Thanks jparker - interesting reference in the link above about Nissan’s and Subaru’s differential systems.

Also while browsing the net for WRC transmission information, I stumbled across this not very new link, which I am sure it must have already been posted around here, but I will do it for reference, containing interesting info about the diff settings of the previous generation active diff cars:

http://wrcbehindthestages.blogspot.c...ravaganza.html

Also pondering few points about differentials below:

1. WRC cars up to and including 2016 – no centre diff, i.e. spool – the rules read:

Front/central/rear differentials
Only the housings and mechanical limited slip differentials
homologated in the WRC extension may be used (without any
modification).
"Mechanical limited slip differential" means any system which
works purely mechanically, i.e. without the help of a hydraulic or
electric system.
A viscous clutch is not considered to be a mechanical system.
Any differential with electronic management is prohibited. The
number and the type of the plates are free.

The significance is that the rules specifically mention mechanically locking central differentials as being allowed, but the common knowledge suggests they were not utilised, i.e. the competitors decided it is better to have a spool (locked front and rear transmission paths) rather than mechanically locking central differential.

2. Torque split of a spool often given as 50:50 – to state the obvious, this is of course seldom correct. It might apply for near perfect conditions where all the gearing is the same, tyre dynamic dia’s are identical and none of the tyres are slipping, otherwise greater proportion of the torque will flow towards the axle/wheel with greater grip/resistance.

3. I was thinking how would I approach the challenge if I was given the task of designing an independent clutch packs type central diff, what would it take? And then I thought the chances for this working on a current rally car are probably greater than implementing it on a road car. A current rally car has got to cover, let’s say, 350 competitive km per event, which is equal or less than F1 race + Qualy + FP3, which in turn is what the transmission there has got to cover in a race weekend – now, before anybody objects saying the transmission stresses in F1 are lower, I am pretty confident it will be the opposite due to the higher engine power coupled with the enormous downforce levels - the stresses on a F1 differential would be higher than WRC car central differential (which also does not see all the gearing of the complete WRC car transmission). With that in mind, in my opinion, to design a reliable independent clutch packs only central diff, will take proper sizing and material selection for the discs as well as sufficient cooling and top that up with plenty of hours on the transmission dyno and the test course. It is maybe unlikely that current WRC teams’ budgets stretch that much, but it is good to dream.

4. Continuing from point 3 above, I suspect the biggest hurdle in making an independent clutch packs central differential work is the actual control strategy – in what manner do you actually use such device. There is a lot of experience in making hydraulically operated epicyclic diff work, but hardly any when it comes to making the clutch packs one working. I suspect the number of options going from the normal hydraulic epicyclic diff to the clutch pack one will increase exponentially. So although the clutch packs will give you way more opportunities, they will be easier to become lost in what you are doing, which means the epicyclic diff will be much easier to work with.