958 V8 TDI Cayenne with Low Range ??? Lets do it

All right, as much as I love the car stock, with its great traction control, air suspension and all that, a true offroad vehicle really does need a bit more.  They do 95% of tasks very well, sand, mud, hill climbing, high speed gravel, towing and touring. But without low range you cannot reliably tow a camper or van, up steep hills in the bush.  The transmission is borderline overheating just with a loaded car in the super steep high country, adding a trailer of sorts would be the death of it.

Things I've learnt about the Active transfer case in the 958.  The V6s have a similar setup to the Touaregs, ie a torsen centre diff, non active and outputs to front and rear diffs at the same ratio, 3.27:1.  It perform pretty average most of the time except when it really gets steep.

The V8 TDI, GTS and Turbo petrols use an active centre, 100% rear wheel drive with a clutch pack to drive the front output when needed.  Here is the weird thing, the transfer case chain is driven by a 33 t sprocket, the output is a 37 t sprocket.  That's right, the front drive shaft spins slower than the rear.  To counter this the diffs are, 2.92:1 in the rear, and 2.58:1 in the front.  This gives a slight overdrive to the front when the centre is locked.  WTF.

My real goal at the start was to install a locking rear diff.

So how do we do this?

1. Buy a 2.92 rear locker from a 958 Turbo with PTV.  Hard to find but not impossible.

2. Buy a 3.27 rear locker from a 4xmotion Touareg.  Easier to find, but then the front diff to match the transfer case ratio needs to be 2.9 which doesn't exist.

3. Buy a 3.27 rear locker, 3.27 front and change the transfer case from a V6 diesel.  Easier to find but I'd be losing a centre lock up capability.

4. Buy a 3.27 rear locker, 3.27 front and change the transfer case to an original 7L or 955 type.  Really easy to find.  Hard to integrate though.  But l have the best of everything. Low range, centre lock up, rear lock up, and a 10% lower ratio for the bigger tyres!

So option 4 it is.  Let's do this, it may be a world first.

Steps to complete:

1. Complete emulator to trick the current servo into thinking it has a working TC attached.

2. Purchase Low range transfer case and reverse engineer the wiper type location sensor.

3. Create the arduino control unit and test.

4. Install transfer case, front and rear diff. 

5. Custom front and rear drive shafts.

6. Redo custom front and rear drive shafts because they vibrate.

7. Install custom controllers and enjoy the new lockers and low range.