Gearing Down
Lethal Performance seeks lower e.t.’s with a Super 8.8 gear swap
By Steve Turner
Photos by Steve Turner and courtesy of Ford Performance Parts
Back in the old days, one of the first Fox Mustang mods was a gear swap. Moving to a numerically higher gear ration would help these cars get out of the whole quicker, and simply make the cars feel faster, and it was often that better feeling that encouraged the swap. When it comes to modern Mustangs, they are already pretty fast from the factory, and they often come equipped with the gears we used to swap in.
Case in point is Lethal Performance’s 2015 Mustang GT. It rocked 3.73 gears right from the factory, which definitely feels and sounds great. However, when you car is making over 800 rear-wheel horsepower and its mission is lower quarter-mile e.t’s, the question of the right ratio gets a bit more complicated.
Now, there is a science to picking the right gear ratio and the proper tire size to arrive at the ideal ratio for your combo’s goals. That said, the robust Super 8.8 center section in the 2015 Mustang doesn’t quite have the number of ratio options that the venerable Ford 9-inch has—not yet anyway. That means making the best of what’s readily available.
During the drag testing of its Whipple-supercharged GT, Team Lethal hotshoe Jeremy Martorella was tickling the rev limiter at the top of fourth gear going through the traps. He didn’t want to shift that late and hurt the car’s performance, but dancing with the rev limiter was not ideal either.
“We decided to swap gears because we were running out of gear with the 3.73s at the track. They worked well for when the car was stock, but with the power we're making with the Whipple the gears were too short,” Jared Rosen of Lethal Performance explained. “The 3.73s basically kept us from passing the traps in the desired rpm range that we would really benefit from. The 3.31s allowed us to go through the gears without having to shift as fast and to pass the traps in Fourth without hitting the limiter.”
As you can see, the swap helped the car run 9.7 at 150 mph…
[video=youtube;na7_tA2cxuc]https://www.youtube.com/watch?v=na7_tA2cxuc[/video]
Ford Performance Parts offers 3.55 gear sets (PN M-4209-88355A; $249), 3.73 gear sets (PN M-4209-88373; $249), and an install kit (PN M-4210-B3; $150) if you are looking to go the other way. In the case of the Lethal car, we documented a set of 3.31s from the local dealer being installed at Power by the Hour in Boynton Beach, Florida, using the FPP install kit.
The Lethal 2015 GT has since been sold to make room for new projects, but if you are looking to switch Super 8.8 gear—no matter the ratio—the swap specifics are the same. So, if you are considering the swap now or just want to be ready down the line, follow along with our photos. And, if you do make the swap, you'll need to use the Whipple Flare to correct the speedometer for the new gears.
If you are looking to upgrade your S550 with 3.55 (PN M-4209-88355A; $249) or 3.73 (PN M-4209-88373; $249) gears, Ford Performance Parts has the gear sets and the install kit (M-4210-B3; $150) necessary to swap them in. Lethal Performance was actually making the move to 3.31 gears, but the process and parts look the same.
Here’s a look at Ford Performance Parts 3.55 gear set for the Super 8.8 rearend in the 2015 Mustang. It includes the ring, pinion, and necessary bearings. According to Ford Performance, these gears receive OE-quality surface finish, hardening and manufacturing consistency, which is said to make them the most rugged and quietest gears on the market.
FPP says this install kit is required to install the new gear set. The kit includes new OEM pinion bearings and races; OEM diff bearings and races; pinion and diff shims; a crush sleeve; a pinion nut, a pinion seal, ring gear bolts, and gear-marking compound. It is definitely a complete kit.
Power by the Hour technician Donnie Renfrow starts the process by draining the differential fluid from the Super 8.8 center section.
Next Donnie removed the cat-back exhaust and unbolted The DriveShaft Shop one-piece aluminum driveshaft to clear the way for the removal of the center section. He used a Bungie cord to hold the shaft up so it didn’t dangle.
A first for us was seeing someone take apart the halfshafts on an S550. Donnie unbolted the D.S.S. halfshafts on each side and the Super 8.8 was almost ready to come out.
With the driveline connections free, Donnie could finally unbolt the center section from the cradle. Obviously you want to support the carrier before you remove these bolts.
After disconnecting the vent tube and removing the cradle bolts, Donnie lowers the Super 8.8 center section out of the Lethal 2015 Mustang’s cradle.
After years of watching gear swaps in 8.8-inch rearends, we can appreciate that the cast aluminum cover employed on the S550 Super 8.8 is both beefier and more complex than the covers found on solid-axle 8.8s. Much of this is because the cover incorporates the mounting points for the carrier. After removing its retaining bolts, Donnie removes the cover with a few gentle taps from a mallet.
You will want to mark the differential bearing caps so that you know which is left and right, as you’ll want to reinstall them on the same side you removed them from so that they will seat properly.
Clearly, the Lethal Mustang doesn’t have a lot of miles on it, but it does have a quite a few drag strip laps on it. Part of its street cred is that it retained the stock limited-slip differential. No spools here.
Donnie used a puller to push out the factory pinion gear. He then removed the factory pinion cups and would replace them with new cups included in the FPP kit.
Next, he removes the ring gear bolts. You could re-use these fasteners, but the FPP install kit includes fresh bolts.
It’s not a pretty process, but Donnie used a puller to extract both of the factory differential bearings. The FPP install kit provides a fresh set, so don’t worry if yours are damaged coming off the diff.
With all the factory parts disassembled, Donnie embarked in the install by measuring to see which pinion shim would be required. Then he installed the shim and pinion bearing using a hydraulic press. He followed the same procedure on the differential bearings. Using a pinion-depth gauge, he determined which shims were required for the pinion gear.
With the new pinion bearing and crush sleeves in place, it was almost time to tap the pinion into place. Before we got to that point, however, your author suggested that this would be an opportune time to clean and paint the center section. Yup. Even 2015 Mustang Super 8.8s get rusty. Now, thanks to Power by the Hour, the Lethal GT has a nice, black Super 8.8. That will definitely make the car faster—or at least look better.
After tapping in the pinion, Donnie reinstalled the pinion flange. You’ll want to check the torque on the pinion flange nut before you remove the stocker. When you reinstall the flange, you’ll want to torque it to 5 in-lb more than the rating you recorded on the stocker.
With the stock ring gear removed, Donnie installed the new ring gear using the fresh hardware supplied in the FPP kit and a dab of thread-locker on each bolt. He torqued the ring gear bolts to 100 lb-ft, then installed the diff.
With the proper shims in place, Donnie snugged the bearing caps. Then he torqued them 71 lb-ft. With the bearing caps torque, he double-checked backlash measurements to ensure they were within spec. If you don’t get this right, you’ll get noise and premature wear.
With all the proper shims in place and the fasteners torqued, Donnie marked the bolts with paint to show they have been torqued. He did so on ever fastener along the way, which is a sure sign of a fastidious technician.
Donnie applied a 2mm-wide bead of silicone sealant around the perimeter of the Super 8.8 housing. Then he bolted the cover back on and torqued the fasteners to 34 lb-ft.
With the Super 8.8 reassembled, Donnie bolted it back into place and reinstalled the halfshafts, driveshaft, and exhaust system. He torqued the differential mounting bolts to the prescribed 129 lb-ft of torque. The driveshaft-to-pinion flange bolts get 41 lb-ft of torque. After everything was properly torqued, Donnie replenished the differential fluid, and the Lethal GT was racetrack-ready. It eventually ran in 9.7 at 150 mph before being sold to make way for some new Lethal project cars.
