What Rear diff choices are there for the IRS from Factory Five

[Jkviper]

Hi,

I am still in the planning stage before I order my Coupe and could not seem to find this info and have a few questions:

1)If you order the Complete kit and select part number 60380 - 2015 Mustang-based Independent Rear Suspension Kit, what type of diff and gearing can you choose from?

2)If it turns out that I supply my own diff, do I just order part number 16126 - 2015 Mustang IRS Center Section, Spindles, and Hubs? Does this include the drive shafts?

Thanks for the help.

JK

[wallace18]

FFR can supply the rear parts. They have a list of what parts you need and what they supply. It is all listed on the order sheet in the brochure as well on their website. On my MK4 427 customer build, my customer chose to have Mike Forte supply the necessary parts. Not sure of all the ratios offered. My guess would be what the 2015 Mustangs came with.

http://www.fortesparts.com

[edwardb]

Hi,

I am still in the planning stage before I order my Coupe and could not seem to find this info and have a few questions:

1)If you order the Complete kit and select part number 60380 - 2015 Mustang-based Independent Rear Suspension Kit, what type of diff and gearing can you choose from?

2)If it turns out that I supply my own diff, do I just order part number 16126 - 2015 Mustang IRS Center Section, Spindles, and Hubs? Does this include the drive shafts?

Thanks for the help.

JK

1. Any of the 2015+ Mustang IRS center sections will physically fit. There are a number of variations. Aluminum and iron case. Either works. The aluminum version is lighter of course. Initially Factory Five only recommended the iron case. Now they say either is OK. Ratios available are 3.15, 3.31, 3.55, and 3.73. All are limited slip. The 3.73 uses a Torsen setup using gears (much more expensive, now getting hard to find), the balance are traditional Ford Trac-Lok clutch based setups. The 3.15 version you may want to avoid if you have any idea of ever changing the ratio. It's the only one that can't undergo a ratio change. The 3.55 version, either aluminum or iron, is probably the most common. You will need to determine based on your engine, trans, and expected use which ratio would be best for your build.

2. The 60380 option provides the IRS configured chassis and every part required except the center section and axles/hubs. Includes all control arms, CV axles, related hardware, etc. The 12126 option is the center section and knuckles/hubs. Those are standard Ford parts and available through multiple channels. Even salvage if you want to do some hunting. The Factory Five price is OK for new. Some just want to have it part of their kit delivery and be done with it. Just be sure they are supplying the ratio that's appropriate for your build if you go that way.

[Jkviper]

Thanks Edward, really appreciate the detailed response.

[qwezxc12]

JK,

Some minor additional info. The 2015 IRS Ring and Pinion gears are referred to as "Super 8.8" - they are not the same as a regular Ford 8.8 gears, so be clear if you order any separate from the housing and dif. Neither Ford Racing or any aftermarket suppliers (that I could find anyway) are selling gear-sets in a numerically lower ratio than 3.55... which makes sense as pretty much all F-series truck and Mustang owners go to a taller gear than stock.

If you want to go the other way, like me, you'll need to find an OEM set. I bought a loaded Ford Racing IRS with the Torsen diff and 3.73s, but wanted to try a shorter gear based on my tire dia, trans, and rpm preferences.
I ordered a set of 3.31 gears from TascaParts: RING & PINION - FORD (FR3Z-4209-D)

[CraigS]

Did you verify that the 331 ring gear will fit the Torsen center section? I helped a friend swap gears in his IRS and wasn't real happy w/ the carrier bearing shims. I am used to the old 8.8 which had a super selection of shims. The shim kit he got had a minimal selection w/ a lot of thick shims and not a lot of variation.

[edwardb]

Some minor additional info. The 2015 IRS Ring and Pinion gears are referred to as "Super 8.8" - they are not the same as a regular Ford 8.8 gears, so be clear if you order any separate from the housing and dif.

Good point. This isn't a new article, but explains and shows the difference between the original 8.8 and the newer "Super" 8.8. http://www.fordnxt.com/tech-stories/...nion-for-s550/. Pretty interesting. Couple years ago I did a gear swap in a solid axle older style 8.8. Wasn't too bad other than I had to get a pro to press the pinion bearing on for me. I did some brief research on changing gears in the Super 8.8. A lot of similarities. But does require a case spreader to get everything out. At that point, I no longer considered it a DIY project. I too have the 3.73 center section for my Coupe build. I'm going to leave it as is for now

[qwezxc12]

Did you verify that the 331 ring gear will fit the Torsen center section? I helped a friend swap gears in his IRS and wasn't real happy w/ the carrier bearing shims. I am used to the old 8.8 which had a super selection of shims. The shim kit he got had a minimal selection w/ a lot of thick shims and not a lot of variation.

Craig,

I'm at that stage right now; I just dropped off the gear set last night to the shop doing the work. As edwardb said, I did not consider this to be a DYI job.
The 3.73, 3.55, and 3.31 are all supposed to fit the Torsen, it's the 3.15 that doesn't.

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The kit I got included the new gear set, M12 ring bolts, pinion nut and spacer, pinion head and tail bearings, and seal. I have the original shims from the diff when it was disassembled and the shop has some. The diff felt great from Ford; ~.008" backlash IIRC.

[Jim1855]

I have the aluminum case, 3.31, Torsen. The full build was supplied by Mike Forte. Call Mike.

And a cheap promotion. I have an iron case, 3.55 Traction-Loc in the box.

Jim

[KirkS]

I got a 3:31 iron case that came out of a 2015 from midwaymustangparts. They're out there, don't know how rare. I want a cruiser, not a drag racer.

Collecting parts for a future Gen III build, Kirk