Project FD RX7 Restomod: Part 15 – Building the Ford 8.8 Differential with a Detroit Truetrac LSD and WPC

FD RX-7 Ronin Speedworks Ford Explorer 8.8 Diff and Eaton Detroit Truetrac LSDWe take a junkyard Ford Explorer 8.8” differential and transform it by installing a Detroit Truetrac limited slip differential, hand deburr the ring and pinion gears, WPC treat the gears and bearings, and modify the aluminum housing to fit the high-power goals, handling and drivability requirements of our FD RX-7.

The Ford 8.8” axle has been around since the 1980’s and has been used on everything from Mustangs to F-150s, Explorers and Crown Victorias.  The legendary performance of the 8.8 and the sheer number of cars that use this axle has created a massive aftermarket which has greatly driven down costs.  Gear sets can be found in almost any ratio with prices starting at less than $200, and there’s a wide assortment of limited slip differential options, rebuild kits, and upgrade kits on the market.

The solid axle Ford 8.8 has been the swap of choice for many domestic cars and trucks, and the Independent Rear Suspension (IRS) Ford 8.8” Differential is a staple in high horsepower road race, drag, and drift builds from import Supras and S2000s, to BMWs and domestic cars.

As soon as we realized IN PART 14 that upgrading to a Ford Explorer 8.8” differential was going to be a similar price as bracing the stock diff housing and installing a chattering aftermarket clutch-type differential, we immediately knew we were going to use the Detroit Truetrac Limited Slip Differential.

Detroit Truetrac LSDThe Detroit Truetrac LSD (by Eaton) has always been one of our all-time favorite limited slip differentials.  The handling, performance, character, and smooth operation makes it perfect for street use, and performs incredibly well on the track.  The way the Truetrac handles high torque and puts down power in a consistent, predictable, and easy to control manner makes this helical (gear-driven) LSD one of the best modifications you can do to an S197 or S550 Mustang.

Eaton Detroit Truetrac Limited Slip DifferentialUnfortunately, this LSD is available mostly in domestic applications from Ford, GM, and Chrysler.  We were extremely excited that with the Ford 8.8” swap, we can use this limited slip diff in our RX-7.

Eaton Detroit Truetrac LSDWe have used the Truetrac in many different cars, ranging from our Project Budget 400whp S197 Mustang Track Car, to daily-driven 600-800whp Mustangs, as well as a 540hp Roush Stage 3 Mustang that we finished 1st in class and 3rd overall in the grueling Tire Rack One Lap of America.  Needless to say, the Detroit Truetrac was actually a major driving factor in the decision to upgrade to the Ford Explorer 8.8” differential.

For more information, we went into extensive detail of how the Truetrac functions and performs in THIS ARTICLE on Project Budget 400whp S197 Mustang Track Car.

2010 Ford Explorer IRS 8.8 DiffWe bought this 2010 Explorer differential on Ebay for $250 shipped.  But any 2007-2010 Explorer diff will work.

11 comments

  1. Looks like a win-win upgrade (adding a clutch-type LSD, reducing the assembly’s weight, and improving durability). Really enjoying this project – thanks for sharing all the progress!

  2. It’s always great to hear Mario’s named mentioned from TSR. He’s an artist with a welder. I love all the work he has done for the Datsun community.

  3. Awesome to see the attention to detail on the ring and pinion gears! All that work will increase the fatigue life by magnitudes.

  4. Love it. I’m still waiting to see a brake rotor WPC treated to see how much longer the brake distances would be.

  5. Whats the difference in length, axle centre to driveshaft flange between the two diffs? Currently have an aftermarket clutch type diff in the OEM housing that I don’t really trust behind a T56 Magnum. It’d be a shame to bin/remake the carbon driveshaft shaft…….they look pretty close!

    1. The Ford Explorer 8.8″ diff flange (using the Ronin mount) is 0.425″ rearward/closer to the axle centerline than stock. It sounds like your best bet is to get a 0.425″ spacer to adapt your driveshaft to the Ford 8.8 diff flange.

  6. Perfect – thanks Billy. That’s better than trying to go the other way! Looking forward to your next instalment.

  7. While the out side of the assembly might be larger, it’s no guarantee of greater strength. A measurement of the gears and the bearings would be needed to make that claim. The fact the Ford unit is lighter is also a factor. Also the original mounting with the PPF is quite strong, as the lever arm of the PPF is much longer than the bushings located right at the companion flange.

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