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.