Looking to upgrade gears from 3.15 to 3.55. Need advice.

[CaptFlan]

So I have a 2015 Mustang V6 automatic with 3.15 gears and want to swap to 3.55 gears. I have been trying to research the best way to approach this, but I am finding a lot conflicting info. On American Muscle's site, they say that I can swap out the gears, I just need to also swap the differential, and of course get an install kit. I can still use the aluminum housing of my existing differential. CJ Ponys, however, says differently. They say you have to switch to the cast iron housing to changes gears, ie you would have to switch out the entire differential. This doesn't make a lot of sense to me. I have gotten on eBay and looked at differentials that have 3.55 gears in them, and it is very clear the housing is aluminum. (Cause they ain't got a spot of rust on them!) I also seen some 3.55 differentials with the cast iron housing as well. I am wondering if the cast iron housing if for manual transmissions? Has anyone does this upgrade using their existing 3.15 differential housing? Thanks!

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[K4fxd]

The aluminum and cast iron housings are the same.

The 3.15 uses a different "carrier" or differential unit. You will need to buy either a trac-lok or a torsen differential unit in order to install 3.55 gears in your aluminum housing.

 

[TeeLew]

The easy way to do it is to find a rear diff housing that already has the diff and gear you want. It's not tough to find a 3.55 housing loaded with a torsen. Just think about how many end up crashed...

 

[CaptFlan]

The aluminum and cast iron housings are the same.

The 3.15 uses a different "carrier" or differential unit. You will need to buy either a trac-lok or a torsen differential unit in order to install 3.55 gears in your aluminum housing.

Thanks for the input! It didn't make sense that the housing would be different.

 

[CaptFlan]

The easy way to do it is to find a rear diff housing that already has the diff and gear you want. It's not tough to find a 3.55 housing loaded with a torsen. Just think about how many end up crashed...

Yeah, I was toying with that idea but a lot of the current offerings on eBay seem a bit sketchy. No seem willing to give me a straight answer on how many miles the differentials have or the VIN of car they cam out of. And apparently the flange that connects to your drive shaft is different for automatics vs manuals, and there doesn't seem to be an easy to tell the difference, at least not that I have found.

 

[TeeLew]

It doesn't matter which you get. The flange is an easy fix. Do what you want, but this *is* the "Easy Button." Add far as mileage goes, why does it matter? This is not a fragile component on the car. Put fresh oil in it and there's no way to tell whether it has 10k miles or 100k.

https://thepartsfarm.com/search?type=product&q=Mustang+differential+

 

[Cobra Jet]

Going from memory, I think the 2015-2017 GT PP MT82 pumpkins were supposedly iron w/optional Torsen.

The 2015-2017 GT w/PP and w/ 6R80 was iron with standard trac-lock. The 2015-2017 iron case was bare, so if looking at it, the case will be “rust” color.

The 2015-2017 GT W/O PP but with the 6R80 were all aluminum housings.

2015-2017 EB 6R80 with or without PP andthose with MT82 were strictly aluminum case w/ standard traction lock.

There were many options for the 2015-2017 rear pumpkin depending on the platform chosen. So I am not sure if I’m covering all available case/gear types....

Reference these threads to help you out some:

Older thread discussing the 3.15 rear diff:
https://www.mustang6g.com/forums/threads/3-73-to-3-31-or-3-15-swap.119318/#post-2498556

Lots of rear diff/gear ratio tech here:
https://www.mustang6g.com/forums/threads/swapping-rear-end-gears-on-a-pp.11162/

----

I just posted this the other day for another M6G member too, which may or may not help you:

The 2018-2019+ GT PP or EB PP’s all come with iron pumpkins and Torsen.

The 2018-2019+ iron case will be coated black with a cast aluminum rear cover. Also to note, the 2018-2019+ GT/EB PP rear covers will have the included threaded port for the rear axle temp sensor.

A) The 2018-19+ GT w/ MT82 came w/ Torsen and 3.73’s.

B) The 2018/19 GT or EB w/10R80 came w/ Torsen and 3.55’s.

---
TIP:
- If the rear diff has Trac-loc guts, you won't be able to see through it with axles removed. There will be a "bar" in the hole which is for the spider gears.

- If the rear diff has Torsen guts, you'll be able to see through it with axles removed.

---
and.... everything you ever wanted to know about a Torsen differential:
https://www.mustang6g.com/forums/th...d-slip-differential-work.110850/#post-3451290

 

[TeeLew]

JohnVallo really knows the driveline stuff. Aluminum housings only came in cars with automatics, but, unless you're drag racing or doing a lot of clutch drops, the aluminum housing is plenty strong and it's lighter.

The Traction-Lok is what it is...not a Torsen. I'm going to play with it to make it better, but it will always be a relatively simple differential. It will just be better than the factory provided.

 

[CaptFlan]

So following up on this, I did go ahead and get a complete housing. Interestingly the housing is cast iron and it came out of a 2017 EB auto. I was really excepting it to be an aluminum housing. The flange does match to what I currently have, 113mm diameter with a 95mm bolt pattern. But it does have one problem, they is a chunk out of the flange. I am not sure how big of a deal that is. I am worried it could unbalance the flange and cause issues. Anyone have any thoughts on this? Am I over thinking this? I know I can get a replacing flange, but that is more money and time. Thanks.

 

[TeeLew]

File the fracture smooth so there are no stress concentrations or burrs and then send it.

 

[TeeLew]

But make sure you don't do it, again!

 

[CaptFlan]

But make sure you don't do it, again!

I didn't do it! I swear! lmao!

 

[mc68386]

Where'd they find that? Next to the Titanic?
While it's out how about some sandblasting and a coat of paint?

So following up on this, I did go ahead and get a complete housing.

 

[K4fxd]

While it's out how about some sandblasting and a coat of paint?

I think a complete tear down and inspection before it gets installed is in the cards.

 

[CaptFlan]

Where'd they find that? Next to the Titanic?
While it's out how about some sandblasting and a coat of paint?

So funny thing, the eBay seller I bought it from, gave me the wrong carrier. That carrier is off a 2015 GT that was a manual, hence the looks like a giant troll turd "cast iron body". I am taking back Monday to exchange for the correct one which has an all aluminum body.

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