Differential swap on a 6R80 car. Easier than expected.

[2015Etrac]

I've been researching a differential swap for my car for some time now. I wanted to swap the 3:15 out for a 3:55 diff. There was some mixed info on what was needed to adjust the automatic cars for the change in gears. I finally got around to swapping out the rear this weekend for a 3:55 from Ford Performance and it was easier than expected. Adjusting for the new 3:55 gears is as easy as changing 4 numbers with Forscan to "1063" which is the code for 3:55. Also, several places list that a new pinion flange is needed because the 3:55 diffs from Ford Performance come with a manual flange, however this was not the case, so I wasted $80-90 on a Ford Performance flange. The differential already came with the correct flange for my 2016 6R80 GT, so you might want to look into the beforehand to save yourself some time and money. I wouldn't have tried the job myself, luckily my brother-in-law is a mechanic and we used his shop but it should be a pretty straight forward job for you guys who have done suspension work or some mid level mechanic stuff. As for the rear, it's a noticeably "snappier" and I feel like the transmission doesn't hunt for gears as much as it did with the 3:15, which I remember some others who have done the swap noticed as well. It's also nice that Ford Performance paints their rears and pinion flanges, unlike the factory, so it should, hopefully, look good for awhile.

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[ZenkaiRacer]

Nice, this is my next mod. Where did you get the complete 3.55 diff from?

 

[glockholiday]

Was it worth the money, notice a big difference? Seems if I were to be dropping the $$$$ I'd go with 3.73 and a 400-500rpm change instead of only a 200-300 rpm difference.

 

[2015Etrac]

Nice, this is my next mod. Where did you get the complete 3.55 diff from?

American Muscle had the best price and free shipping, so I bought it there. However, the first one they sent me came in a torn box, soaked in diff fluid, with a broken flange. They wouldn't let me drop it off at their warehouse which is close to my home so I had to purchase a second one, have that shipped to me, and use that box to ship the broken one back. Of course the second one shipped upside down so the bottom with the staples was the top, and open when it arrived, which meant I couldn't use that box either. It might be worth shopping around. I wouldn't recommend AM. There's several places that sell them if you search for 3:55 Ford Performance loaded differential.

 

[2015Etrac]

Was it worth the money, seems if I were to be dropping the $$$$ I'd go with 3.73 and a 400-500rpm change instead of only a 200-300 rpm difference.

If it wasn't a daily, I probably would have went with the 3:73, but most people told me that 3:55 was the better option, at the time. I think the 3:55 on the 6R80 is close to 3:73 on the 15-17' MT82 cars. Also, I wanted a new complete rear and I'm not sure they sell them for the 3:73? I've only seen new 3:55 differentials available from Ford. I could have gone used but I'm leery of putting used drivetrain stuff on my car. But you're right, there's not a ton of difference between the 3:55 and 3:73.

After driving the 3:55 for a day or so now, I feel like I could have went with the 3:73, but I'm not really sure if the difference between the two would be big enough to justify spending the money for the new loaded 3:55 diff and then factoring in 3:73 gears and an install.

 

[Rick#7]

The only loaded 3.73 diff's you'll find have the torsen differential in them, since the only factory option for 3.73 gears was with the performance package which included the torsen. IIRC, FP had a loaded 3.73 torsen diff in their catalog along side the 3.55 traction-lok loaded diff, but I don't know if they still do.

If someone wanted 3.73 gears with a traction-lok, (arguably a better diff for the street than the torsen) the only way to get it would be to buy the appropriate traction-lok and 3.73 gears separately, then pay someone to properly set up everything in a carrier. With the labor cost at a decent race shop being what it is, the 3.55 loaded diff's are a much more cost effective route, so you really have to want 3.73's for some reason to pay the extra bucks for the small advantage over 3.55 gears.

Buying a new loaded 3.55 diff, then paying to have 3.73 gears installed would really be a waste of money. Used 3.15 diff's are everywhere, and a new FP traction-lok is only $210 retail, so you could pick those up for maybe half the cost of a loaded 3.55 diff, or at least a couple hundred cheaper.

Ideally, you might find a used, low mileage 3.31 loaded diff for about the same money as a 3.15, then you wouldn't even have to buy a new traction-lok, you could just get the 3.73's installed.

 

[2015Etrac]

The only loaded 3.73 diff's you'll find have the torsen differential in them, since the only factory option for 3.73 gears was with the performance package which included the torsen. IIRC, FP had a loaded 3.73 torsen diff in their catalog along side the 3.55 traction-lok loaded diff, but I don't know if they still do.

If someone wanted 3.73 gears with a traction-lok, (arguably a better diff for the street than the torsen) the only way to get it would be to buy the appropriate traction-lok and 3.73 gears separately, then pay someone to properly set up everything in a carrier. With the labor cost at a decent race shop being what it is, the 3.55 loaded diff's are a much more cost effective route, so you really have to want 3.73's for some reason to pay the extra bucks for the small advantage over 3.55 gears.

Buying a new loaded 3.55 diff, then paying to have 3.73 gears installed would really be a waste of money. Used 3.15 diff's are everywhere, and a new FP traction-lok is only $210 retail, so you could pick those up for maybe half the cost of a loaded 3.55 diff, or at least a couple hundred cheaper.

Ideally, you might find a used, low mileage 3.31 loaded diff for about the same money as a 3.15, then you wouldn't even have to buy a new traction-lok, you could just get the 3.73's installed.

Yep. I'm not sure If the 3:73 is still available. I haven't been able to find one if they are, but I do remember back when they did sell them. Like you said it was more like $1800 for the 3:73 because it only came with Torsen. The 3:55 definitely is a great deal considering you get a whole differential assembled by Ford and with a warranty.

Can you use the housing of a 3:15 for a 3:55 swap? I thought the 3:15 housing was smaller, or something like that? Also, the 3:15 I had on my GT was aluminum, which is probably slightly better for shedding heat and saves 20lbs, but it's weaker and less desirable.

Looking at the used prices on Ebay I'm pretty happy with the price I paid, which was like $850 shipped. I just wish they were more clear on which pinion it had so I could have saved some more money. It seems like the only option right now for any 3:73 is to buy a used one or buy a new loaded 3:55 and waste a bunch of money swapping one in, if you want new.

 

[Rick#7]

The housing (sometimes called a carrier) is the same regardless of which gears are inside, the only difference is aluminum for AT cars and iron for MT cars. The traction-lok differential which goes inside the housing is gear specific, one part number for 3.15 gears, and a different part number for 3.31 and numerically higher gears.

So, if you didn't mind the down time on your car while the shop was swapping the gears, you could just buy the gear set and new traction-lok, and let a competent shop remove the loaded diff from your car, install all the new stuff into your housing, then reinstall it back into your car for you. Still more expensive than buying a 3.55 loaded diff and swapping it in yourself, but an option non the less, and a lot less chance of scraping up your knuckles LOL

 

[2015Etrac]

The housing (sometimes called a carrier) is the same regardless of which gears are inside, the only difference is aluminum for AT cars and iron for MT cars. The traction-lok differential which goes inside the housing is gear specific, one part number for 3.15 gears, and a different part number for 3.31 and numerically higher gears.

So, if you didn't mind the down time on your car while the shop was swapping the gears, you could just buy the gear set and new traction-lok, and let a competent shop remove the loaded diff from your car, install all the new stuff into your housing, then reinstall it back into your car for you. Still more expensive than buying a 3.55 loaded diff and swapping it in yourself, but an option non the less, and a lot less chance of scraping up your knuckles LOL

haha yep. From the start I knew I wanted a new loaded diff. from Ford. I know some shops do great work with gear installs, but there's also a lot of shops out there that do sub par work when it comes to installs, and I didn't want to chance it. The last thing I wanted was gear whine on an otherwise perfect daily driver.

I just checked Ford Performance site, it looks like the 3:55 is the only complete option they offer now. Then again I wouldn't pay an extra $900 or so for a Torsen 3:73, even if they still had it.

I feel like the 6R80 works better with the 3:55 for some reason. The shifts feel smoother and it doesn't seem to be hunting for gears like it occasionally did with the 3:15. Maybe it's just in my head or it could be smoother due to it being new, but my car only has 18k miles on it, so the 3:15 wasn't worn out or anything.

 

[Emilbadal]

@2015Etrac I’m planning on doing the same thing. My 3.15 diff is going out and I’m thinking of buying the same diff as yours. I’m just wondering how much heavier is the iron case than the aluminum case? Also what did you else had to buy to do the installation?

 

[Southview]

How is first gear? It is basically useless with my 3.15 auto. Shifts way to quick into 2nd. I was also thinking of doing the 3.55 swap.

 

[Jackson1320]

The only loaded 3.73 diff's you'll find have the torsen differential in them, since the only factory option for 3.73 gears was with the performance package which included the torsen. IIRC, FP had a loaded 3.73 torsen diff in their catalog along side the 3.55 traction-lok loaded diff, but I don't know if they still do.

If someone wanted 3.73 gears with a traction-lok, (arguably a better diff for the street than the torsen) the only way to get it would be to buy the appropriate traction-lok and 3.73 gears separately, then pay someone to properly set up everything in a carrier. With the labor cost at a decent race shop being what it is, the 3.55 loaded diff's are a much more cost effective route, so you really have to want 3.73's for some reason to pay the extra bucks for the small advantage over 3.55 gears.

Buying a new loaded 3.55 diff, then paying to have 3.73 gears installed would really be a waste of money. Used 3.15 diff's are everywhere, and a new FP traction-lok is only $210 retail, so you could pick those up for maybe half the cost of a loaded 3.55 diff, or at least a couple hundred cheaper.

Ideally, you might find a used, low mileage 3.31 loaded diff for about the same money as a 3.15, then you wouldn't even have to buy a new traction-lok, you could just get the 3.73's installed.

Everything I’ve read says that 373’s and 355 will not fit inside of the 315 carrier. Also do you know how much it cost to have a set of gears installed

 

[Rick#7]

Labor for a gear install can run $500+ at a competent shop, give or take. That doesn't include the cost of the gears or the installation kit (gaskets, washers, crush sleeves, etc).

As far as the gear fitting in the housing (or case, or carrier, whatever you want to call it), I haven't read anything to convince me there is any difference in any of the housings based on gear ratio, just aluminum for AT cars and iron for MT cars.

I believe the issue comes from inconsistent use of terminology regarding the rear end parts. Some sites refer to the housing as a case, or carrier, or even use the term "differential" to describe the entire assembly. Some site may also call the differential inside the housing a carrier, to add to the confusion. I'm not going to say one is right or the other is wrong, however for the purpose of this discussion, the differential is the limited slip device that the ring gear bolts to and sends power to the axles. The housing (also called a carrier) is what the complete differential and pinion gear gets assembled in.

As I mentioned before, the differential has to be changed when going from 3.15 gears to any numerically higher gears, however any super 8.8 differential can be mounted in any super 8.8 housing.

 

[JohnVallo]

The inside clearance of the aluminum housing is much less to add strength, therefore ring gear diameter is limited, which limits to only a 3.15:1 ratio.

Wrong, do some research please

 

[Rick#7]

The inside clearance of the aluminum housing is much less to add strength, therefore ring gear diameter is limited, which limits to only a 3.15:1 ratio.

The ring gear diameter is 8.8", regardless of the ratio, hence the name "Super 8.8". The ratio is determined by the number of teeth on the ring gear and pinion gear, but the ring gear diameter is always 8.8".

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