Pretty much everything on the 2015 Mustang is brand new. This platform brings new technologies and new performance to the Mustang brand. One of the first things we heard, even before the cars started hitting dealer lots, was that the ring and pinion gear sets were an all new design, and that old gear sets were incompatible. We recently received a ring and pinion set for our Project 5-Liter Eater EcoBoost Mustang from Ford Performance. While we’ll be covering the build of our IRS 8.8-inch rearend in an upcoming tech article, we wanted to get an early look at the gears themselves.
The 8.8-inch solid rear axle made its debut in the Mustang in 1986, showing up in any model equipped with the 5.0. While the housing has undergone some various changes in width, brake compatibility, speed sensor pickups, and even an IRS version from 1999-2004, the center section has remained largely the same through the years. That center section has been a mainstay of Mustang performance for nearly 30 years. For 2015, however, changes were made to the 8.8-inch housing that dictated an all new ring and pinion gear set.
According to Ford Performance’s Jesse Kershaw, the new ring and pinion gear sets are known as the Super 8.8-inch. Kershaw says these are designed to handle more horsepower and torque than previous generation 8.8 gear sets, but Ford has yet to release the official numbers. Ford Performance had these gear sets ready to go as soon as the 2015 Mustang hit the streets, knowing that many enthusiasts would be looking for an upgrade. Currently, the only ratios offered are 3.73 and 3.55, although Kershaw says a 4.10 set could be available in the future.
It’s worth noting that although the gears for the IRS 8.8 are different than the outgoing solid rear axle they’re still manufactured in the USA at Ford’s Sterling axle plant.
We happen to have a set of solid rear axle Ford Racing 3.73 gears sitting on the shelf, part number M-4209-88373. We also happen to have a set of the new 2015 Super 8.8-inch ring and pinion gears in 3.73, part number M-4209-88373A. We pulled both sets out of the box for some side by side comparisons so we could share with our readers exactly what is different physically.
The pinion gear is the piece that is most obviously changed from the solid rear axle (SRA) 8.8-inch rearend. At first glance, you’ll notice the IRS pinion gear is significantly longer than the outgoing version. The SRA 8.8 pinion gear measures 8.31-inches (211mm) long verses the IRS pinion gear length of 9.125 (231.77mm). The difference in length is due to the deeper case design of the new IRS 8.8-inch housing.
The diameter of the pinion gears is also significantly different. Where the pinion bearing seats, the SRA diameter is 1.6-inches (41.3mm). The IRS pinion gear has a diameter of 2.0-inches (50.9mm) at the pinion bearing seat area.
These differences also require a new crush sleeve and a new pinion nut. Although we didn’t measure the crush sleeves, the differences are pretty obvious. The inside diameter is different between the two to accommodate the different pinion gear shaft sizes. The IRS crush sleeve is also huge by comparison to that of the SRA unit.
The threaded diameter of the IRS pinion is different than that of the SRA pinion. This requires a new pinion nut. Ford Performance includes thread locking compound on the new nut. The SRA pinion nut measured 27mm from one flat surface to the other. The IRS pinion nut measures 28mm in this same measurement.
The ring gears are also different as well. The 8.8-inch rearend gets its name from the ring gear diameter. While the outside diameters are the same, and the gear tooth pattern appears nearly identical there is at least one other difference. We could not get a precise measurement on the height differences (not for a lack of trying), however its enough to be visibly noticeable.. Notice also that the shape of the backside of the ring gear for the IRS 8.8 appears to have a smaller beveled section than the SRA ring gear. We did not have any ring gear bolts to check to see if they were different as well.
We know that Ford has a new installation kit and side bearings for the IRS 8.8-inch rearend. A new Torsen differential is also available, as part number M-4204-MT. It looks like the older SRA style differentials are not compatible with the IRS 8.8, as the differential is 34-spline, and the new ring and pinion design may also be incompatible with SRA differentials.
Kershaw says Ford has several parts in the works for the S550 Mustang and its IRS housing, including a Torsen differential, heavy duty axles, 3.73 and 3.55 ring and pinion gear sets, and they’re working on selling complete ready to go 8.8 center sections. We’ll be building our 8.8-inch IRS rearend for an upcoming story, and we’ll detail what’s new and different right here when we do.