Since a Cayenne is not the easiest conversion target, I have to ask... why the Cayenne specifically, and not any of the many other medium-to-large SUVs? For that matter... why this type of vehicle? I understand wanting more comfort than a sports car, but there is a wide range of vehicles between a Boxster and a Cayenne.
- Limited Range due to weight.
It's not just weight... it's also large frontal area, mediocre aerodynamics, and resulting high aero drag. On the other hand, high weight also means high weight-carrying capacity; a large body also means a lot of space for batteries. This doesn't work if you want to use a stock pack for a smaller production car, but on the other hand twice the weight of a Leaf and half the range might work out. - Cayenne is Auto only.
The obvious solution is to not use the original transaxle.- A manual transaxle from another VW/Audi product could work, although none will be direct bolt-in (assuming that the Touareg and Q7 were auto-only as well) because this platform was not used for any other model lines.
- A single-speed system could be set up with one or two (for 2WD or 4WD) complete motor-transaxle units salvaged from a production vehicle... the most obvious match would be to build a Cayenne-bodied Tesla Model S dual-motor, but that would be expensive and a lot of work. Any two-motor system means two controllers/inverters, and coordinating them.
- Enough power to move the Cayenne at a reasonable clip.
It's hard for me to believe an AC51 would be enough for this much vehicle. Perhaps two of them (one front, one rear) would work?