3.73->3.55 difficulty (Torsen 2015 MT-82)

[NightmareMoon]

I'm considering changing gear ratios to try to get more top speed in 2nd (Autox car).

How bad/expensive is it to swap the 3.73 gears in my Torsen to a 3.55 or a 3.31 (keeping the Torsen, obviously)? The other routes are maybe changing to a Tremec or even a trade with a 2018+ MT-60 owner, IDK.

Time/labor costs?

Is there a Ecoboost torsen 3.55 swap which is equal strength for hard/track use?

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[MRGTX]

HARD.
Well, not necessarily "hard" but there is a lot of labor involved and to do it yourself, you will need a pretty good garage setup with plenty of space, preferably a lift. You'll need to pull the driveshaft, unbolt and drop the complete rear suspension cradle/assembly...then when installing the new ring/pinion, you will need to properly set the gear mesh (can be tricky if you've never done it before)....and you may need a re-alignment when you put it back together. IMO, it's a big pain in the rear (pun intended).

If this is your daily driver, you will almost certainly be without a car for a while as I don't think most folks can do this in one day. If you're having this done by a shop, they're probably going to tell you to just bring the center section as they won't want to deal with the rest.

Am I saying that it's not worth it? Not at all. I've chronicled my adventures with the 4.09:1 in my 2020 which was a much bigger pain in the ass than your job will be and I don't regret it.

As for the Ecoboost swap, are you thinking about saving weight with the aluminum housing? Please verify this first but I believe it is strong enough for stock-ish power levels. If you're running dedicated autocross tires and really hammering the snot out of it, I'm not certain if it's worth the risk, especially since the housing is unsprung weight, exactly where you would want it. Lighter is always better but this chunk of weight isn't as problematic as it would be farther from the drive wheels or farther from the CG.

Comparing this to a Tremec swap? Sheesh...I'd love to hear someone's opinion on that but I can't imagine that it would be easier and it certainly wouldn't be cheaper though it would offer more benefits.

If you decide to go the ring/pinion swap route and you are looking to save a few bucks on the parts, drop me a PM. My 3.55s are just sitting in a box. They have ~2,900 miles on them, never thrashed. Something like $75+shipping? Trying to keep it around half the cost of new parts...let me know if you want them.

 

[TeeLew]

I'm considering the same, but 3.55-->3.31. It nets 5 mph of top end speed and will likely help reduce wheelspin. It does seem like a lot of work for a limited gain. I think if I'm going to do it, I would just get a used AL housing and swap the diff.

FWIW, just because it's from an Eco doesn't mean it's an AL rear housing. Manual cars are iron housings just like GT's. I'm guessing they're iron for dipshits doing clutch drops. I'm not going to be doing that and the weight reduction would be a good secondary reason for the change.

 

[Jeff's FRC]

I had my 3.55 swapped to 3.31 last winter. All in all was around $1k for parts and labor. Used my Whipple flare tool to reprogram the bcm for the new gears.

 

[Jordan @ Lethal]

"IMO, it's a big pain in the rear"

 

[WildHorse]

HARD.
Well, not necessarily "hard" but there is a lot of labor involved and to do it yourself, you will need a pretty good garage setup with plenty of space, preferably a lift. You'll need to pull the driveshaft, unbolt and drop the complete rear suspension cradle/assembly...then when installing the new ring/pinion, you will need to properly set the gear mesh (can be tricky if you've never done it before)....and you may need a re-alignment when you put it back together. IMO, it's a big pain in the rear (pun intended).

Wrong. You don't need to drop the cradle , nor do you need a lift . I did it in a cramped garage on the floor . Driveshaft-brakes-upper/end link-lower control arm- knuckles- half shaft- axlebacks-diff . You need to remove all those parts even if you drop the cradle . Also with the cradle you need to unbolt the shocks and lots more . Took 30 mins a side, then another 20 mins to drop the axlebacks/diff . Setting up the gears in a super 8.8 is no different than setting other gears.

https://www.mustang6g.com/forums/threads/going-from-3-31-to-3-73.147455/#post-3069697

 

[NightmareMoon]

HARD.
Well, not necessarily "hard" but there is a lot of labor involved and to do it yourself, you will need a pretty good garage setup with plenty of space, preferably a lift. You'll need to pull the driveshaft, unbolt and drop the complete rear suspension cradle/assembly...then when installing the new ring/pinion, you will need to properly set the gear mesh (can be tricky if you've never done it before)....and you may need a re-alignment when you put it back together. IMO, it's a big pain in the rear (pun intended).

If this is your daily driver, you will almost certainly be without a car for a while as I don't think most folks can do this in one day. If you're having this done by a shop, they're probably going to tell you to just bring the center section as they won't want to deal with the rest.

Am I saying that it's not worth it? Not at all. I've chronicled my adventures with the 4.09:1 in my 2020 which was a much bigger pain in the ass than your job will be and I don't regret it.

As for the Ecoboost swap, are you thinking about saving weight with the aluminum housing? Please verify this first but I believe it is strong enough for stock-ish power levels. If you're running dedicated autocross tires and really hammering the snot out of it, I'm not certain if it's worth the risk, especially since the housing is unsprung weight, exactly where you would want it. Lighter is always better but this chunk of weight isn't as problematic as it would be farther from the drive wheels or farther from the CG.

Comparing this to a Tremec swap? Sheesh...I'd love to hear someone's opinion on that but I can't imagine that it would be easier and it certainly wouldn't be cheaper though it would offer more benefits.

If you decide to go the ring/pinion swap route and you are looking to save a few bucks on the parts, drop me a PM. My 3.55s are just sitting in a box. They have ~2,900 miles on them, never thrashed. Something like $75+shipping? Trying to keep it around half the cost of new parts...let me know if you want them.

Very helpful thanks. I’ll ping you about the parts later!

 

[Mattwood440]

I'm considering changing gear ratios to try to get more top speed in 2nd (Autox car).

How bad/expensive is it to swap the 3.73 gears in my Torsen to a 3.55 or a 3.31 (keeping the Torsen, obviously)? The other routes are maybe changing to a Tremec or even a trade with a 2018+ MT-60 owner, IDK.

Time/labor costs?

Is there a Ecoboost torsen 3.55 swap which is equal strength for hard/track use?

I had a respectable shop do it to my 2015 pp for less than $900. Gears, bearing, and seals. I think auto trans cars and the newer ones with rev matching need software changes but not us

How do I like it? Around town it's not great because you're right on the border between 5th - 6th at 45mph and I live in hills so I'm constantly shifting. I wanted it for drag racing and it worked

 

[Brazos609]

Just swap in a complete intact 3.55 Torsen differential out of a 18+ auto PP1, that's my plan. I have the 3.55 differential and a TR3160 in the garage waiting.

 

[Burkey]

Just swap in a complete intact 3.55 Torsen differential out of a 18+ auto PP1, that's my plan. I have the 3.55 differential and a TR3160 in the garage waiting.

Aren’t the auto housings alloy rather than steel?

 

[Brazos609]

Aren’t the auto housings alloy rather than steel?

No, the 3.55 Torsen equipped differential from the automatic PP1 is steel.

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