Hoping to do the same...

 

I think I got my diff for around $450 off eBay, and it was low mileage.

 

The only one I have seen so far is a c4s and I do not think that's the right one.

 

If it looks like this and has the little wiring pigtail in the middle, it's the right one.

 

Damn, that thing looks heavy.

 

My 996 Turbos range from stock to around 550whp, and I love the full mechanical AWD system. Rear end will push out and dance a little under WOT boost and then the front grabs and it just goes.

Personally, I don't want the 997 system 🤷‍♂️

 

Same weight as the 996 unit, and not all that heavy. 40-50 pounds maybe? If you include the cardan shaft and axles, the whole system only weighs around 70 pounds if I recall correctly.

 

Captain you have done exactly what I have considered doing for a while! Good to know my theories would have played out!

My research has shown the 991.1 front diff to be considerably cheaper than the 997 PTM diffs.
I would love to confirm the 991 dimensions still fit into a 996.

My ideas for this diff are for the safari builds. In low speed mud a manual locking function would be ideal. My concern is with the final drive ratios. Most safaris are running a square tire setup. The 996’s 3.44/3.44 works ok for this, but I haven’t seen any data to prove a square setup will last on the 997’s 3.33/3.44 ratio.

I’d like to determine whether the PTM will burn up with a square setup over time.
I would consider machining together 3.44 996 diff/pinions into 991 housings, and packaging it with the necessary hardware to drive the diff.

 

Why couldn't you run a square setup? AWD wants/requires? same size front/rear. Square is perfect.

 

Because the 997 turbo isn’t a “square” setup. It has 25.5” front tires and 26.3” rear diameter tires.

 

Interesting, I wasn't aware. That still seems within the acceptable % . I'd give it try.

 

I think you'd be safe running square with a 997 front diff in the scenario you describe. It would be similar to running locked hubs in a truck: just fine in the dirt where a bit of slip is much easier for the tires but a bit harder on things when driving on pavement. Also, I'd imagine if you had it locked for offroad driving that would only be for relatively short periods of time and not at particularly high (or sustained) speeds. And finally, there is a temp probe built into the 997 diff that you can wire to the controller that will turn the diff off if it hits a certain temperature threshold (the factory PTM module does this on the 997). I have the wires run to accomplish this on my car but I haven't finished hooking it up yet because I've been lazy about ordering the correct resistor for the the pull-down circuit.

 

The more you know...

 

I agree, if purely on the dirt at dirt speeds it would be a non-issue. When these cars are “dual purpose” mall crawlers they are being driven dynamically on pavement at freeway speeds. At 80 mph+ I would start to get more concerned about the induced permanent 3% speed difference in the clutch plates. Obviously if you left the clutch open above a certain speed or whatever then it’s no problem, but if it’s trying to function dynamically like a road car I’m not sure. It IS liquid cooled and monitored like you mention so one would hopefully notice a trend during testing with a square setup.

 

Running a taller rear tire is preferable from a performance standpoint and better than the old school square set up.