1992 Ford Mustang Rear End Upgrade - The Last Stand

Over the last several issues, we've been tearing into a certain '92 Mustang coupe that we dubbed the "Little Juiced Coupe," due to its reliance on a Nitrous Express nitrous-oxide system for quick elapsed times. We've added hustle to the muscle with a slew of aftermarket parts including GT-40X cylinder heads, a Cobra intake manifold, and roller rockers-all from Ford Racing Performance Parts-as well as exhaust mods from DynoMax and a new transmission and converter from TCI Auto. We've also bumped the nitrous jets from 100 hp to 150 hp, and while we've taken this coupe from low-15-second e.t.'s to high-12-second times, we've always known there was more in the combination.

We've fought traction issues, transmission issues, deadline issues, and bone-headed MM&FF staff-member issues in the last few installments, and haven't quite made the quarter-mile elapsed-time progress that we had hoped. We think we've finally got all of our ducks in a row, however. In this installment, we added more rear gear to the notchback, going from a 3.55 ratio to a 3.73:1, and we also stuffed a Detroit Truetrac differential between the Moser axles to make sure all of the power goes out through both wheels.

Our juiced coupe gets recognized quite often at our home track, no doubt because of its Saleen 10th Anniversary-esque paint scheme. We still have the Pony wheels and another stock black hood for when we need to go incognito.

Since we were pushing a lot of torque on the bottle, we thought it was time to beef up the rear axle assembly, so we ordered a 3.73 ring-and-pinion gearset from Ford Racing Performance Parts, a Detroit Truetrac limited-slip differential, and Moser 31-spline axles from Summit Racing.

The Detroit Truetrac is designed for smooth operation, and it performs as an open rearend until its fully automatic limited slip is needed. The Detroit Truetrac was the first helical gear differential ever introduced into the automotive aftermarket as an Eaton brand, and its design eliminates the need for wearable parts, resulting in maintenance-free traction.

We have the NX nitrous system set up on a window switch that starts the nitrous-oxide delivery at 3,000 rpm and turns it off at 6,000. We went with a TCI Saturday Night Special torque converter with a 400- to 500-rpm increase in stall speed to help get us into the nitrous faster, and as much as AOD-equipped Fox-bodies like a 3.73-4.10 rear gear ratio, we also bumped the 3.55 to a 3.73 to get the sauce chilling the combustion chambers sooner.

Shawn Caputo of Shawn Caputo Motorsports in Hudson, Florida, has worked on late-model Mustangs for more than 15 years, and his clientele consistently supplies him with a plethora of Fox-body Ponies, though Caputo does work on the occasional cammer colt. In just a few hours, he had installed the Ford Racing Performance Parts ring-and-pinion gears, Detroit Truetrac diff, and Summit Racing-supplied Moser axles. He topped off the 8.8 with several quarts of Royal Purple gear lube, as well as a Trick Flow Specialties rear-axle support.

Fortifying the 8.8 rear axle is Shawn Caputo of Shawn Caputo Motorsports in Hudson, Florida. The first steps in the axle overhaul include supporting the rear of the car, removing the wheels and the brake drums-or rotors and calipers, if so equipped.

After draining the fluid and removing the differential cover, Caputo removed the center pin, followed by the C-clips and then the axleshafts.

The main caps were next to come out, followed by the differential itself.

Before we hit the track, we did take care of one small issue-the rear exhaust system. Over the course of the buildup, the exhaust was removed numerous times, and the constant torqueing of the system, combined with its age, proved too much for the stock flow tubes, and they cracked along the factory weld seam. It was a no-brainer to order up a 2.5-inch Flowmaster system to replace the worn-out parts.

The time finally came for us to hit the track, with the lightweight Weld Wheels and sticky Toyo tires beneath us. We found that the car responded best when shifted manually, and our first pass of the night was a 13.56 at 104 mph on motor alone. Our next attempt resulted in a 13.81 at 101 mph, which was due to a bad manual shift of the AOD transmission. Hey, that's what test-and-tune events are for.

We then cracked open the Nitrous Express bottle and promptly laid down a best-ever 12.19 at 117 mph. On our second nitrous pass, we decided to try letting the transmission shift by itself, and the Pony trotted to a 12.21 at 118 mph. We opted to shift it manually from here on out. The next run started with a not-so-great burnout, which resulted in tire spin and a slower 12.40 at 118-mph effort.

We forgot to buy new studs for the Moser axles, so we reused the stockers by installing a lug nut backwards and then tapping them out. Depending on your wheel selection and class rules, you may need to upgrade to longer studs, which will protrude through the end of the lug nut.

Notice the size difference between the stock 28-spline axleshaft (left) and the new Moser 31-spline piece (PN MSR-A883141) from Summit Racing. The Moser axle is also constructed from forged steel and features hardened bearing seats. We opted to keep the four-lug configuration as we already had three sets of four lug wheels to use on the car, depending on its intended purpose.

Here you can see the obvious differences between the stock Traction-Lok (left) and the Detroit Truetrac (right, PN DTL-913A561). The beefier case is able to handle more abuse and keeps the internals from moving in directions they shouldn't be under load.