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Differential Upgrade - Part One: May 2012

Here is the video version of this update. Take a look, I bet you'll get a kick out of it!




Ok Folks, after a long hiatus filled with many miles of fun, I'm finally back wrenching on the Turbo Miata! So after all of the fun that I've had with this great little car over the years, there's something that I've been putting off- replacing the wimpy stock differential. Heck, I'm surprised it's lasted this long! I sure haven't been easy on it, and I have a history of breaking rear ends.



Before we get started, I had my little plastic garage buddy DJ Torch providing the soundtrack for this thrash session. Here he is in action. How the heck does he manipulate an iPhone (or anything, really) with those "hands"?



I started by pulling old Sunflower into the garage and up into the air thanks to our new BendPak two post lift! This was the first time that the Miata has gone for a ride on the XPR-10A and you could almost hear the big machine chuckling as it lifted the paltry 2300 pound Mazda in the air.



Here are all of the parts we'll need to replace our weak and wore out differential with something much better and stronger- A Torsen Torque Sensing limited slip unit from a newer Miata! I'm lucky that I went this long without breaking this old, tiny diff, as they've been known to blow apart on completely stock cars!

To make the job easier, the differential unit, axles and driveshaft are sold as a complete swap kit by the friendly folks at The Parts Group. In addition, I added a set of Energy Suspension polyurethane mounts, a quart of redline synthetic gear oil and some new power plant frame mounting bolts, just in case.

Another great benefit is a numerically lower gear ratio! My stock 1.6 liter differential has a 4.30 to 1 ratio, while the new Torsen is 3.90 to 1. I believe this taller gear will suit the turbo engine's powerband far better than the old setup. Speaking of the gearing change, the rpms at 70mph will be dropped from a buzzy 3,600 to a much more tolerable 3200rpms. The speed in gear will also be noticeably higher.



My first task was to remove the stock rubber mounts from the new Torsen unit.



To help these stubborn pieces out of their home, I sprayed them with the Revolutionary, Fabulous, As Seen on TV, PB Blaster and let them soak overnight.



There are a variety of techniques to remove these, some even involving fire. I just relieved the pressure of the steel collar with a hammer and chisel, and lightly tapped around the edges.



With the old gummy bear mounts out of the way, it was time for these stiffer pieces from Energy Suspension. These are said to improve handling, reduce wheel hop, improve shifting and make you an all-around better person.



After a generous dollop of silicone lube, I pressed them into place.



For easy access to everything, I unbolted the exhaust where it meets the downpipe and carried it off. It's time like these when band clamps and one piece exhausts really come in handy!



Now we have a nice, clear path to the diff.



Next up, I removed the four driveshaft nuts & bolts and got it out of the way. Just a very simple tip, set your emergency brake to make loosening these bolts a breeze. Then I unbolted the passenger axle from the differential and got rid of that too.





While the passenger axle slipped right out, the driver's side was a completely different story. I tried a variety of different techniques, including beating the crap out of it, but nothing worked. It was stuck to the hub! After a quick Google search, it appears this is kinda common. I can understand, the part has been in that same spot since it was built back in 1991/92.



After a little break, and unbolting the driver's side hub/axle, it was time to remove the differential. As you can see I chose to lower the assembly with the PPF attached. That wasn't my plan, but became necessary thanks to a busted mounting bolt.



Here's a look at it. Even with a good soak in the PB Blaster, the bolt head snapped clean off.



So here's where we'll leave off this time. I fully expected to get this job done in a few hours, but the little Miata didn't want to let go of it's rear end so easily. Next time we'll finish up this job and take it for a spin!



Well that's it for now, thanks for reading!

-Tom