We’ve been living dangerously with Project Magneto, our 10R80-equipped 2018 Mustang GT ever since we put on the meaty Mickey Thompson ET Street R 305 drag radials. Even with the Steeda Stop the Hop Kit, the sticky tires tested the viability of the factory aluminum center section and Traction-Lok differential on mid-10-second passes. With the intention of cranking up the boost even further, we searched for a budget S550 rearend upgrade.

Since the manual-transmission cars came with iron Super 8.8 center sections, and better still – the Performance Pack cars also came with Torsen differentials – picking up a used center section from a wrecked vehicle was the obvious path. I found a Mustang recycler within 30 minutes of home that was selling one for $550. It had only slightly fewer miles than my stock one, but thankfully the Torsen diff is a gear-type, so it doesn’t wear out like the Traction-Loc clutch-type.

All our used Performance Pack S550 rearend needed was a ring and pinion swap, some fresh seals and bearings, and fresh fluid. Though 3.55:1 may not seem like a lot of gear, the VMP blower and 10-speed combination warrants a max of 3.31s or you risk going into an Overdrive gear at wide-open throttle (delivering a quick death to a 10R80 automatic). Since 3.31:1 gears are OEM Ford ratio, I found a Ford dealer online and ordered an install kit from Ford Performance (who also sells brand-new, complete 3.55 Performance Pack rearends separately).

Ulterior Motives

If there was one complaint with driving Project Magneto on the street, it was the noticeable NVH introduced by the differential bushing insert kit. It funneled gear noise straight into the cabin. Since that aluminum diff was living on a hope and a prayer, I was OK with it for a while. However, with the piece of mind of an iron diff I decided it was time to quiet this thing down a little and would sacrifice a little performance to swap in some urethane. Steeda had sent the urethane insert along with the Delrin version as part of the kit, with the warning that unlike the cradle lock-outs, these would increase NVH.

Although I endeavored to work with the best rearend guy I could find in the Charlotte area to swap the gears, I didn’t want to leave anything to chance, so I also had the ring and pinion micropolished. Micropolishing not only extends the life of the hardware and and enhances durability, but it also increases oil retention on the teeth of the gear, leading to smoother and quieter operation. Essentially, it removes all the sharp peaks on the surface of the metal. That means less friction, and less heat – all good things. Opting for this treatment sort of blew our whole budget theme, but if a job is worth doing – it’s worth doing right the first time. I definitely didn’t want to pull these gears back out to do it later.

Prepping the Diff

Step one was taking the crusty old center section to a local builder, Robert Knipp, giving it a bath, and a fresh coat of paint. He also removed the limited-slip along with the ring, pinion, bearings, and seals. Before installing the micropolished pinion, Robert placed it in an oven to warm up, to make it easier to slip onto the diff, which took about 40 minutes at 200 degrees.

Securing the ring gear to the Torsen diff requires 100 lb-ft of torque and red Loctite. Before the final installation of the pinion, we test-fit it to ensure correct shimming and good tooth engagement. A used pinion bearing is pressed in during this process.

Robert found a .010 backlash, which was sufficient. Backlash is the distance you can turn the pinion before the ring gear moves. Too tight and it can cause too much heat and break. Teeth get full engagement on OEM gears, which is nice – while aftermaket gears have to break-in first, according to Robert (our installer).

After test-fitting, we were ready for full assembly and so Robert took everything back a part. The pinion was crushed to 18 in-lb, 20 in-lb is ideal. He crushed the pinion nut on the edge to lock it in place.

The differential took some mallet massaging before settling into the housing. The caps are side specific so it’s best to mark them and the case to ensure you get it right. He treated the caps with red Loctite and secured them in place with 80 lb-ft of torque. Then Robert performed one last gear-mesh check.

A thin bead of RTV on the housing seals the cover with 30 lb-ft of torque, using an inverted Torx socket on the cover bolts. Finally, he sealed the bottom plug, while leaving the top plug removed until we installed and filled the new center section.

Robert recommended I change the oil after 500 miles, so I placed an order with Liqui-Moly (no friction modifier is needed for the Torsen by the way).

Installation Surprise

Normally, swapping a S550 rearend is about a 3- to 4-hour job. But, of course, with hot rodding, you need to be prepared for anything. After the techs at Pro Dyno in Ford Mill, South Carolina, removed the exhaust and peeled away the Steeda components, we found a problem. The OEM rubber differential bushings, among the few left in the car, failed. Completely replacing them is a giant pain, so much so that Ford doesn’t even sell them and instructs its dealership techs to replace the whole subframe. But this is no dealership, so we had a set of complete urethane replacements overnighted from Summit Racing and got to work cutting out the factory bushings.

The subframe is massive lifting and holding it in a press isn’t a great option. A hole saw and reciprocating saw are pretty much the best options. However, keep in mind there is a bonded metal collar that also needs to be removed. This takes a lot of elbow grease and patience.

When at long last the broken bushings were replaced, the S550 rearend was put in its rightful place and so was the subframe. Once buttoned up, we used Liqui Moly’s GL5 Fully Synthetic Hypoid Gear Oil in 75W-140 to fill the pumpkin. It exceeds factory requirements for protection and can endure the most extreme applications. Its wear resistance and ability to reduce gear noise made it an even more attractive choice. The Liqui Moly bottle even comes with a nipple on the top, making pouring easier if you don’t have an electric pump or Liqui Moly’s hand pump to push it into the diff.

Pulley Time

As much as I like to test incrementally, we were starting to run out of season. And I knew I’d need a little more power to deliver a significant reduction in Magneto’s elapsed time. Its best run to date was a 10.6…

With the stock air box and throttle body, you can’t exactly just slap a party pulley on and let it rip. Justin Starkey at VMP Performance recommended we step up to a 3.3-inch pulley for a pound or two of additional boost. Using simple math, we calculated an estimated gain of 25 to 50 horsepower – putting us close to 700 horsepower at the tire. The VMP TVS2650 has a lot more left in it from what we can tell, but a larger throttle body, air intake, and race fuel would be required to push the platform. I liked how the car drove on the street now. Anecdotal reports say maintaining the stock throttle body was the way to go for optimal street driveability. Thus, I decided to push this stock intake tract as far as possible. Step one was the smaller pulley paired with pump fuel enhanced by a trusty does of BOOSTane for increased octane. At a half a tank, a 32-ounce can put us between 103 and 104 octane according to the company’s handy chart.

Track Time

As we entered the winter months in the south, it is hit or miss on the track. Some just shut down, but Darlington Dragway says if it’s above 50 degrees, we are racing. Thankfully on Sundays, during the day, that is a real possibility. In our first attempt, we got four runs in. For the first two, I simply aired down the rear tires, added some BOOSTane to the tank, and let it rip. The crew worked hard to keep the track as sticky as possible, even though the glue required enough heat to be effective. Dragging helps.

Though I didn’t hit it quite as hard out of the hole as the last outing, clearly it liked the cool air. Magneto cruised to a new personal best 10.42 at 131 mph. I came around through the staging lanes and immediately made a second attempt. Being more aggressive on the launch paid off, and the S550 responded dropping from a 1.50 short-time to 1.439. We’ve gone faster but it was a step in the right direction, and given the weather – it might have been all the track had to give. On a pass, it ripped to 6.479 in the eighth and 10.0875 at 134.85 mph to the quarter. Winner, winner chicken dinner.

Being so close to the nines, I changed the shock settings to see if I could get a little faster off the line. Alas, I went in the wrong direction and as the temperature dropped the track got worse. On the first pass I spun slightly, and on the second pass I spun even worse – going a 1.44 and then a dismal 2.19 to the 60-foot. Game over. Through video review, it also appeared I was going through the traps in Seventh gear.

Knowing this combo was so close to going 9s, I wanted to make one more attempt after working with Dan at Pro Dyno to make a small adjustment to the tune, but it was no matter. Slightly warmer weather and the same result basically, 10.12 at 134.97 mph. According to most drag racing calculators, the 50-horsepower gain seems correct as we are now over 700 horsepower at the tires. Magneto is heading to Pro Dyno soon to verify that, and up the ante again with a little sauce.