Controlling your 2015 Mustang’s Super 8.8 with diff bushing inserts
By Steve Turner
Photos by Steve Turner and courtesy of Steeda Autosports
Sure the factory Independent Rear Suspension under the latest Mustangs raised the bar for pony car handling. However, as we now know, the things that make it livable for everyday drivers allow many of the subassemblies to move. Moving means that geometries change, and bad things can happen when your Mustang is driven in anger; bad things like wheel hop and loss of traction.
You can order the Steeda S550 Mustang Differential Bushing Insert System in three flavors: Urethane (PN 555-4439; $99.95), Race aluminum (PN 555-4440; $94.95) and a complete set (PN 555-4442; $169.95), as shown here. This kit is designed to minimize differential movement, improve traction, and reduce wheelhop. They are available this way, because you could swap in the race inserts for the track and reinstall the street or performance inserts for daily use.
One of the major contributors of wheel hop and lost traction is differential movement. The stock rubber bushings that isolate the differential from the cradle are not only rubber, but they are peppered with voids that provide additional give. Sure, they do a great job keeping things quiet, but when you decide to use that Launch Control, things start to move.
You could cut out the stock bushings and replace them with more rugged replacements, but that’s a lot of work, and you are sure to introduce some additional noise, vibration, and harshness into the picture. That’s why Steeda Autosports developed its S550 Mustang Adjustable Differential Bushing Insert System. This bolt-on solution bolsters the factory bushings with urethane (PN 555-4439; $99.95) or aluminum (PN 555-4440; $94.95)  inserts that reduce differential movement.
We told you about this system when it was released and we recently had the opportunity to visit the Steeda campus in Pompano Beach, Florida, to watch technician Steve Chichisola install the system on the company’s in-house EcoBoost Mustang.
For its EcoBoost project, Steeda chose its Street urethane diff inserts, which feature a durometer rating of 80. They will shore up the movement of the Super 8.8 without increasing any of that much heralded noise, vibration, and harshness.Steeda’s EcoBoost Mustang already wore a number of the company’s signature suspension upgrades, including its handling springs and rear sway bar.Start by putting the car on jackstands or a lift, if you have access to one. Once the car is in the air, support the IRS subframe with a separate jack. Then you can remove the rear subframe bolts (but leave the fronts in place) and let the subframe droop, as Steeda’s Steve Chichisola does here.It’s not included in the diff insert kit, but this car already had Steeda’s Subframe Alignment Kit (PN 555-4438; $39.95). If your car isn’t so equipped, this would be a good time to add these inserts, which shore up any misalignments in the factory fit of the subframe and keep them locked in place. These inserts simply slide into the mounting holes and are secured with the stock fasteners.Steve moved on by zipping out the rear diff bushing fasteners with an air ratchet.We had never seen them before, but this EcoBoost Mustang was equipped with rather bulky vibration dampers that mounted to the IRS subframe via the rear diff bushing bolts. Steve says they are only on some cars, so they must be added to cars that have a detectable resonance when they are checked out at the factory.One look at the stock, rubber differential bushing and you can see why the Super 8.8 might get frisky when you drop the hammer. Not only is rubber made to deflect, but the voided areas also allow these bushings allow even more movement.Prep the Steeda inserts by lubing the mounting hole with the supplied silicone grease and sliding in the provided metal spacers. The rear inserts are thinner, so they use the shorter spacers.The Steeda inserts are designed to fit into the stock bushings like a puzzle piece, but because the stock bushing differ on cars with the bolt-on dampers, you will need to install a washer on the insider of the insert to ensure that it fits properly. Cars without the dampers, just use a washer on the outside of the insert. Either way, bolt the insert in using the bolt provided in the Steeda kit, and make sure you apply a bit of blue thread locker to the threads. Then you can bolt the rear subframe back up.Moving to the front, Steve removes the front subframe fasteners and lets the front of the subframe droop. Naturally, you want to make sure it is still supported by a jackstand and that the rear subframe is all bolted up before you do this.The front diff bolt threads into the subframe from the back to the front. Steve zips out the long factory bolt.Steve replaces the factory fastener with a shorter Steeda bolt.After installing the spacer in the insert, Steve slid the shorter bolt through the insert and slipped it into place. The passenger side is the easier side, while the driver side may require a bit of prying on the fuel tank shield to let you slip in the insert and bolt. Don’t try to put the insert and bolt in separately. It won’t go in that way.With the diff bushing inserts installed, Steve bolted the front of the subframe back up to the chassis. He had a few more steps here, as the car already wore Steeda’s subframe bushing supports (PN 555-4437; $144.95).Steve reinstalls the Steeda IRS subframe support braces (PN 555-5754; $194.95) and the assembly process is just about complete. Obviously, the Steeda EcoBoost already had these installed, but they would make another good while-you’re-there upgrade to accompany the diff bushing inserts.Steve completes the installation by torquing the front and rear differential retaining bolts to 129 lb-ft. The new shorter bolts that hold in the front diff bushing inserts don’t secure the diff, so they should only be tightened to about 30 lb-ft. However, you are going to be hard pressed to fit a torque wrench in the space left, so you’ll have to rely on your mechanical feel. Make sure they are tight, and you might want to double-check them after you put some miles on your car.
