The S550 Mustang represented major changes to the Mustang platform. One of those changes has been both bemoaned and embraced by Mustang fans and the media is the Mustang’s new independent rear suspension (IRS). While this new rear setup does utilize an 8.8-inch diameter ring gear for the rear differential, that is the only similarity between the reared center section and Mustang rearends dating back to 1986, when the venerable 8.8 rearend first appeared under the rear of the Mustang.

This new version of the 8.8 rearend requires not only unique rear axles or half-shafts, but also its own specific set of parts. Everything from the ring and pinion and differential down to the shims and crush sleeve are unique.

With this design so new to the market, Ford was the only game in town for upgrading the rearend on our 2015 Mustang, Project 5-Liter Eater. We reached out to Ford Performance and received a new 3.73 ring and pinion, Torsen T2 differential, and Ford Performance axles.

If you're familiar with the ring and pinion in the 1986-2014 Mustangs equipped with the 8.8-inch housing, you'll recognize the obvious differences between the new S550 ring and pinion gear set, and the older style set. The two are not compatible, nor are their shims, crush collar, or bearings.

The Gear Dilemma

Project 5-Liter Eater is more about handling finesse than all out straight-line brute force. In the autocross world where the car regularly competes, that means we need to gear the rearend for the right combination of low-end grunt, and top-end pull. Since most autocross courses rarely see speeds over 60-70 mph, we wanted to be able to get as close as possible to that mark with just our first two gears. The 3.55 gears for our EcoBoost-powered Mustang was originally equipped with were working, but weren’t giving the car the low end grunt and pull-away power we wanted. At the time of this install the only other two options were to go with a higher (numerically lower) 3.31 ring and pinion, or go with the lower (numerically higher) 3.73 gears that the Mustang GT Performance Pack comes equipped with. We chose the 3.73 route Ford Performance PN M-4209-8373A. The 3.73 gears are not an OEM option on the EcoBoost Mustang.

 

To get things going, we need to drop the entire rear subframe out of the car. The forward differential bolts cannot be removed with the subframe in the car.

Ford Performance’s Jesse Kershaw says, “The S550 Mustang platform utilizes a completely new rear differential and independent rear suspension. The newly designed ring and pinion sets are the strongest gears ever offered by Ford Performance, and should stand up to a lot of abuse, while offering reliable operation on the road. Of course enthusiasts installing these gears will need to have a Ford dealer or aftermarket supplier recalibrate the car’s electronics to compensate for the gear change.”

Kershaw says the gears undergo a proprietary hardening and finishing process that helps make them not only stronger but also ensures higher quality and quieter gear. “We actually call this the Super 8.8, and these gears are some of the strongest and quietest we’ve ever offered,” Kershaw says.

We're using an aluminum center section from a Mustang GT to speed up our build, since we can bench-build this center section while the rear subframe and center section in our project car is being removed. London Chassis Dyno's Chad Epperson placed this unit on a nearby crate for tear down. After draining the gear oil and cleaning off the OEM sealing material, Epperson used a dial gauge to measure the differential's backlash at 0.011-inch,

Since these gears and the new 8.8 are unique themselves, installation also requires a completely different set of shims and bearings than previous generation 8.8-equipped Mustangs. For that, Ford Performance has a complete solution with PN M-4210-B3, the Super 8.8 ring and pinion install kit. This kit includes all the bearings, shims, and even an extra crush sleeve, needed to install or upgrade the ring and pinion gears in the S550’s 8.8.

A case spreader is recommended by Ford to service the center section in the 2015 and newer Mustangs. Using brute force to pry the center section out could potentially damage the case by either cracking or stressing it, or by expanding it beyond the factory specifications of 0.020 inch. Since Epperson didn’t currently have a case spreader in the shop, and there wasn’t one to be rented, purchased, or borrowed nearby, he fabricated one using steel and Grade 8 hardware.

Traction Force

Seated within the Performance Pack EcoBoost’s rearend center section is a limited slip differential that is based on the same design as many other clutch-type limited slips differentials Ford has offered as standard or optional equipment on Mustangs since 1986. We need something that will both stand up to the additional power we plan to put to the rear tires as our project progresses, and something that will maximize the application of that power to the ground. The Torsen T2 differential PN M-4204-MT, which is also standard on the Mustang GT Performance Pack was the answer.

The Ford Performance Torsen T2 differential (PN M-4204-MT) is standard equipment on the Mustang GT Performance Pack. This differential is unique to the S550 center section,. Designed to accept the 34-spline half-shafts, this new differental is not backwards compatible to older solid rear axle (SRA) Mustangs. The Torsen differential for SRA-equipped Mustangs is not compatible with the S550 center section as well.

This gear-type differential has no clutches to wear. It allows us to put more power down through the straights, and thanks to its gear design, will allow us to apply power more effectively through a corner since its design allows power application to both wheels even in a turn. The 2015 Mustang utilizes a unique Torsen differential that isn’t compatible with previous generation Mustangs, or vice versa. This differential accepts 34-spline axle shafts and is only compatible with the differential in the new S550 chassis.

“Torsen has been a supplier to Ford for a number of years now … these differentials offer a substantial improvement over the standard limited slip unit by allowing the car to put more power to the ground, and improving durability. The full-time torque-biasing of this differential keeps power to the ground, and allows for better acceleration characteristics through improved traction,” says Kershaw.

This cut-away display of the S550’s center section gives a better look at the inner workings of the Torsen T2 differential. Rather than clutches or cones, this differential uses gear sets and frictional forces to lock and unlock the rear axle. With no friction material to slip, the Torsen T2 can send added power to the wheels more of the time, including in a cornering maneuver. Because of this, owners should experience a longer service life even in higher horsepower applications.

Just like with SRA 8.8 rearends, getting the shim off the pinion gear requires a gear puller and either a large hydraulic press or a bearing puller. The shim that is installed on this gear is a good starting point to set the pinion depth properly with the new gear set.

The weakest link in the S550 Mustang’s rearend has proven so far to be the rear axles or half-shafts. The smaller diameter of these axles, and the material chosen for them at the OEM level, has lead to numerous enthusiasts snapping these axles as soon as power is increased, or sticky tires put to work.

Constructed from a proprietary alloy, and featuring one-piece billet ends, the Ford Performance half-shafts are a considerable upgrade over the OEM parts. Ford Performance rates these shafts at 900 horsepower, which should be more than enough for the goals of our EcoBoost project car.

Ford Performance knew the half-shafts would be a potential issue for many enthusiasts from the beginning, so it had replacement heavy-duty half-shafts available since the debut of the new Mustang. The new half-shafts part number M-4130-M are built by G-Force for Ford Performance. These half-shafts feature severe duty, serviceable CV joints that will stand up to not only the abuse but can be serviced and even rebuilt of necessary. The inner and outer stub ends are each constructed from a single billet, adding further strength.

The half-shafts were one of the first parts that Ford Performance began offering for the S550 Mustang. Ford Racing’s Kershaw says, “We worked closely with G-Force engineering to develop these half-shafts. We knew Mustang enthusiasts would start turning up the power and were going to need the parts to put that power down, especially the guys running sticky tires at the drag strip.”

The Build

We headed to London Chassis Dyno, in London, Kentucky, where owner Chad Epperson and Willie “Red” Taylor took on the task of building our Mustang’s rearend while we photographed and offered encouragement, and opinions from the sidelines.

The half-shafts can be replaced easily enough, probably in two hours or less for many enthusiasts with the use of only a floor jack and jackstands. Servicing the center section beyond just a fluid change however, requires removal of the entire assembly from the car, which is made easier by removing the rear subframe assembly.

There is one special tool the Ford service manual says is necessary for servicing the rear differential: a case-spreader. In the past, we’ve always used a prybar to get the differential and ring gear assembly out of solid rear axle 8.8 rearends. However, those cases are steel, and the design dates back to the 1970s. Depending on the vehicle, a S550-based Mustang will have either an aluminum or steel housing. The tight tolerances of this housing dictate the use of a case spreader. We’ve heard of enthusiasts using a long pry bar and getting the differentials out of these center sections. However, we were warned by Ford that failure to use a case spreader and forcing the rear differential out may result in bearing or other issues with the rear differential post-installation.

With the ring and pinion installed and the case spreader still in position Epperson installed the Torsen T2 differential and ring gear, along with the shims and new bearings into the center section. The ring and pinion, as well as the shims, install similar to older SRA ring and pinions. Backlash was set at 0.008-inch, the gear mesh pattern was verified, and new RTV silicone was used to seal the rear cover. Once the center section was reassembled, Willie 'Red' Taylor installed the center section into the rear subframe along with its new BMR bushings.

Since we did not have access to a case spreader, Epperson built one using angle steel and Grade 8 hardware, combined with his welding skills. Ford specifies that the case can only be spread a maximum of 0.020-inch, beyond that you risk damaging the case, so care must be taken when removing the rear differential.

To speed up our build and give us a possible performance advantage, Jordan Weir, of Weir Racing provided us with an aluminum rear differential housing from a ’15 Mustang GT. This allowed us to build the rear differential and swap it with our car’s original steel differential more rapidly. We were conducting this job at the same time we were installing our BMR Suspension components.

We installed the half-shafts into the rear differential prior to raising the entire assembly back into the car. This made verifying that we had good engagement of the lock clips on the axles easy.

Prior to removing the limited-slip differential, we measured rearend back-lash at 0.011-inch. We noted this for setting up our new ring and pinion. The differential must be removed from the car to conduct this type of upgrade, since the rear suspension subframe will not allow full access to the center section.

Aside form the physical differences in the parts, and the presence of axle bearings on the outside of the center section, this rearend build is much like any other 8.8-inch rearend build to date. We set our backlash at 0.008-inch, somewhat tighter than the original, checked our gear tooth pattern to ensure the proper pinion gear depth, and then closed the differential up and installed it back into the rear subframe.

If you have access to a case spreader, you can build the S550’s differential in about eight to 10 hours, including removal time. Like previous 8.8-inch rearends, the most difficult job is setting backlash and pinion depth.

Driving

With the rear subframe and all components reinstalled in the car, we used our SCT X4 to update our PCM for 3.73 gears.

High-Performance Gear Oil

We chose to break-in our new rearend parts with semi-syntehtic 75w90 gear oil. After 500 miles of break-in driving we then switched over to Driven Racing Oil’s GO 75w140 synthetic gear oil, available from Jegs.com as PN 04330. Lake Speed, Jr. tells us that Driven’s GO works well with Torsen and other gear-type differentials. The high temperature tolerances of this gear oil are ideally suited for a car that sees both daily driving, and high RPM, or high-speed driving and high loads on the differential.

After setting our rear axle ratio to 3.73 with our SCT X4 tuning device, we took Project 5-Liter-Eater for a test drive. On the street, there’s little difference in the way the car feels most of the time. There is a noticeable improvement in acceleration characteristics — had you told us the car came with 3.73s and we didn’t know better, we’d believe it. The EcoBoost loves this gear ratio.

The Ford Performance Torsen T2 differential may be slightly noisier than a traditional clutch-type differential, about what you’d expect from an upgraded differential. However, it’s difficult to determine that with certainty at this point since this project was performed at the same time as our rear suspension upgrades. We spoke with Ford engineers and they agreed much of what we’re hearing in terms of additional NVH in the cabin is due to changing out the differential bushings rather than additional or new noise caused by the differential upgrade itself. Kershaw informed us that Ford has tested each of these components one at a time. He went on to tell us that the axles added no additional noise to a car when tested by Ford Performance engineers. Ford found that changing the subframe bushings had a minimal NVH increase. Changing the differential also had a minimal impact on NVH. Changing the differential bushings in all of Ford’s tests had a substantial impact on NVH in terms of increasing these noises. It’s reasonable to assume that any increase of in-cabin NVH can be largely attributed to our upgrade to stiffer polyurethane differential bushings which will transmit more noise from the road, the differential, and the suspension through the car than the OEM parts would have.

With regard to our BMR Suspension bushings, specifically those for the differential. We’ll take the tradeoff of an increase in NVH in return for the reduced deflection, better handling and acceleration benefits we’re seeing with that upgrade. The bushings do an excellent job of mitigating wheelhop, and providing better response from the drivetrain in just about any driving scenario. Combined with the changes we made to the rest of the suspension and the rearend, this is an excellent package, allowing the car to perform markedly better.

On the autocross the gearing and differential upgrades were just what we were looking for. Acceleration is tremendously improved, and as best we can tell we only gave up about 2 mph before hitting the rev limiter, meaning we can make it to about 62 mph in second gear. The 3.73s lend to better starting line grunt, and when we exit a corner it’s actually easier to modulate the throttle — we’re not waiting for the car to accelerate, and the new gearing actually gets the car accelerating more rapidly. We can also tell the Torsen differential is working its magic as well. Power application is more linear and predictable as we work back into the throttle in a corner. Starting line traction is markedly improved as well; typically we want to spin the tires briefly as we launch, and the Torsen differential allows us to do so without worry of only one wheel having traction.

The 3.73 gears worked so well in our upgrade we were left wondering why this is not a Factory option on the EcoBoost, especially with the Performance Pack and manual transmission. The car has suffered no noticeable loss of fuel economy, and its low-gear performance on the autocross has been remarkable.

With our rear differential, axles, and gearing now squared away, we’re ready to begin adding even more power to Project 5-Liter Eater, and continue our quest to see just how far we can push the performance of our EcoBoost Mustang.