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.

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.

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.

With the bottle pressure still hanging out around 900 psi, we headed up to staging once again. The Little Juiced Coupe chirped the tires a bit at the nitrous hit, but charged to a 12.23 at 118 mph. Attempt number seven started off much the same way, with a little chirp, chirp, chirp when the nitrous oxide started flowing. We decided to try short-shifting the car at 5,200 rpm on the stock tach, and it turned out to be the wrong decision, as the quarter-mile time fell off to a 12.45 at 113 mph. Bottle pressure was down to 850 psi, but more importantly, it was time to head home. We managed to squeeze in one more pass, albeit on motor alone. At the stripe, the black notchback tripped the clocks in 13.40 seconds at 104 mph.

Keep in mind that this latest track outing was made in late June in Central Florida, and ambient temperatures were in the low 90s, with about 70 percent humidity. Though we often test at Englishtown's Raceway Park in New Jersey, where the air can be staggeringly good, this was not the case with our Southern-based project car, and thus the times were probably more representative of the national average than one might expect.

Our efforts to dramatically improve our 60-foot times at the track have fallen short of expectations, as we have only whittled it down from the 2.2 range to the 2.0 time. The higher stall speed and increased gear ratio should have us in the 1.65-1.75 range, but we've yet to get there despite all of our attempts. After reviewing the timeslips, it would seem that the car is running its best short times on motor alone, which possibly suggests that we may not have our MSD Digital 6 set optimally when the nitrous is armed.

We're giving up about 0.4-0.5 second in e.t. with the slower short times, and we're going to get the car on the chassis dyno to see if it's a tuning issue in the chip or with the Digital 6. In the meantime, we're pretty satisfied with its performance, given it has been running in the heat of the Florida sun. We look forward to the cooler, winter track days where it should easily dip into the 11s without the quicker short time. Though we're bringing this project to a close, you never know when it might pop up again for another test, but for now, we'll see you at the track. Be sure to check the MM&FF Web site and blogs for new results.

On-Track Performance Table