So here is the story:

Original 4.44 differential worked in the car with zero issues for two years of racing. I then swapped in the 4.77 gears and the next race weekend the diff overheated badly and was locking up way too much cause the car to push badly in turns. So I bought a whole new rear diff from Mazda that already had the 4.77 gears in in from the factory. Ran this diff for a year with zero issues.

A few weeks ago I decided to swap in the 5.12 gear set. I went to a different shop this time - one that specialized in only rear diffs - figured they would do it right. I broke in the diff with some street driving (three 20 minute easy drives per the shop's recommendation). After the first race practice session I could smell hot gear oil - not good. After qualifying the car was really hard to push onto the scales. After a second practice session the diff was clunking as I drove through the pits and would bind up so bad it almost would stall the car. Guess that diff is toast also.

Do you think both shops just set up the diffs wrong? Why are factory diffs working fine and then when I have gear sets swapped burning up?

I've been using the same high quality gear oil each time.

Any advice?

 

Find a shop that know what they are doing ....and install a diff cooler

I had way over 300 degree diff temps on hot days...and they don't live long like that

When we setup mine the last time we heated it up ( in the oven ) to set the clearances hot...wife hated it....but it works well

 

That wasn't much break in, from the Mazdaspeed Motorsports tech info page:

As a rule, differential temperatures should stay below 200° Fahrenheit range. Consistent temperatures above this level can reduce the service life of the limited slip and differential significantly.
A new differential and limited-slip unit should be broken in over a period of approximately 200-300 miles. Operating temperatures that slightly exceed 200° F during this period are normal. A 9OW hyoid gear oil should be used for the break-in period. Thereafter, we recommend switching to a synthetic gear lube that is compatible with limited-slip differentials. Torsen-Gleason differentials do not require limited-slip gear oil.

Setting Up the Ring and Pinion
Use the stock factory specifications in your shop manual for adjusting the ring & pinion. Attaining the proper tooth contact pattern is essential to obtaining the optimum service life from the ring & pinion. For racing purposes, a tooth pattern that contacts at the edge or "toe" side of the teeth is preferred. This type of contact pattern will make more noise than a "centered" contact pattern; however, it will reduce the chance of tooth breakage. Gear marking compound will provide the most accurate display of the ring-to-pinion gear tooth pattern.
After obtaining the desired tooth pattern, follow the recommended preload specifications in your factory service manual. We recommend use of a new crush collar each time you rebuild the differential. Replacing the pinion bearings is also recommended. See the Mazda Special Tool section in Appendix A of this website for a listing of differential tools available (to be added at a future date).

 

Thanks for the advice guys. I guess I'll get it rebuilt by someone who HOPEFULLY knows what he's doing and break it in very carefully. I hope the ring and pinion is not toast. The 5.12 gears are SO expensive.

 

So... it turns out the limit slip unit destroyed itself and some pieces were jumping around loose.

Unfortunately Mazda has ZERO limited slip cartridges in the USA so I'm having to go with aftermarket to get the car put back together in time.

Any recommendations for a good diff for Road Race usage? The OSGiken looks good and some recommend the Cusco as well.

Not that I ever had any problem with the performance of the OEM limited slip diff when working correctly....

 

OS Giken, none better.

 

But I have a low mileage OE LSD if you'd rather not spend the money