Tony at Steelie Gears swapped a torsen centre from a manual non-turbo supra into my open diff (JZZ31, same as JZA80). So I kept the same ratios with a torsen.
If the whole diff pumpkin is a direct swap into your car then you could just swap your torsen centre into the new open diff.
If it isn't a direct swap then the you probably have 3 options,
1. You could ask Jason from Jap spares if the diff is going to be compatible, but last time I tried to ask him about a fitment issue his response was so bad that I decided never to go there again. I will hazard a guess that he will tell you to look it up on the internet, even if he knows the answer.
2. Ask Jap spares if you can take the diff to Tony at Steelie Gears so he can have a look at it and tell you if he thinks swapping the crown and pinion will work.
(Jap spares is about 2 mins drive from Steelie Gears.)
3. Buy the diff and chance it.
Cheers
Edward
Diff info
http://www.supraforums.com/forum/showth ... f69acab7d3
Science never sleeps...
Current JZZ31 soarer 2jzge-T (project), Crown Athlete 3.5 (Daily)
RVR Turbo (idle)
Carmy Wagon 2.5 (lent to mum).
Previous. A32 maxima (dead) 190e Merc (dead)
Terrano (dereg for 10 years, still being driven in west Auckland)