1992 Ford Mustang Rear End Upgrade - The Last Stand
We Wrap Up Our Little Juiced Coupe Project With A Rearend Upgrade And A Flat-Out Track Flogging.
From the October, 2008 issue of Muscle Mustangs & Fast Fords
By Steve Baur
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...
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...
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...
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...
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...
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...
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...
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...
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.
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.
The supplied ring gear bolts...
The supplied ring gear bolts for the 3.73 gear (PN M-4209-F373N) were a little on the short side, so Caputo used longer, hardened bolts to draw up the ring gear. He then reinstalled the Grade-8 hardware.
The Truetrac uses this little...
The Truetrac uses this little metal satellite-looking object, along with the C-clips, to keep the axleshafts in place in the housing.
Once you have the satellite...
Once you have the satellite installed, secure it with the included snap ring and you're done, aside from adding the fluid.
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
|FRPP underdrive pulleys, 3.55 gears, Flowmaster mufflers, off-road pipe||15.12/92 mph|
|Above plus 100 hp of nitrous oxide||13.62/104 mph|
|Cobra intake, equal-length shorty headers||14.55/97 mph|
|Above plus 100 hp of nitrous oxide||13.06/111 mph|
|MSD Digital 6||14.67/96 mph|
|Above plus 100 hp of nitrous oxide||13.21/109 mph|
|Above plus 150 hp of nitrous oxide||13.04/112 mph|
|GT-40X cylinder heads, 65mm throttle body, C&L 76mm MAF,|
SCT chip FRPP 1.7:1 roller rockers arms
|Above plus 150 hp of nitrous oxide||12.68/115 mph|
|Strange adjustable shocks, UPR lower control arms||13.78/101 mph|
|Above plus 150 hp of nitrous oxide||12.62/115 mph|
|TCI Auto AOD and 2,200-rpm stall converter||13.89/102 mph|
|Above plus 150 hp of nitrous oxide||12.75/114 mph|
|Weld Wheels, Toyo Slicks, 3.73 gears, Detroit Truetrac||13.40/104 mph|
|Above plus 150 hp of nitrous oxide||12.19/117 mph|
To give the differential caps...
To give the differential caps a little extra support, we sprung for this aluminum Trick Flow Specialties differential girdle (PN TFS-8510500). After installing the cap studs, Caputo applied silicone gasket maker to the cover, and then installed it on the car with the provided stainless steel hardware.
We've had the coupe in pieces...
We've had the coupe in pieces so much lately that the exhaust started falling apart near the mufflers. Given the power that the car was making, we opted to replace the Flowmaster-equipped, stock 2.25-inch system with Flowmaster's 2.5-inch American Thunder setup (PN FLO-17213) from Summit Racing. These days, two-chamber Flowmasters and Weld Draglite wheels are vintage Fox-body attire, but they continue to deliver time and again.
With our Weld Draglites from...
With our Weld Draglites from Summit Racing and Toyo Tire package, the coupe charged to a 13.40 at 104 mph on motor. Despite the 3.73 gear change, 60-foot times were still a slow 2.05-2.09 seconds. We backed up the 13.40 with a 13.56 at 104. We could do a lot better with a higher stall-speed converter, but we like the present 2,200-rpm stall that we are currently using. In the future, we may put the car on the dyno and datalog it to see if there's any tuning to be done through the SCT chip. We've made a number of changes since the last time we were on the rollers, including larger injectors and a mass air meter recalibration.
The fortified 8.8 was filled...
The fortified 8.8 was filled with Royal Purple's Max Gear 75W/90 gear lube. The Truetrac requires no friction modifier.
Toyo sent us a set of its...
Toyo sent us a set of its new Proxes drag rubber, with 26x9x15 slicks for the rear, and 25x4.5x15 front rollers. Toyo also makes a 26-inch front tire, but we thought the smaller one would look better, and if anything, maybe weigh a hair less. After about three passes, the slicks were broken in and working great. They were steady on the top end as well. Toyo has a small, but optimized, range of sizes in its slick tire line, and they're adding new sizes all of the time.
We made two naturally aspirated...
We made two naturally aspirated passes before our car owner could no longer stand to run without the juice. Opening the Nitrous Express bottle with the 150hp jets in the nozzle dropped elapsed times from 13.40 to 12.19 seconds. Miles per hour increased from 104 to 118. We've got power for 11.60s; we just need to get the short times into the 1.70-second range. Perhaps a progressive nitrous controller is in order. Still, we did well, taking this 15-second sled and shaving nearly 4 seconds off its quarter-mile time.