5.0 Mustang & Super Fords
Install a Limited-Slip Differential
An installation of a limited-slip differential into a '96 GT
Mustang street enthusiasts like to debate which supercharger is best, drag racers are keen on internal engine parts, and road racers--strangely enough--seem fixated on limited-slip differentials. That's because an open differential spins the inside rear tire on corner exit to the point of hopelessness, while a completely locked differential promotes tire and time-scrubbing understeer. A limited-slip differential that splits the difference is best from a power and handling standpoint, but often the trick is finding one that works that you can afford.
Followers of our '96 GT open-track machine know it's aimed at serious road-course handling, which means having some sort of limited slip on hand. As the car's suspension was being heavily modified by Maximum Motorsports with its full line of handling tricks, the 8.8-inch axle in our project car remained filled with a drag/off-road-oriented Powertrax limited slip and 4.10 gears. These are great devices, but are not optimized for the open tracking we have in mind for our hot-to-trot g-machine.
The Powertrax provides the elusive and desirable lockup of the rear axle that positively provides drive to both rear tires. It's a godsend on the dragstrip, but its built-in system of ramped teeth angles means it unlocks and locks up again in midcorner if the throttle is adjusted in midcorner. At street speeds, this is no big deal--resulting in little more than a cluck and a wiggle when negotiating 90-degree intersections. At road-racing track speeds, however, the lock/unlock accompanying throttle movement can upset the car just when you've got it hung out on the edges of its tires. As we've proven ourselves fully capable of high-speed spins through the high-desert chaparral on our own, we figured we might as well avoid any mechanical help in that direction.
While there are numerous differentials to choose from in selecting a road course-compatible diff, this time around we selected the good, old Traction-Lok. Yes, the same limited-slip Ford fits optionally to Mustangs on the assembly line. There were a few good reasons to go with the Traction-Lok, not the least being availability and price. It also does its job quietly.
As does the T5 transmission in Fox chassis cars, the Traction-Lok has a divided reputation. Some folks say it quickly starts slipping and doesn't offer sufficient lockup; others are fairly happy with it. Our experience has shown that tire-smoking throttle-jockeying is the main Traction-Lok killer. Prowl the streets with burnout intent and a Traction-Lok is not going to last. Rely on it to provide protection against time-robbing wheelspin from your smooth driving on a road course, and they work fine. As with any clutch-equipped device, it's clutch slippage, and hence heat, that is the Traction-Lok enemy.
We don't want to overlook the Traction-Lok price advantage. Thanks to Ford's economies of scale, they are half the price of some of the high-zoot limited slips, and they're available everywhere. They can be had with either stock 28-splines or 31-splines to accommodate heavier axles. Installation is as simple as stock R&R of the differential carrier, which, if not exactly a home-garage job, is a familiar assignment with many shops. Traction-Loks are quiet, street-friendly units too.
We're not naive enough to figure our Traction-Lok will last forever; any plate-type clutch limited slip is even-tually going to wear out. Certainly our track driving will task the Traction-Lok fairly hard compared to simply squirting around on the street, so we know we'll be seeing it again. At that time, we'll employ an old Traction-Lok trick of fitting extra clutches to the unit. This gives more lockup at the expense of mild street manners--in other words, a packed Traction-Lok scuffs rear tires while negotiating parking lots and the like. In our track-oriented GT, however, we doubt we'll mind the occasional scratch from the rear tires as we sit inside a rollbar in high-rise aluminum race seats suspended over stiff Bilstein shocks.
While moving to the Traction-Lok, we also moved down a notch from 4.10 to 3.73 gears. There are two reasons for our change. First, on some of the faster road courses, 4.10 gears are too short for the longest straights. The engine hits redline in Fourth, forcing a shift into Fifth. That's a problem because Fifth gear is so tall compared to Fourth that even with the 4.10 rear axle ratio, the engine bogs slightly and time is lost to the lack of power and the time spent shifting. Fifth gear is also not as mechanically strong as Fourth in the T5 and T45 gearboxes in late-model Mustangs, so staying out of Fifth is a common road-racing/open-tracking strategy.
We're also considering adding a supercharger sometime in the future, which then makes the 4.10s definitely too short. A blower and 3.73 gears ought to be just right, so let's say we're also preparing for the future.
There's always a flip side, however, and we have to say the 4.10s are nearly perfect for our present combination. They give impressive pull, from the drop of the throttle all the way through the tach. Furthermore, with the 4.6 modular V-8's smooth personality, the revving V-8 seems unconcerned about the rpm. Thus, the 2,800-and-higher freeway-cruising rpm from the 4.10s doesn't seem so frantic after you've lived with it for a while, and the around-town grunt is strong.
Now that we've tried the 3.73s, we realize they've uncovered the flat power our Ford Racing Performance Parts-headed-and-intaked Two-Valve 4.6 delivers before it gets revving. Acceleration right off the bottom definitely feels lazier than before, while the top-end pull remains basically the same. That said, the freeway cruise is considerably relaxed, and whereas before we had to stick to the slow lane and 65 mph at best, we can now zoom along with the rest of the world. For a car that needs to drive itself hundreds of miles--one way--to the road-racing circuit, that's a plus.
In the photos, we're hitting the highlights of the Traction-Lok, the gears, and their installation at axle specialists DriveTrain Direct. We don't expect anyone to try these jobs at home because having a large number of shims and somewhat specialized tools on hand is necessary. That's easy for an axle, gear, and differential specialist such as DriveTrain, but not practical for the do-it-yourselfer.
Horse Sense: When we pulled up to DriveTrain Direct for our gear change, the customer before us was just pulling out in his Four- Valve Cobra. A week earlier he had 3.73 gears installed; now he was back for 4.10s. We were coming in with 4.10s and wanted 3.73s. To save a few bucks, we opted to take the week-old 3.73 gears in our car. But the episode shows how modular Cobras and GTs differ in their power delivery. The higher-winding Cobras want, and can use, plenty of gear, and 4.10s are common with them. GTs have less rpm to work with and are often equipped with 3.73s. Neither modular has the torque of a 5.0, which does well in all-around driving with 3.55s.