Inside the "Super 8.8" Rear End


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Feb 9, 2015
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Venice, FL
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'16 GT350, Shadow Black, Chassis: G1245
In 2015 Ford Introduced a newly designed 8.8” Rear-End they called the “Super 8.8”
It replaced the older 8.8” rear that had been in production for some 25 years, and parts from these two do not interchange.
The newer “Super 8.8” comes in 2 flavors
1. A solid axle model designed for the 2015+ F-150 light duty pickup.
2. An Independent Rear Suspension (IRS) model designed for the S550 Mustangs.

A Little History:
This new design change possibly all began with the Pinion Bearings.
The beefier front/outer & rear/inner pinion bearings in the Super 8.8 were carried over from the previous 9.75” Ford rear-end.
The pinion bearings, crush sleeve, pinion seal, and pinion nut all came from the earlier 9.75” model (i.e. 2012 Ford Expedition, Lincoln Navigator).
A newly designed ring and pinion was also introduced for the Super 8.8, along with new 34 spline differentials.

The F-150 pickup model and the Mustang model share the following compatibility:
Most all internal parts inside the carriers do interchange, -Except the differential itself-
The F-150 uses c-clips holding the axles in the diff, which is different than the Mustang IRS diff. which uses a spring snap-in design for the axle clips.
*While using vastly different rear covers, the 12-bolt pattern for the rear covers is exactly the same on both.*

The S550 Super 8.8

The Super 8.8 in the Mustang came with two different housings, Aluminum and Iron. The Aluminum housings were placed in the Automatic Transmission cars, while the Iron housings were used in the Manual Transmission cars.
The bare Iron housing on the Left is 24 lbs. heavier than the Aluminum bare housing on the right
This Iron housing is going to receive all new bearings

Carrier Comparison-1200x600.jpg

The Ford Traction-Lok differential would be standard on the S550 cars with the Torsen as a specific option.
Only with the Iron housing you could get a Torsen differential (with the 3.73 axle ratio). Starting in 2018 Ford also made the Torsen available (with the 3.55 axle ratio). The Torsen T2R differential isn’t manufactured by Ford, it is a registered trademark and made by a company named ‘JTEKT’. And by-the-way, the KOYO bearings in the OEM Super 8.8, they are also made by ‘JTEKT’.

The Torsen (Differential) on the Left weighs 26 lbs., while the Traction-Lok (Differential) on the Right weighs 18 lbs.
Differential Comparison-800x600.jpg

Here is a T2R Torsen Differential taken apart. I have had several of these apart and every time upon inspection they still look like new.
Every Torsen Differential also has a unique Identifying Serial Number stamped into its housing
Torsen Dissembled-800x600.jpg

Gear Ratios:
Factory OEM gear ratios for the S550 consisted of: 3.15, 3.31, 3.55, 3.73.
The 3.15 gear ratio used a different differential. (The flange offset for the ring gear was different to accommodate the larger pinion head diameter of the 3.15). The 3.31 and-up ratios are all interchangeable with their differentials.
US Gear also makes (For Ford) a 3.73, 3.90, and a 4.09 ratio. Other aftermarket gear ratios available are: 4.11, 4.56, and 4.88
The Differential on the Left is a 3.15 Axle Ratio (Ring and Pinion), Notice the difference in position between it and the Differential on the Right (Non 3.15 Axle Ratio)
The ring gear bolts and the diff flange are closer to the left housing cap on the 3.15

Flange Clearance Comparison.jpg

US Gear also sells a Polished Version of their Ring and Pinion Sets (Using an ISF process to obtain a mirror-like finish on the surface)
Here is a 4.11 Lightning Series Ring and Pinion from US Gear
Lightning Gearset 4.11-500x600.jpg

For a Ford Parts List/Breakdown see the attached "Super 8.8 parts" .pdf file attached
(at the end of this post)

Ford Performance: M-4851-M8A pinion flange kit

For those who are just wanting to use the kit above to replace the Pinion Flange,
I'd like to make a couple of comments concerning my experience with the Collapsible Spacer (Crush Sleeve):
The crush sleeve on the Super 8.8 cannot be removed/replaced from the front.
What I mean by that is, if you remove the pinion flange, pinion seal, oil slinger, and front bearing cone, the crush sleeve will not clear the front pinion bearing cup and come out from the front.
In order to use a new crush sleeve you will need to remove the rear cover, remove the differential, and remove the pinion shaft from the housing (from the rear), to get the crush sleeve off the pinion shaft.

A Deeper Dive:
When removing the Pinion Flange from the S550 you will most certainly need a puller, especially on the 2018 and up Flanges (They fit really tight)
I use this puller set which is widely available (Speed Shops, Internet, Parts outlets)
Gear Puller Tool Set-800x600.jpg

When removing the Pinion Flange, Never hammer on the Flange.
Also be careful not to drop the Housing or Flange, or you could have a situation like this.
Broken Pinion Flange-800x600.jpg

One of the first things I do after removing the Rear Cover is mark the Housing Caps (Left & Right)
The Aluminum Housing Caps are already marked, but on the Iron Housing Caps they are not.
I always use Number Stamps to mark the Caps
Diff Bolt Cap Numbering-400x600.jpg

When disassembling the S550 carrier I always use a 0-1" micrometer to measure and then mark the Pinion and Differential Shims.

One of the best investments I made recently is a Bench-Top 10 ton hydraulic press
This is a JEGS Model: 81636 (Under 200.00). It works really well for this kind of work.
Here I am pressing off the inner pinion bearing
Press and Pinion Bearing Removal-1500x600.jpg

The Carrier/Housing Pinion Bearing Cups/Races should always be replaced when replacing the Pinion Bearing Cones/Rollers.
The Cups are pressed in the housing, but can be easily removed with a long drift punch and hammer.
To make the installation of the new cups easier I fabricated my own cup drivers from old pinion bearing cups and 1/2" drive sockets. Some buy/Use the Cup Driver Sets which also work well.
Homemade Cup Driver-Install-1-1500x600.jpg

Homemade Cup Driver-Install-2-1500x600.jpg

Differential Bearing Removal:
The removal of the differential side bearings can be rather difficult at times
Here are two methods of removing the side bearing Cones/Rollers
If these two methods don't work, you can PM me on a fail-proof third method to remove them
Diff Bearing Puller-4-874x600.jpg

Differential Bearing Installation:
Diff Bearing Install-1-800x600.jpg

Axle Bearings and Seals:

Axle bearings and seals are easily replaced while the carrier is out of the car.
I always measure the old bearing depth before removing it so I can install the new bearing in exactly the same depth.
Axle Bearing Depth-800x600.jpg

I use an old Differential Bearing Cup to seat the new Axle Seal, It is a perfect fit and doesn't bend the raised flange of the seal.
Axle Seal Install-2-800x600.jpg

Painting the Iron Housing:
I definitely believe in painting the Iron housings, black seems to be the best color
GT350's and the PP GT's come painted from the factory.
Without a doubt, the best paint for the housings is the POR 15 caliper paint
Housing Caliper Paint-800x600.jpg

Gear Oil and Wear:
If you ask 10 car enthusiasts' what Gear Oil they prefer, you'll probably get 5 or 6 different answers.
Nothing wrong with the Motorcraft Full Synthetic. My favorite is the BG Ultra Guard LS.
They offer it in 75W 90 (PN 751), and 75W 140 (PN 753)
Here in Florida the temperatures' usually range between 60 and 90 degrees, so the 75W 140 works well for me.

The Ring and Pinion on the Left is a New Ford 3.73 gearset. Notice the Black Oxide finish on the factory OEM Gears.
This black finish wears off slowly upon break-in and leaves a nasty black residue in the gear oil.
That's why I recommend you change your differential gear oil at 5-10k miles, although Ford doesn't see this as causing any problem at all.
Ring and Pinion Comparison-1500x600.jpg

Vibration Concerns:
The 2015-2017 S550's used a unique balancing scheme for the Pinion Flange and Driveshaft.
The big yellow paint marks on each needed to be lined up for the best balance.
In 2018 Ford released a new and improved Pinion Flange that was more Zero-Balanced.
I like to mark the Driveshaft and all 6 holes on the Pinion Flange with number stamps.
In case I have a vibration problem I can use the numbers to clock the driveshaft in its best non-vibrating position.
Pictured here is a QA1 Carbon Fiber Driveshaft with the 2018-up Pinion Flange.
Pinion Flange Marking-6-1858x600.jpg

After torqueing the Carrier cap bolts, I use gear marking compound to make sure I still have a good contact pattern for the Ring and Pinion.
Gear Pattern Coast Side-800x600.jpg

Here is the Rebuilt Carrier Assembly (3.73 w/Torsen) ready to go in.
It has the New Finned Aluminum Rear Cover designed by:
"Ish" owner of the FluiX (Fluid Dynamic Experimentation and Technology) Company in Germany.
I will be testing this rear cover, and recording temperature comparisons with temps taken from a stock OEM cover
Ready For Install-800x600.jpg


Early information announcement links for the Super 8.8:

2020 Ford Performance Catalog:

2015 Mustang Rear end Build:

Discussion on OEM Rear Covers:

Torsen Info: Torque-Sensing (full name,manufactured by the Gleason Corporation

My First Gear Change (3.73 to 4.11):

My Second Gear Change (4.11 to 4.56):

Additional cooling and fluid capacity solution for the S550 Mustang ‘Super 8.8’ rear end:‘super-8-8’-rear-end.117720/

The Black Oxide Finishing Process:



Last edited:

WD Pro

Well-Known Member
Jan 18, 2018
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United Kingdom
Lime GT
Vehicle Showcase
Excellent info ! :like: :sunglasses:

I’ve still not got the balls or the tools to attempt this myself though ... :cwl:

WD :like:


Well-Known Member
Oct 12, 2020
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2019 GT Premium PP2
Thats awesome! Great info on designs.

Makes me wonder if at 16k miles, I should have my rear diff oil changed...


1 5 4 8 3 7 2 6
Feb 9, 2015
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Venice, FL
First Name
'16 GT350, Shadow Black, Chassis: G1245
Build Complete.jpg

For a good temperature comparison of this New Rear Cover with OEM, I figured the best test would be going with a three-pronged approach.
  • Extended Interstate drive
  • Back road aggressive session
  • Drifting and autocross event
I made a 1020 mile round trip to Fort Walton Beach FL from Venice Fl. recently
I had made this same trip in Aug 2018 with my OEM rear-End w/Stock Rear Cover and had recorded some diff temps. Mostly all interstate driving with occasional higher speeds.
Aug 2018 -Diff Temp Average = 202, Highest reached = 220 Air Temp 90
Nov 2020 -Diff Temp Average = 194, Highest reached = 210 Air Temp 88

In the FWB area there are 2 large military installations, (Eglin AFB, and Hurlburt Field)
The roads are pretty desolate at times, plenty with low traffic, some winding.
I was able to do some aggressive back road driving again while there.
Aug 2018 -Diff Temp Average = 224, Highest reached = 235 Air Temp 90
Nov 2020 -Diff Temp Average = 216, Highest reached = 222 Air Temp 88

About a year ago I participated in a drifting event at a local asphalt oval race track in Central FL
Jun 2019 -Diff Temp Average = 255, Highest reached = 275 Air Temp 85

For the final leg of testing me and couple of my buddies recently set up an autocross in a large abandoned parking lot.

We did several runs and some drifting in 25-35 minute sessions.
Nov 2020 -Diff Temp Average = 238, Highest reached = 255 Air Temp 88

In conclusion I believe this newer Diff Cover definitely offers some additional cooling for the Mustang S550 Rear-End and I would definitely recommend it, not only for its great look, but also for its functionality. It positively has been an improvement for me over OEM and I am very happy with it.


Well-Known Member
Jan 22, 2018
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2015 GT Premium PP Recaro
I'm sure I'm not alone in being interested in more details about the FluiX cover.

Cobra Jet

Well-Known Member
Feb 12, 2015
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2018 EB Prem. w/PP and 94 Mustang Cobra
Great thread @JohnVallo - lots of tech and details for future reference!!


New Member
Jun 26, 2017
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Tacoma, WA
2016 Mustang GT
Great info. I am about to change my 3.55 trac lok rears to a 3.31 Torsen rear. I will be taking it to a shop with the proper tools to get it done. This is beyond my level of expertise.


Oct 13, 2018
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2018 Mustang ecoboost base w/ performance pack
I have a question. My 2018 3.55 Torsen differentials pinion seal is leaking, and i want to change the seal myself, however if that collapsible spacer has to be changed, then i will not even attempt it. I'm hoping i can just pick up a seal and be done with it. I appreciate any info. Cheers