Switching from 3.15 to 3.55 Gears

RaceRed5.0

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What do I need because I know the 3.15 differential does not work with the 3.55 gears is that all I need to switch out or is there more? Thanks!





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timd38

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Just buy the 3:55 diff from Ford already assembled for $800.
 
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RaceRed5.0

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Yeah I've seen that on AM but I was wonder if I don't have to change out the housing and just replace the gears and differential.
 
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RaceRed5.0

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VooDooDaddy

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You'll also need an install kit which includes a crush sleeve for the pinion and a shim kit..., AND you'll have to pay someone to install all the parts.

After it's all said and done, you'll be at or near the price of the complete differential that you can simply buy (complete) from mother FORD.

You can install the complete diff yourself.
 

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You'll also need an install kit which includes a crush sleeve for the pinion and a shim kit..., AND you'll have to pay someone to install all the parts.

After it's all said and done, you'll be at or near the price of the complete differential that you can simply buy (complete) from mother FORD.

You can install the complete diff yourself.
^ This. Plus whenever I've had gears installed in other cars I've had some amount of gear whine. There is even a market to sell your used take off complete differential so that puts it way ahead of getting them installed.
 

TripleB

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It's the housing that's different.
3.31, 3.55, 3.73, 3.91, 4.09 ratio gears will not fit the 3.15 carrier (Housing)
The actual differential (inside the housing), are all compatible for the Super 8.8 Mustang IRS rear end (34 spline)
Wait, so I can’t change my 3.73 gears to 3.31? I need a whole new pumpkin?
 

08StangGT_CS

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It's the housing that's different.
3.31, 3.55, 3.73, 3.91, 4.09 ratio gears will not fit the 3.15 carrier (Housing)
The actual differential (inside the housing), are all compatible for the Super 8.8 Mustang IRS rear end (34 spline)
Wait, so I can’t change my 3.73 gears to 3.31? I need a whole new pumpkin?
The way I read it is you can't change 3.73s to 3.15s but you should be good for all the rest in the list.
 

timd38

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If you buy the 3:55 from Ford Performance and have an auto trans, you will need to change the flange for the driveshaft.
 

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Y'all making this WAY TO HARD. It is much easier to drop the center chunk and replace it then buying gears and setting up in housing in car...MUCH. If you do it right you won't have much $$$ investment either. Ebay it...I picked up a complete 3.31 center chunk for $400 and sold my 3.73 center chunk for $600. I've seen people do the opposite as well. If you are going for the straight line with launching get the steel case housing, if you are a twisty guy get the Aluminum housing.
 
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RaceRed5.0

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So I am really looking at just buying the whole housing with the auto flange kit? How much does it normally cost to have this installed because I won't be doing it myself?
 
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RaceRed5.0

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Well I went ahead and changed the diff and at first glance it doesn't seem to be worth it, I am not regretting it though yes I do notice a difference. Can't really say there is difference between the 0 to 60 times of course traction is still an issue maybe more so now. I do notice a difference from a 20 to 30 roll than I did from my 3.15's having the power there when I step on it.
 

EcoSwag1990

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Well I went ahead and changed the diff and at first glance it doesn't seem to be worth it, I am not regretting it though yes I do notice a difference. Can't really say there is difference between the 0 to 60 times of course traction is still an issue maybe more so now. I do notice a difference from a 20 to 30 roll than I did from my 3.15's having the power there when I step on it.
I was very close to making the switch about a year ago and never did anything. What parts did you get? Also, how much did your installer charge you?

I thought there was a need to flash the BCM to update for the gearing, did you use the Whipple flare tool?

Thanks in advance!
 
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RaceRed5.0

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I was very close to making the switch about a year ago and never did anything. What parts did you get? Also, how much did your installer charge you?

I thought there was a need to flash the BCM to update for the gearing, did you use the Whipple flare tool?

Thanks in advance!
I went to a place called Hillbank Motorsports by me here in Cali they charged me $300 for work. I ordered the entire Differential and they had to replace the manual flange that comes with it and put an auto one in, I could of bought the differential from AM or CJ Pony but it wouldn't be much of a difference because I would still have had to bought the flange they charged me about $940 for the diff so in all just over $1,300.

They did have to retune my car but I have the Ford Performance Pack 2 so he just retuned it with the tuner that came with it.
 
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Rick#7

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It's the housing, (i.e. carrier/pumpkin) that's different.
3.31, 3.55, 3.73, 3.91, 4.09 ratio gears will not fit the 3.15 carrier (housing)
The 3.15 carrier is different from all the rest.
The actual differentials (-inside the pumpkin, the part that the ring gear bolts to), whether it be (Torsen, Traction lock, limited-slip, Open) are all compatible for ALL the 2015-up Super 8.8 Mustang IRS rear ends (34 spline).
This isn't completely accurate. If you look at any ad for Ford performance gears or differentials for the 2015-up IRS, they all mention the factory 3.15 geared differential is not compatible with the 3.31-numerically higher gears, and you must change the differential when replacing the 3.15 gears, however I haven't seen any of them say anything about the housing.

I haven't changed rear gears in my car so I won't comment on whether the 3.15 housing itself is compatible or not with 3.73's, but the actual differentials (-inside the pumpkin, the part that the ring gear bolts to), absolutely is NOT compatible for all gears, it has to be changed when changing the 3.15 gearset.
 

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