continued from above
Here are more weights from the the 1999-03 Mustang Cobra 8.8" aluminum rear housing (62.9 lbs, at left) and the "medium case" BMW E36 housing (76.9 lbs, at right). The weight for the "large case" E46 M3 unit is above, at 100.4 pounds. All of these weights are complete housings with the factory rear covers, factory gears, and factory differential units. The BMW housings are heavier because they are cast steel - which is stronger than aluminum - but what usually breaks is the actual ring gear.
And while the external dimensions on the medium case BMW unit and the Ford 8.8" unit look similar, the 8.8" ring gear translates to 223.5mm. And this is one of those cases where "bigger is better". Not to mention there are countless ring and pinion gear sets available at a fraction of the cost of the limited number of BMW gear ratio options for either the 188mm or 210mm housings. And limited slip units are more abundant (and less costly) for the 8.8" vs the BMW units.
So like the engine and drivetrain in this build, we're looking at a domestic part for a lower cost and stronger differential, with more plentiful gear/diff options and a lighter weight by nearly
37 pounds.
We happened to have an 8.8" aluminum diff housing from a 2015-up Mustang S550 chassis, and its a thing of beauty. We mocked it up next to the 210mm M3 diff (above left) and it looked like it might work. Then we pulled the subframe out of the car, pulled it apart, and bead blasted it to raw steel. That's when we noticed - this S550 housing isn't going to ever fit. The front mounting "feet" on the S550 housing are inside the pick-up points of the rear suspension and the middle of the case hits at the back. This would require a completely scratch built subframe and new control arm lengths - and we don't need those kinds of hassles.
Enter the 1999-04 Ford Mustang Cobra aluminum 8.8" housing. I picked one up used because we couldn't find new stock of the OEM housing or even the old Ford Racing part number kit - popular with Cobra kit car builds for years. The M3 rear subframe was clamped to the fab table and Ryan started cutting...
The front of the 99-03 Cobra 8.8" housing is much narrower than the 2015-up S550 housing, so after some minor clearancing and reinforcement, the front was able to slide between the suspension pick-up points. The front sections needed a good bit of fabrication and even custom machined front mounting bushings, but it all went together smoothly. The centerline of the drive flanges was matched to that of the BMW unit and it was squared up before any of the mounts were designed.
The rear mounting was a bit trickeir. The 99-04 Cobra "clamp" mount was similar to the E46 non-M rear cover, but was never a popular choice and this cover/mount had failures in the Cobra Mustangs (below left). The later Explorer 8.8" rear cover (below right) mimicked the E46 M3 "ears" but they were in the wrong plane fore-aft and the wrong width. It was going to take massive modifications to make that work, so we punted on these.
Instead Jason recommended an aftermarket steel Ford 8.8" solid rear axle cover that has the same bolt pattern and shape as the 99-04 8.8" IRS housing. This was a fabricated unit for off road use, so it was beefier than the thin stamped steel OEM covers used on solid axle 8.8s.
continued below