Of all the "improvments" Mazda has made on the Miata since 1991, there are three that I want for my car- chassis braces, cheap & available from the aftermarket- super light alloy wheels, no brainer- and the Torsen Limited Slip Differential, ouch! The diff isn't as easy or cheap as the braces or wheels, but it is certainly possible to update a pre '94 Miata to the '94 & later differential. The first thing to realize is that the diff is a larger, hopefully stronger unit. You'll need to change out the driveshaft, the whole diff, and the halfshafts. If it is at all possible, you want to get these parts used. When they're available, Mazmart sells kits with "everything you need" for $1000. I checked all the local junkyards, noone in South Florida seems to keep Miatas around. I checked Mazmart & Mazda Recyclers repeatedly, and finally ran out of patience. I wanted to have the limited slip unit in before getting my turbocharger, so I broke down and ordered the parts from Tommy Grimes at Roebuck Mazda. I did manage to buy the diff used, but I still needed the rest of the parts including these: Part Number Qty Description ------------+----+------------------- MA02-27-600A 1 Casing, Diff. M055-27-270C 2 Diff Yoke MD02-25-500 2 Shaft, Drive MF04-25-100 1 Shaft, Propeller-Frt 9956-21-400 2 Gasket 9951-11-800 1 Plug 0810-17-121 1 Plug, Magnet 0730-26-060 1 Breather, Air G003-25-421 2 Clip MA02-27-238 2 Seal, Oil NA01-28-890 2 Rubber, Mtg. P012-25-123 4 Bolt 9078-12-830 8 Bolt M053-27-604 1 Bolt M053-27-605 1 Bolt 1368-25-124 12 Nut 9S82-B0-1103 12 Washer, Spring

The next trick is to get a different ratio gear for the differential - the stock '90-93 Miatas came with a 4.30 ratio, the '94+ come with a 4.10 ratio - there are 3.90 gears for RX-7s that fit the Miata Torsen unit, and that's probably what I'll do, though after driving the turbo for a week, I think my ideal ratio would be 3.50 which would make 4th gear like 5th is now, and have the following effects on the speed in gear table:

Gear  4.30  4.10  3.90  3.50
1st:   34    36    37    42
2nd:   55    58    61    68
3rd:   79    83    87    97
4th:  106   111   117   130
5th:  132   138   146   162 @ 7000 RPM

The Torsen is in, with a 3.90 (I think) and it is awesome!

I say I think it is a 3.90, because Mazda Competition sold me gears they said were 3.90 ratio, my mechanic counted the teeth and told me it was a 3.90 ratio, but the speedometer is only 5% slow when compared to the mile markers on US1 to the Keys and my CD player's song clock... i.e. 3 miles at 60MPH should take 3 minutes, but they really only take 2:51, 9 seconds or 5% too little. Now, I never did the speedo calibration test before changing the diff, so maybe it used to read 5% fast. One of these days I'll roll the car down the driveway one wheel turn and see if the driveshaft turns more or less than 4 revolutions.

Chris Harris races RX-7s in SCCA for RoTech, he also does mechanic work out of his shop in Fort Lauderdale (954)587-1503. He installed both of my clutches, shocks, sway bars, and did the differential swap - and did a great job at a reasonable price. If you're in (or near) South Florida, Chris is a great Mazda mechanic, and an excellent alternative to the high buck speed shops.

I don't have any hard performance numbers on the new differential, but the subjective evaluations are this:

• Will stay hooked up through 1st gear on good pavement, even at 12psi boost
• Far less spin happy than the open 4.30 diff, especially in the wet
• Much better control when the rear end does break loose
• Doesn't scream past redline in 5th gear anymore, just walks
• Overall, gives the car a far less beastly demeanor

---

Also on the MangoCats website: Art, software and 20 acres on Peace River for sale.

---

Last updated May 13, 1998