Mustang IRS details Needed

[BrewCityCobra]

Thank you again for all of the help on my earlier thread - to that point I'm convinced that IRS is the way to go.

So that leads to another question - what should I be looking for on the used market?

A few specs for context:

- Motor is likely going to be a 427W with TKO 600 with a .64 5th gear (Forte).
- Autocross and track days are in this car's future so I suspect some type of locking rear diff is a must.

With that said, is there any places I should be looking aside from Ebay? I've been finding a good number of 3:17 rears that seem to fall in my budget but am not sure if they are actually locking or open. Would that fit my needs or do I need to accept that a more expensive style is needed for my setup?

Any help is greatly appreciated.

-Brew

[Hoooper]

All 2015+ mustang diffs are limited slip, so any 3.17 will have the clutch type LSD. Only 3.73s have the torsen as far as I know.

You can also look at LKQonline. Since Mustangs are everywhere there is a pretty good selection of low mileage diffs to choose from for relatively cheap

[edwardb]

As stated, all 2015+ Mustang IRS are limited slip. The 3.73 ratio is Torsen (gears...) and the rest are the standard Ford Traction-Lok clutch pack setup. Available ratios are 3.15 (not 3.17), 3.31, 3.55, and 3.73. The main housing is typically aluminum. Some 3.55's are iron case as are all 3.73's since it's considered the highest performance option also with Torsen. All use a cast aluminum rear cover.

Originally, when the re-designed suspension was introduced, Factory Five only recommended the iron case. They later changed that to say either is OK. The concern was breaking the mounting ears off the aluminum case. I've personally not heard of any instances of that happening with any version. They are very robust. Especially for these cars which are significantly lighter than a Mustang.

The reason the 3.15 is widely available and cheapest is that it's standard equipment on 4, 6, and 8 cylinder Mustangs with automatic. Plus it's also the least desirable for the aftermarket. I don't know the details, but the ratio is unable to be changed. For every other ratio, there are gear sets available to change the ratio. So a 3.31 could become a 3.55. A 3.73 Torsen could become a 3.55, etc. With the 3.15, that's it. If you're sure 3.15 is OK (some use it...) and you don't ever plan to change it, then go for it. If you order the optional IRS diff and knuckles from Factory Five, they supply the 3.55 ratio. Which is probably the most common. For your 427W, 3.31 would probably be a strong consideration as well. Do the math and look at the whole package: Engine + trans + diff ratio + tire size + intended use. I like to evaluate various combinations with this tool from Tremec: https://www.tremec.com/calculadora.php.

[BradCraig]

I did a 3.55 and it seems to be a good middle ground with the TKO. Not sure I would want to do any less IMHO. I went with a full IRS pallet from Parts Farm in GA, $600. Hard to beat the price and you get the knuckles and brakes.

[CraigS]

315 will be fine w/ a 427. I had one w/ my 408. I kept thinking I would like to change to a 331 but never actually did. The included Traclok, or whatever Ford calls it, works fine after 3 seasons of autocross on Hoosier A7s. Be sure to get a salvage unit w/ the brakes included. They work very well and Hawk has a lot of their compounds available for them. One very nice advantage compared to my previous solid axle was that I no longer get pad knockback so brake pedal stays exactly the same though out an autocross run. This IRS was designed for performance oriented Mustangs so it is quite heavy duty in a 2500# FFR.

[toolerwayne]

With the .64 ratio overdrive 5th gear is a bigger jump up from 4th than the jump between all the other gears. in other words, as you shift up the ratios are evenly spaced 1-4. then 5th is like you went into 6th. That is what i have. Its the one thing i would do different. With the .82 ratio overdrive the gears are evenly spaced. Instead you can go up on the rear axle ratio to get the same top speed and have a 1st gear that is better too. I have 3.55 rear and i shift to second before i am through an intersection. If I had the 3.27 rear and .82 overdrive my 1st would take me faster and 5th would sound and feel correct. Power is not an issue. For the new mustang IRS I would choose 3.31 with .82 overdrive. Just my opinion

[Hacksaw84]

Thank you again for all of the help on my earlier thread - to that point I'm convinced that IRS is the way to go.

So that leads to another question - what should I be looking for on the used market?

A few specs for context:

- Motor is likely going to be a 427W with TKO 600 with a .64 5th gear (Forte).
- Autocross and track days are in this car's future so I suspect some type of locking rear diff is a must.

With that said, is there any places I should be looking aside from Ebay? I've been finding a good number of 3:17 rears that seem to fall in my budget but am not sure if they are actually locking or open. Would that fit my needs or do I need to accept that a more expensive style is needed for my setup?

Any help is greatly appreciated.

-Brew

Join Facebook groups for mustang parts. This has been my #1 place to find good deals. Here are a few options but there are many more:

"S550 Mustang Parts, Cars, Classifieds"
"Coyote Parts Swap"
"Anything S550 2015+ Mustang"
"2015+ Mustang S550 Parts and Cars Classifieds"

I did a quick search and here was an example of one. Its just the diff, you need a few more parts to make the IRS complete. Also you need to specifically look for one out of a manual car. I believe they have a different mounting flange.
132763

[chmhasy]

https://www.car-part.com/

for parts use
Axle Assy Rear, or Carrier, or Differential Assembly