Tech | Drivetrain Upgrade
Our '68 Torino GT is factory original in every way except for a 4V carb and intake. At just over 100,000 miles, the drivetrain seems to be fine, making no serious noises. We're planning on some power upgrades in months to come and when the opportunity arose to visit Currie Enterprises of Anaheim, California, to have the company inspect the rear axle on our car, we we're glad to visit.
Currie is a time-tested leader in rear axle technology, and it is the only outfit we know of that covers such a wide scope of third member possibilities. Currie won't try to sell you an axle that will support 1,000 horsepower for your weekend-driven 289 Mustang, but rather recommend just the sort of upgrade that's appropriate to your application. In our case, we wanted to upgrade our existing unit to handle perhaps an expected maximum of 250 horsepower at the tire. Currie can build an 8-inch axle for any project, but it is reluctant to do so for projects where more than 300 rear wheel horsepower is anticipated.
To get started, the driveshaft...
To get started, the driveshaft is removed. We were amazed that the completely toasted rear U-joint wasn't making more noise. The driveshaft was set aside for U-joint replacement and the transmission output was plugged.
When a Mustang was equipped with a K-code engine making 271 horsepower, the car got a 9-inch axle. We wanted to see what Currie would recommend and what procedures that it would perform to turn our 8-inch differential into a bolstered brute, and we learned that the real secret to building a durable axle is careful component inspection and assembly procedure.
Although we don't plan on using our car for drag racing, we wanted the strongest possible assembly using our stock components and a Currie TSD or "torque sensing differential." This differential would replace the open factory unit and provide traction to both wheels simultaneously, while being quiet in the turns. In our past experience we've had great success with the Currie TSD and we were jazzed to have one installed into the peg-leg Torino.
The next step is to get the...
The next step is to get the axles out. The axle retaining hardware is accessed through the service hole in the axle flange. Sometimes a slide hammer is needed to extricate stubborn axles. A few blows and the bearings separate from their seats and the axles can be removed from the car. The leaking axle seals are removed with a smaller slide hammer as well.
There are different kinds of performance and for this car we want open road prowess. The car doesn't have an overdrive transmission (yet) but with the car's 2.79 gears and relatively tall tires, the car goes 75 miles per hour at 2,400 rpm. We'd like to have the added acceleration that a 3.50 gear would provide, but with a 1:1 final drive ratio at the transmission, the long legs would be gone. Nothing takes the fun out of a road trip for us like having big rigs blow by you like you're tied to a tree. For this reason we elected to stay with the stock gears and will consider changing to a deeper ratio if the car ever receives an overdrive transmission.
For higher performance applications using an 8-inch axle, Currie now offers an aluminum gearcase casting capable of supporting more horsepower. Once we get into the category of 9-inch axles, the range of selection at Currie is almost endless. With its line of excellent 9+ products, as well as modern 8.8-inch axle possibilities, Currie Enterprises can construct an axle that's perfectly suited to your application and your budget.
All retaining nuts were removed...
All retaining nuts were removed from the circumference of the gearcase, and a large screwdriver serves as a wedge to separate the case from the axle housing. You better have a good grip on this thing before you try to remove it, as you wouldn't want it to get away from you. The casting was sent over to the other end of the shop for service.
Before tackling the center...
Before tackling the center section, we decided to go through our axles. There wasn't any damage to the splines so we decided to reuse them. Here, the technician presses the old bearings off of the axle.
Careful inspection is an important...
Careful inspection is an important part of Currie's procedure and here the axles are checked for straightness after being chucked into a lathe. The axles both checked out within spec.
Next, the axles were set into...
Next, the axles were set into another lathe where the bearing surfaces were polished to ensure an easy and snag-free installation. Finally, the new axle bearings are pressed onto the axle shafts.
Perhaps the most important...
Perhaps the most important consideration when building a mild 8-inch axle is the casting itself. If you have a '65 or '66 Mustang, then your axle uses the single-web casting similar to the one shown at the left. While adequate for normal use, you can see that this piece won't be nearly as strong as the '67 and later casting shown at the right. The extra webbing strengthens the casting substantially, and it is the piece you want if you're interested in a more stout 8-inch axle. Since our car is a '68 model, it already came with the improved casting. After the '66 model year, the single web casting was relegated to six-cylinder use only.
Over on the bench our gearcase...
Over on the bench our gearcase is being disassembled. The differential main caps have already been removed and now the differential itself is slid out of the gearcase for inspection and rebuilding.