Rear axle differential - manual vs auto?

[Lidodrip]

Hello All,

I am looking at getting a used rear axle differential for my upcoming roadster build and I was wondering if it makes a difference if it comes from a manual or automatic donor? I am planning on using a coyote/tko600/IRS if that makes a difference. Thanks for any help, much appreciated.

Thanks,
James

[edwardb]

No, it doesn't matter. In the past, automatic Mustangs used an aluminum case IRS diff. Manual shift the iron case version. But either will work fine. The iron case is 10-15 lbs heavier, but no big deal. The pinion flange adapter might be different between the two. But Factory Five will provide the right one.

Your focus needs to be on the proper ratio. FWIW, for that combination (Coyote, TKO, IRS) my #8674 build was 3.55 and in my experience was perfect.

[Lidodrip]

Thanks for the information edwardb. I have thoroughly enjoyed your many posts and build threads. My plan was to use a 3.55 - glad to hear you liked it. This is all new to me and I am having a lot of fun learning about everything. This forum has been fantastic. My kit arrives 1/9/21 - can't wait!

[progmgr1]

IIRC, the 3.55 gear ratio was not available in the aluminum housing. Also (again relying on fading memory) the two housings use different gear sets that are not interchangeable. It's my understanding that the cast iron diff / gear set is stronger and the aluminum diff should only be used for naturally aspirated applications. Keith HR #894

[edwardb]

IIRC, the 3.55 gear ratio was not available in the aluminum housing. Also (again relying on fading memory) the two housings use different gear sets that are not interchangeable. It's my understanding that the cast iron diff / gear set is stronger and the aluminum diff should only be used for naturally aspirated applications. Keith HR #894

Not sure about most of that. When I did my first IRS in 2015 (20th Anniversary Roadster when Factory Five introduced the new design) the aluminum and IRS case discussion was automatic vs. manual shift on the Mustang. As I noted in post #2. My only option at that time was to find something in salvage, and I was fortunate to find a zero miles takeout. Turned out to be a 3.55/iron case diff. But 3.55 aluminum case versions were available at that time too. But at that time Factory Five only recommended the iron case so that's what I used. Their concern was not about anything internal. But the possibility of breaking the mounting ears on the aluminum case.

They later changed and said either aluminum or iron case was OK. There have been multiple model years since then, so can't confirm exactly what combinations Ford is selling now. But what I can say is the only ratio that's incompatible is the 3.15 ratio. Those internals aren't compatible with any of the others and those diffs can't be changed. Since that ratio is only used in V-6 and Ecoboost automotic Mustangs, maybe it is lighter duty. But I don't know that for sure. I'm 99% certain any of the other ratios can be changed to any of the others. None of the replacement gear sets I've seen show any limitation on which case can be used.

But most of this is maybe a moot point for the OP. I'll stick my original advice -- go with 3.55 and either aluminum or iron case. For anyone that doesn't want to mess with salvage, https://performanceparts.ford.com/part/M-4001-88355B is what Factory Five supplies if you select their option. Or you can find them elsewhere for just over $800. Not bad for new in box IMO. Iron case 3.55.

[GoDadGo]

Question: Is the 3:15 IRS a Posi, Ratchet, Limited Slip, Sure Track, or is it an Open Style rear end?

If it isn't an open unit, it may be a good option depending on what transmission you plan to run.
A TKO Road Race has a shallow .82 overdrive with tighter ratios than the standard TKO unit.
In addition, what is your expected primary use for the car because it does make a difference?

> Cruising
> Auto-Crossing
> Road Racing
> Drag Racing
> Cross Country Touring

Personally, I think 3:15 gears would work well with the right 5 speed, but you need to determine what you want to use the car for first.

Good Luck From The Dark Dart Side!

Steve
https://www.factoryfive.com/whats-new/customer-steve-l-graduates-his-mk4-roadster/

[edwardb]

Question: Is the 3:15 IRS a Posi, Ratchet, Limited Slip, Sure Track, or is it an Open Style rear end?

If it isn't an open unit, it may be a good option depending on what transmission you plan to run.
A TKO Road Race has a shallow .82 overdrive with tighter ratios than the standard TKO unit.
In addition, what is your expected primary use for the car because it does make a difference?

> Cruising
> Auto-Crossing
> Road Racing
> Drag Racing
> Cross Country Touring

Personally, I think 3:15 gears would work well with the right 5 speed, but you need to determine what you want to use the car for first.

Good Luck From The Dark Dart Side!

Steve
https://www.factoryfive.com/whats-new/customer-steve-l-graduates-his-mk4-roadster/

All 2015+ Mustang IRS diffs are posi. All are the typical Ford Trac-Lok disk style units. Similar to what's in the solid axle 8.8. Except the 3.73 which uses a Torsen gear unit. Side note -- my previous builds were all Trac-Lok style. My Coupe is the 3.73 Torsen. Other than the fact it put a hole in my wallet, I like it. Not sure how much it's warranted for my driving. But no regrets.

[GoDadGo]

Here are the numbers if you plan to run 3:15's with the TKO-600 that sports a .82 O/D.

2,350 RPM
120.03 MPH 1st
130.41 MPH 2nd
144.91 MPH 3rd
157.48 MPH 4th
170.10 MPH 5th

6,000 RPM
151.14 MPH 1st
177.65 MPH 2nd
114.66 MPH 3rd
146.98 MPH 4th
178.98 MPH 5th

Like Paul (Edwardb) I'm running 3.73's but I'm running a 6-Speed with a .75 5th and a .50 6th so I'm only turning 1,700 at 70 MPH.
Hope this helps and let me know if you want me to run other ratios.
We're all here to help each other.

[Jim1855]

Forte built an aluminum 3.31 Torsen unit for me.

Yup the steel is stronger but probably only matters if you're drag racing with lots of HP/TQE and making hard launches on slicks.

An iron center with 3.73s & the Torsen weighs 105#. The aluminum with 3.31s & Torsen weighs 82#.

In a previous life & car I switched from 3.73s to 3.27s, wasn't ever sorry but had 440hp 435tqe at the wheels. Driving preferences, 1st gear ratio, available hp/tqe, and RPM rev limit will have a lot to do with YOUR selection.

Jim

[Lidodrip]

Question: Is the 3:15 IRS a Posi, Ratchet, Limited Slip, Sure Track, or is it an Open Style rear end?

If it isn't an open unit, it may be a good option depending on what transmission you plan to run.
A TKO Road Race has a shallow .82 overdrive with tighter ratios than the standard TKO unit.
In addition, what is your expected primary use for the car because it does make a difference?

> Cruising
> Auto-Crossing
> Road Racing
> Drag Racing
> Cross Country Touring

Personally, I think 3:15 gears would work well with the right 5 speed, but you need to determine what you want to use the car for first.

Good Luck From The Dark Dart Side!

Steve
https://www.factoryfive.com/whats-new/customer-steve-l-graduates-his-mk4-roadster/


Thanks Steve. The intended purpose is primarily cruising. It seems from what I found on this forum, most with a street-build Coyote and the TKO 600 find the 3.55 a good match. While I think it is unlikely that I will ever change ratios, the fact that you don't have that option with the 3.15 makes that choice less desirable. I see that Tremec came out with a new transmission, TKX, that looks interesting. There are a couple of gearing options with the TKX - looking forward to hearing more about this transmission.

James

[Lidodrip]

Here are the numbers if you plan to run 3:15's with the TKO-600 that sports a .82 O/D.

2,350 RPM
120.03 MPH 1st
130.41 MPH 2nd
144.91 MPH 3rd
157.48 MPH 4th
170.10 MPH 5th

6,000 RPM
151.14 MPH 1st
177.65 MPH 2nd
114.66 MPH 3rd
146.98 MPH 4th
178.98 MPH 5th

Like Paul (Edwardb) I'm running 3.73's but I'm running a 6-Speed with a .75 5th and a .50 6th so I'm only turning 1,700 at 70 MPH.
Hope this helps and let me know if you want me to run other ratios.
We're all here to help each other.

Thanks, very useful information.

[Lidodrip]

Forte built an aluminum 3.31 Torsen unit for me.

Yup the steel is stronger but probably only matters if you're drag racing with lots of HP/TQE and making hard launches on slicks.

An iron center with 3.73s & the Torsen weighs 105#. The aluminum with 3.31s & Torsen weighs 82#.

In a previous life & car I switched from 3.73s to 3.27s, wasn't ever sorry but had 440hp 435tqe at the wheels. Driving preferences, 1st gear ratio, available hp/tqe, and RPM rev limit will have a lot to do with YOUR selection.

Jim

Hmmm, an aluminum 3.31 sounds interesting. Thanks for the suggestion Jim.