Rear Differential Upgrade - Project Silver Stealth Stang

A few issues ago we introduced Project Silver Stealth Stang, and the goal is to pump life back into a tired '99 Mustang GT. The high-mileage Two-Valve modular-equipped Mustang was a plain Jane car that was destined to visit the car crusher in a few years as its service was nearing an end, thanks in large part to the 160,000-mile odometer reading. Ken Miele, from our Yo, Ken! tech column, picked it up rather cheaply ($5,800) and we started tossing on reasonably priced parts. The '99-'04 Mustangs are great project vehicles thanks to how well they respond to modifications; even the most minor changes show an effect. The New Edge era of Mustangs have become very reasonably priced, when compared to other high performance cars in the used car marketplace. The '99 cars went on-sale a decade ago, and by now most units have racked up high mileage. Our last project car report saw the addition of an exhaust system (Ford Racing shorty headers, Bassani x-pipe, and cat-back system) to help the Two-Valve modular engine breathe easier. We had previously opened up the intake system with a JLT cold air kit, a TFS 70mm throttle body, and a TFS upper plenum. Our accessories take less horsepower away from the 4.6L engine thanks to a set of underdrive pulleys by TFS. Other modifications include 3.73 gears and a complete front and rear suspension system by Hotchkis. The best time at the dragstrip was a 13.97 at 97.50 mph. Each engine mod didn't yield monstrous gains but we didn't care that overall peak horsepower and torque results were not glamorously better. Our true test with this car has been at the track. The mid-range gains were outstanding from the exhaust upgrades and overall, the car picked up three-tenths of a second. That is a huge success and a testament to how well this car responds to minor modifications. The computer carries a custom tune by Radical Racing's Craig Radovich. He mostly corrected the air/fuel ratio and adjusted the shift points for the auto transmission. The car shifts at 5,800 rpm at WOT.

The DynoJet chassis dyno showed 234 rwhp and 272 rwtq with the above modifications. Seeing those numbers, we knew there was more left. This article is focusing on getting the drivetrain to be more efficient in transferring the power to the wheels. The first order of business was in the rear, where a set of 3.73 cogs resided for the initial testing. Gearing is extremely important for accelerating a vehicle down the track. While 3.73 gears are the most popular choice for modular and pushrod powered Mustangs, it is not best for all-out performance. In part one of this series, we added a set of 18-inch American Muscle deep-dish wheels as well as a set of Nitto 555 tires. The Nitto tires are great in wet and dry weather, the only problem is that the overall height of the 285/40-18 rear tires knocked down the gear ratio. The Nitto tires check in at 26.97 inches tall, while the stock tires (245/45-17) roll at 25.67-inches. The extra 1.3 inches of height reduced our overall gear ratio, which hurt performance. "The ideal gears for a Two-Valve Mustang, especially an automatic one, are 4.10s," proclaims Radovich.

Taking Radovich's advice, we ordered up lower gears as well as a new Ford Racing Trak-Lok, installation kit, bearings, and axle seals. We tapped Downs Ford Motorsport (Toms River, New Jersey) for these parts. The dealership's in-house motorsport parts program even offers delivery for local shops. Downs is also one of Ford Racing's biggest dealers, and its stocks all of these parts. One phone call and the parts were at Radical Racing the next day and ready to be installed.

Adding 4.10 gears greatly enhances the acceleration and a definite must-have for modular-powered Mustangs of any variety. The gear helps these cars thanks to the lack of torque from the 4.6L combo. "I know Ken didn't want the extra rpm on the highway, but the 4.10 gears work great in these cars. It helps the car accelerate quicker on the track. Between the weight, the automatic, and the power, the 4.10 gears are definitely needed," comments Radovich. He also went on to tell us that the higher numerical gear ratio will benefit our future modifications, which includes a stroker naturally aspirated Two-Valve engine. Silver Stealth Stang desperately needed the FRPP Trak Lok. It is tougher than the stock unit, which was toasted when we inspected the rear's guts. That is a big problem with high mileage cars, the diffs get wore out from years of abuse. The spider gears were shot when we inspected the internals. While the rear was in pieces, Radovich suggested adding new axle bearings too. The other modifications we made to the driveline included a lighter aluminum driveshaft from Axle Exchange and a looser torque converter from Pat's Performance Converters (P.P.C.). The Axle Exchange aluminum driveshaft is a super nice piece. A neat tidbit about the company is that it is only 15 miles from MM&FF Command Central in northern New Jersey. That worked in our favor by saving money on shipping. We popped in during a lunch break to pick up the driveshaft before heading to Radical Racing. Radovich had this to say about adding an aluminum driveshaft, "Adding a lightweight aluminum driveshaft helps the car accelerate by shedding rotating weight." He also informed us that over time, the stock driveshaft's rubber coupler starts to break apart, causing the 'shaft to become unbalanced. That leads to noise and a rougher ride. That was definitely the case with Miele's daily beater.

I have to admit, to me an aluminum driveshaft is a simple device that gets balanced. That certainly isn't the case, and Neil Welks of Axle Exchange took the time to educate us on just how much goes into a 'shaft. Driveshafts are much more than just some thick-wall aluminum tubing with ends welded on. An Axle Exchange driveshaft costs a little more than the average run-of-the-mill driveshaft, but the extra expense is worth it. There are some products on the market that are far less expensive but it comes at the cost of quality.

Starting with the 'shaft, Axle Exchange uses Alcoa Driveshaft seamless tubing, which is a four-inch diameter with a 0.125-inch wall. The difference between Driveshaft tubing and regular stuff (with the same dimensions) is that the Driveshaft tubing has been accurately measured to carry a consistent thickness throughout the length of the tubing. The other stuff may look identical but inside, the wall thickness can vary greatly.

Moving to the weld yokes, the ones used by Axle Exchange are proudly stamped Made in the USA. Not only does it invoke pride, but it also shows quality. The weld yokes are rated to 4,000 lb-ft, nearly double the stock yokes' 2,300 lb-ft rating. Do not get these ratings confused with torque output of an engine; this rating is used for stress that includes power, weight, and shock. Overseas yokes are rated at a paltry 1,800 lb-ft, far less than even a stock yoke. Most of the cheaper aluminum driveshafts utilize the cheaper off-shore yoke, meaning it is weaker than stock. The U-joints are top-of-the-line Spicer high-performance pieces. The weight difference between the two driveshafts is five pounds, which is rotating weight, and that is a big difference when spinning something.

Two simple changes made a profound difference on the dragstrip as Miele pounded Silver Stealth Stang at Englishtown, right before it closed for the winter. Our initial run, was a best of 13.97 at 97.50 and was accomplished in really good fall weather conditions. Our post testing of the 4.10s and aluminum driveshaft was done in slightly better conditions. Based on the experience of everyone involved in this buildup, we felt the final track testing conditions were worth a tenth of a second quicker than when we ran 13.97. We are setting you up for a major drop in e.t. from just a gear swap and aluminum driveshaft. Miele cooled the car in the same manner, simply popping the hood and letting the car sit for an hour or so before making a run. On track, the car blistered a 13.62 at 99.93 with an outstanding 1.95 sixty-foot time.

"I drove it out softly because I knew the tires would spin easier with the 4.10s," states Miele. It only took two runs to crack off the 13.62. The first hit was a tire-spinning 13.75, which caused Miele to adapt his driving style by rolling into the throttle a little softer on the follow-up pass. He backed up the 13.62 with a 13.65. One of the important factors to getting a car to hook in cold weather is a hellacious burnout, especially with Nitto 555 tires. These aren't drag radial tires, but they respond well to a good smoking. We found that when Miele doesn't do a good burnout, the 60-foot suffers and the tires break loose on the gearshifts.

Our car picked up three-tenths from our previous best, but when you factor in the weather, we feel that adding 4.10s and the aluminum driveshaft was really worth around 2.5-tenths or so. Acceleration is the name of the game and these two simple modifications reduced our dragstrip times without us adding one horsepower under the hood. The great thing about these mods is that the cost is reasonable and they are easy to install. If that was easy, then our next upgrade is going to be even better. In addition to the lighter driveshaft and larger rear gears, we decided to swap in a looser torque converter from Pat's Performance Converters. This would allow the engine to jump right into its powerband and get near peak torque when Miele romped on the loud pedal.

A torque converter is simpler than it is perceived; it's essentially a fluid coupler connecting the engine to the transmission.Transmission fluid is pumped through the converter, and as the engine spins the backside (which happens to face forward), it rotates the pump that pushes fluid through and turns the turbine, and a stator controls the fluid returning from the turbine to the pump. The converter turns the input shaft that goes into the transmission and the vehicle is thrust forward (or backwards). The stall speed is 2,800 rpm, far looser than the stock which was 2,000 rpm. The higher stall speed helps the engine get into its peak torque range quicker and multiply it. That means quicker and faster times.

Due to the weather, we were unable to get the torque converter tested at the track. The Nitto tires bark for mercy now when Miele gets on the throttle hard at the stoplight, which didn't happen with the stock converter. Before the P.P.C. torque converter, he was able to drive it out fairly aggressive. Now the hides just squeal and burn rubber, even with the slightest hint of a jump on the gas pedal. Seat of the pants feel, it is definitely way quicker than before, and we think 13.40s, or even 13.30s, isn't out of the question--not bad for a near 160,000-mile ride.