac97's idea of using the now unused output shaft of the AWD transmission is a good one if a generator can be appropriately sized for the RPM range. I'm not sure on the SW controls the cars have regarding torque split between the rear diff and front diff... i assume some custom ECU work would be required to determine the right amount of torque split to the generator based on the state of charge of the battery.
The 996 has no torque split controller -- it's just a viscous coupler to the front end. Some versions have stability control (keeps the car pointing straight), others have traction control (anti-wheelspin). Traction control is rear-only, it's a virtual LSD using the brakes. But there are no electronics controlling the torque split, only the viscous clutch/coupler - someone quoted 15% torque normally, up to 45% after extended rear slippage.

EDIT Looks like we've got the same ideas here.
 

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Thank you gents. Very interesting about using an older 911 for this project. I haven't verified which years used the VC vs. the MC.

The newer 911 Carrera 4 uses an entirely different AWD system. Instead of a viscous coupling, which responds to slip mechanically, a computer-controlled multiplate clutch actively engages the front differential to transfer torque forward.

 

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A few minutes of Wikipedia checks (might be wrong) tells me that 996 and 997 have viscous clutches, no data on 991 (generation after).

EDIT 993 (1995-1999) had viscous too, generation before that had a real differential.
 

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A few minutes of Wikipedia checks (might be wrong) tells me that 996 and 997 have viscous clutches, no data on 991 (generation after).

EDIT 993 (1995-1999) had viscous too, generation before that had a real differential.
Thanks for the research. I think that might apply to the carrera only, the 997 turbo i think used the multiplate clutch, but the 996 turbo did use the viscous. Very cool.
 
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