There are actually more than 2 options.
Let's start with the small diff as installed on a 90-93 car. All are 4.3:1 ratio. These will break. A few people have been able to make them survive, but they are few and far between.
Available from the factory as an open diff. Has 2 stub axle shafts of the same length.
Available from the factory as a Viscous limited Slip (VLSD). Pretty much an open diff for our purposes, but the parking lot guys love them. Has 2 different length stub axles, one shorter and one longer than the open.
Original aftermarket (from Mazdaspeed) diff unit. Basicly a shrunk down posi-unit out of a 69 Chevelle. Requires special tools and knowledge to properly adjust on installation. Uses the open diff stub axles. Approx cost $1000 parts and labor.
New style after market (from Mazdaspeed) diff unit. I have no personal knowledge
The larger diff came in all 1.8 cars from 94-2005. NA cars (94-97) used a 4.1:1 ratio ring and pinion. 99+ used the 4.3:1 ratio. These are almost in-destructable. I have seen 1 break. For SCCA the 4.3 ratio is required. All accessory parts (axles, driveshaft etc) are interchangable within all years of the big rear end. Nothing from the small rear end is interchangable with the big one.
Available from the factory as an open diff.
Available from the factory with various versions of the Torsen (TORque SENsing) diff, depending upon model year. Most of us can not tell the difference. I know some have tested back and forth and found slight advatages, but not much.
The early cars had stub axles, while the newer ones have one piece axles. The one piecers give better exhaust clearance.
It is very common to purchase both a 4.1 ratio Torsen (fairly easy to find) and transfer to a 4.3 open rear end (again easy to find) since finding a 4.3 Torsen is getting harder and more expensive.
Not a whole lotta SPEC in our rear ends
Dave
Dave Wheeler
Advanced Autosports, the nations most complete Spec Miata shop
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