8¾ Rear Rebuild for Power
Bringing back the power potential in an 8¾-inch differential
Story Dave Verna - June 19, 2011 09:00 AM
Chrysler’s 8¾-inch rearend was in production from the late ’50s until 1974 in the passenger car line. It was used under literally millions of Mopars over the years.
In a street car, with its removable center section, it’s a tough rear to beat. Gear changes can be done in under an hour and you are back on the road. While not bulletproof with slicks and high horsepower, the rear is plenty strong for a weekend warrior that will see the track on occasion.
The rear we are working on has had a hard life under this 1970 Dodge Challenger. It’s still driven (and driven hard) by its original owner. The 383 four-speed car has seen over 114,000 miles with the original drivetrain. The engine and transmission were finally serviced at the 112,000 mile mark and the new-found power, combined with the 26x9-inch slicks, took the 489 cased, cone-type diff to the end of its line. The telltale signs start at the burnout. One wheel would not spin all the time and then the 1-2 shift would not get traction to both tires, causing the car to turn at the shift point. Not very good with concrete barriers on either side!
In 1970, the only sure grip you could get was the cone type. While it did hold up to 41 years of flat shifting and the occasional trip down the strip with slicks, it finally gave up on us. There are a few tricks to get a few more miles of use by machining off a few thousandths and shimming them to get the cone to engage again, but it’s just not the answer. The correct way to repair a cone-type sure grip differential is to replace the unit. We did some research and Eaton has one of the nicest diffs around for our application: the Detroit Truetrac. There is no maintenance on these, just traction! They don’t even require limited slip diff lube. The choice was easy after talking to Eaton, since the helical gear design eliminates the need for wearable parts and the unit has four pinions in the 8¾-inch setup compared to other housing designs. The effective design seemed like a home run in our book!
Follow along as Neal McGrory tears into this and shows you the modern updates you too can perform to your 8¾ rear. We are not going to waste your time with the disassembly but will show you the installation of all the new stuff.
For Your Information:
Eaton Performance / Detroit Locker
Power Train Truck and Auto Parts
Liberty Gear & Axle