Torsen Differential in a GSL-SE

If you're not familiar with the concept of a Torsen Differential, start with this article: http://www.howstuffworks.com/differential8.htm. In fact, you'd do well to go back and read the entire article, not just the section on Torsen differentials. Also, the Mazda Competition Parts catalog describes it pretty well. You would also do very well to look in MazdaTrix' printed catalog for their excellent descriptions and diagrams of differential operation. Also, check out Miata.Net's VLSD Torsen page. Lastly, Zexel Torsen's own web page has some good information for those who want more technical detail.

I should mention that I didn't actually do the work myself in this swap, but I "supervised" every step of the operation :), performed by "Dr." Dave Barniger, at KD Rotary in PA. Actually, Dave did let me turn a few wrenches.

As for the replacement process, it's exactly the same as removing and reinstalling the stock diff. That's covered in Mazda's shop manual, which you should purchase if you don't already have one. (It's MUCH better than any of the aftermarket offerings, and worth every penny of the $80 price.) I would also suggest that you have an experienced professional set the ring and pinion for you. Failure to get it right could damage these rather expensive parts.

David Lane describes what he went through to install his Torsen differential at Paul Mullen's web site. David took some special steps for added durability, considering that he has a Cartech turbo kit on his GSL-SE. I made no such modifications to my axles, and most GSL-SE installations will be the same as mine. So this is, more or less, a drop-in replacement for the stock limited slip diff on 1st gen RX-7s.

Another difference between David's Torsen and mine is that David's car has the earlier Torsen and mine has the later one. I believe that they have different bias ratios, but I can't remember the details. Again, check out MazdaComp's and MazdaTrix' catalogs.

The biggest consideration with this upgrade is not a technical one, but rather the total price -- about $1075, including all parts and labor. That's about $630 for the Torsen diff, plus another $240 for labor, plus a bunch of replacement parts like new bearings and seals and stuff that you would have to replace anyway on the stock diff. (I ordered all parts through Mazda Competition Parts, so this stuff was considerably cheaper than it would have been through most retail channels.) Don't forget that you can probably sell the used stock diff to a Miata owner for $80 or so. (Try E-bay -- it worked for me.) That brings the total cost to just under $1000.

In contrast, the stock diff would have cost about $200 to rebuild, plus installation (which is identical to the $240 for labor that I mentioned above), and would probably need rebuilding after every couple of years of street and autocross duty. So the the initial price difference between rebuilding the stock diff and replacing it with a Torsen is between the $630 part and the $200 rebuild -- $430. Then consider that each disassembly and rebuild will cost that much or more, and after a few years of autocrossing, the Torsen will prove to be a better value. Factor in the cost of your time, and the fact that you won't be driving your car while you or someone else is repairing the stock diff every few years, and this initially expensive upgrade makes a great deal of sense.
 

For the benefit of anyone considering doing this upgrade, here is the parts order that I placed with Mazda Competition Parts for my Torsen upgrade. 

Back to Mazda RX-7