Some info on FB diff stuff
 

http://www.youtube.com/watch?v=2ypp6UPxKFk

i added into my video blog. but i did a side by side comparison between the stock rx7 differential, the Mazda B2600 diff and the kia sportage diff. should be quite helpful. from what i can tell the kia sportage 4.78 ring should be just as strong if not stronger than the stock rx7 ring, via comparing the ratio of the tooth depth on the root width. Obviously the pinion is smaller.