Well I took the plunge and finally ordered the OBX helical LSD that keeps getting tossed around on this forum. I'm pretty sure my "Torsen" rear end that my mechanic from years ago sold/installed for me is an open diff. I did the parking lot test and it failed miserably. Once I get it I'll replace the bolts with something stronger (as I've read this is a failing point on Honda versions of the OBX LSD). I'll post pics once I get it and feedback when it's installed.
******************Update - 04/29/2011
The OBX Diff is installed and functioning perfectly. Here's a quick how to:
1. Buy the OBX Helical Differential
2. Order the bolt and washer kit from here:
3. Open up your new OBX differential.
4. For some reason OBX appears to sometimes assemble rear wheel drive differentials in a FWD configuration. Here is the wrong and right way:
5. If your gears look like the WRONG picture, you need to swap the gears on the top with the gears on the bottom. This is easy to do.
Mine when I first opened it
And with the gears swapped (top to bottom)
6. If your gears are in correct orientation, then just remove the top gears and remove the stock OBX washers. The configuration that worked well for me was all new 6 washers in this orientation: )()()(
7. Reassemble, put blue thread locker on the new bolts, and install in a diagonal pattern. Torque to 28 ft/lbs.
8. To install into your 1.8 diff you will need two new bearings, and a couple of seals. Direct from Mazda cost me $200.
9. BEFORE YOU INSTALL - you might want to take your shiny new differential to a machine shop and get them to measure the tolerance on the surface that the differential ring mounts on. Mine was not machined properly from the factory and was out 0.007, which caused the ring to pop out of the pinion. $100 of machining later and it was perfect.
10. Use the same gear oil that you use in the regular miata diff. I put in Royal Purple Max Gear.
11. Enjoy your new diff.
Bearings & Seals: $200
Bolt kit: $15
Gear oil: $20
That sits at about $485. If you need to machine the unit true then your cost might vary, but mine was $100 which is probably on the higher end of the price. If you consider that if you install a used Torsen or any other diff you will still need the new bearings and seals, then this actually turns out to be quite a sweet deal.
* One misconception to note. This is not a direct copy of the Quaife differential. It actually appears that OBX used some sort of in-house design, and it is unique.