3.55 or 3.73 Rear End?

[TheLion]

I know. I am a terrible person. But seeing the posted "Top Speed" per gear/rear ratio. It says with 3.55's you will see approximate 95mph in 3rd.

That is what my stock non-PP EB runs. So do I have 3.55? I am peg legged bouncing off rev-limiter in 3rd and see 94-95. 18" Stock Goodyears also.

You have a Premium, I believe the Premium does have a 3.55, but could be wrong. Also the 18" wheels are 27.3" roll out, not 27" I used for calculations. Most people are using the Performance Pack 19" wheels with 255/40R19's which have a 27" roll out exactly. 285/35R19's on 19x10's have a 26.9" roll out and 235/50R18's have a 27.3" roll out, tt all affects gearing, but only by a few MPH so it may not be exact.

BTW I found on e-bay a used (10k miles on it) 2015 GT Performance Pack 3.73 rear end for $650 from a crashed car (front end collision). I confirmed as far as I can tell all performance pack GT's have the T2 and 3.73 where the traction lock and 3.55 are the standard equipment. I'll be sending back the 3.73 ring and pinion set from Summit I ordered for $300 as this is a much better deal.

For one, it should have the T2, it obviously has the 3.73 rear end I want and finally I don't have to mess with trying to tear it apart, finding, building or renting a diff case spreader. I can just unbolt the diff, drive shaft and pull / pop out the half shafts and swap them. I can also swap back if I were to have any issues for any reason.

https://media.ford.com/content/dam/fordmedia/North America/US/2015_Specs/2015_Mustang_Specs.pdf

Both the Torsen and the 3.73 are only listed under the GT Performance Package as it's not a factory option, but then again neither are any of the modification's most of us are doing...lol.

Here it is, basking it all of it's cast iron glory: http://www.ebay.com/itm/252469430946?_trksid=p2057872.m2748.l2649&ssPageName=STRK:MEBIDX:IT

Yes, confirmed this came out of a Performance Pack equipped GT, so it's not a conversion. The EB's and V6's probably never came quipped with the 3.73 because they have a slightly taller ratio version of the MT-82 and are more fuel economy minded, also they need to maintain separation between the GT and lower tier mustangs, so options are obviously limited intentionally, the V6 is the biggest proof of this as you can't even get it performance pack...

BTW many sites don't list the EB as compatible, not sure why, because the EB and V6 share the same gearing in MT-82, and all of them have the same axels and drive shafts to my knowledge. A few do however like lethal performance:

http://www.lethalperformance.com/fo...-loaded-differential-housing-3-73-torsen.html

The assembly number for the loaded diff is M-4001-88373T and it should have a sticker on it that say 3L73 and above that S253A:

http://www.ebay.com/itm/2015-15-FOR...ash=item41b03d47d2:g:9~sAAOSw-itXqmj8&vxp=mtr

Even $1k isn't bad for a low mileage rear end as they run about $1800 new. With only 10k miles on it for $650, that's a deal you wont find again!

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[TheLion]

Since I'm some what new to stangs in general, some of these gearing options may be common knowledge, but the 3.73's seem to be a middle of the road gearing as far as common rear end ratios are concerned, but for the 2011 and up it's on the low end:

http://www.americanmuscle.com/mustang-rear-differential-gears.html

 

[Lost]

Since I'm some what new to stangs in general, some of these gearing options may be common knowledge, but the 3.73's seem to be a middle of the road gearing as far as common rear end ratios are concerned, but for the 2011 and up it's on the low end:

http://www.americanmuscle.com/mustang-rear-differential-gears.html

Because awesome torque. With 3.55, it is prolly the perfect balance. As reported, and ESPECIALLY with 3.73, 1st gear goes by pretty damn fast. I can't imagine how completely useless it would be with 4.30's. So the chart you are referencing is not really applicable.

 

[Fast64ranchero]

Bought my PP GT complete drop out with 1800 miles on it for $600 shipped out if a crashed GT. Worth every penny. You will need to swap the yokes if you do the swap on a MT car.

 

[TheLion]

Bought my PP GT complete drop out with 1800 miles on it for $600 shipped out if a crashed GT. Worth every penny. You will need to swap the yokes if you do the swap on a MT car.

Got any pictures of what you mean? As far as I'm aware all Super 8.8 IRS rears use the same 34 spline axles and 30 spline pinion. The drive shaft coupler on the pinion is the same for S550. I'm not aware of any 31 spline IRS. I have the 8.8 3T73 S550 I coming tomorrow (10k miles, so not much use) which came out of a 2015 wrecked GT PP (not sure if it was 6R80 or MT-82, but I don't think that matters as they use the same rear ends sans gearing).

Both the standard Traction-Lok and T2 use 34 spline (although you can buy the S197 variant of T2 in 31 spline for S197's). All lower tier S550's have the standard Traction-Lok which can be had for as little as $179 new. The T2's start out at $1k for the diff only...34 spline is 34 spline. Are you saying you swapped in the T2 from an S197?

http://www.cjponyparts.com/ford-rac...tion-lok-34-spline-8-8-gt-2015-2017/p/M4204M/

http://www.cjponyparts.com/ford-racing-differential-torsen-34-spline-8-8-2015-2017/p/M4204MT/

Also, anybody know the torque socket size on the diff cover bolts? I can't for the life of me find it anywhere, I can find their torque specs on the Ford S550 Ring and Pinion swap instructions, but no mention of their size...

 

[Busser48]

Got any pictures of what you mean? As far as I'm aware all Super 8.8 IRS rears use the same 34 spline axles and 30 spline pinion. I have the 3T73 coming tomorrow (10k miles, so not much use) which also came out of a wrecked GT PP. Although I don't know if it was an auto or manual as I completely forgot to ask the salvage yard.

What's the difference? TO my knowledge all the rear ends have inter compatible parts. Same half shafts, drive shafts and housings are used on EB, V6 and GT. Although I'm not sure if the auto's use something different than the MT's, but I don't see why the rear end would be different.

Keep me posted lion, once u install. This will be my next mod, at the ford place now getting pricing on that and an MGW

 

[Ron@cp-e]

Got any pictures of what you mean? As far as I'm aware all Super 8.8 IRS rears use the same 34 spline axles and 30 spline pinion. I have the 3T73 coming tomorrow (10k miles, so not much use) which also came out of a wrecked GT PP. Although I don't know if it was an auto or manual as I completely forgot to ask the salvage yard.

What's the difference? TO my knowledge all the rear ends have inter compatible parts. Same half shafts, drive shafts and housings are used on EB, V6 and GT. Although I'm not sure if the auto's use something different than the MT's, but I don't see why the rear end would be different.

you are correct on the half shafts
incorrect regarding driveshafts

 

[Busser48]

you are correct on the half shafts
incorrect regarding driveshafts

So Ron in theory, if ihabe the pp EB and want the 3.73 what all exactly would I need to make it work properly

 

[TEXAS HEAT]

So Ron in theory, if ihabe the pp EB and want the 3.73 what all exactly would I need to make it work properly

^^^ This

 

[Ron@cp-e]

So Ron in theory, if ihabe the pp EB and want the 3.73 what all exactly would I need to make it work properly

never swapped one myself
so I can not 100% confirm
but you should be able to swap the yoke so your driveshaft mounts to the new rear
I haven't followed this closely so the splines could be different not completely sure - just never needed to since I made sure I bought the car with 3.55's already installed

 

[TheLion]

never swapped one myself
so I can not 100% confirm
but you should be able to swap the yoke so your driveshaft mounts to the new rear
I haven't followed this closely so the splines could be different not completely sure - just never needed to since I made sure I bought the car with 3.55's already installed

GT MT-82 and EB MT-82 use the same pinion coupler. I suppose we are using different terms, but Coupler is the term Ford used for 6 bolt cone / cylinder that mates to the end of the drive shaft.

I confirmed it was from a manual, silly me, the fact that the entire housing is reddish brown is an obvious give away as to weather it came from a manual or auto AND the other fact that you can't get a 3.73 for the auto as a factory option.

All manuals use cast iron housing with a cast aluminum cover. All auto's use cast aluminum housing and cover. Factory Auto options are available in 3.15, 3.31 and 3.55. Factory manual is available in 3.31, 3.55 and 3.73. 3.73's are only available for GT Performance Pack, it has the T2 diff as well. You can't get the T2 diff with EB's or V6's as a factory option.

I'm sure if you paid enough you could special order one, but that is pretty rare. The MT-82 in the EB's and V6's are also geared a bit taller (higher ratio) than the one in the GT. However with a 7k redline (thanks to LMS), top speed is 152 mph with a 27.3" roll out. Doubt I"ll ever go that fast as I'm mostly interested in auto cross, drag strip, street and eventually when I upgrade the brakes a circuit track.

BTW, if you look at my gearing chart, 1st gear isn't much shorter with the 3.73 than with the 3.55's...

At 7k rpm your hitting 36 in first with 3.73 rear and 37.82 in 1st with 3.55 at the same rpm...not sure why people think that's "super short"...1.8mph difference. But in 3.31 rear your hitting 40.56 mph in first gear! That's pretty tall, which is what I have now, a 3.31.

If you have the factory red line of 6800 your 0-60's will suck because you'll be just a hair shy of 60 mph without shifting into 3rd, but most of us realize 0-60's are pretty useless in gauging real performance.

A Torsen equipped 3.73 rear end from a GT manual will have a rusty cast iron housing (sans not having been though at least 1 winter yet), with a nice clean looking aluminum front cover / bracket, It will have two stickers, one should say S253A and 3L73 just below that on the same sticker (the one which identifies the oil viscosity, on the cover).

The second sticker on the actual cast iron housing will have 8.8 3T73 S550 I on it and a string of numbers above that.

The IRS diff housings are identical for all MT-82 S550's. So are the drive shafts and couplers. The auto does have a different coupler, but also has an aluminum housing, so it's pretty obvious if it came from an Auto or Manual.

Source: Americanmuscle.com

A picture of the actual coupler on my loaded diff...looks exactly like the 6-bolt coupler on my EB.

Source: Americanmuscle.com

Pic of an auto rear end below, note the aluminum housing and drive shaft coupler.

This is an actual pic of the S253A sticker on my IRS diff assembly.

And Finally the 8.8 3T73 S550 I sticker on my use (new) salvage GT diff assembly:

 

[Ron@cp-e]

yeah now that I'm thinking about it
my comment of the axle shafts being interchangeable and not the driveshaft is probably related to the trans yoke and DS length (MT-82 vs A6), not on the rear
sorry for the confusion

 

[TheLion]

https://lmr.com/item/M400188355/201...ng?year=2015&gclid=CN2D3sLYkc8CFQJZhgodJBABQg

https://lmr.com/item/M400188373T/mustang-irs-differential-torsen-373-ford-performance

LMR also lists both housings as compatible with GT, V6 and EB...if there are ANY differences I CANNOT for the life of me find them....please feel free to elaborate.

Ford never offered the 3.73 or the Torsen as a factory option for the S550 lower tier products, aka the V6 and EB in any configuration unless you were to special order one. However ford racing listst both the T2 AND the 3.73 as compatible with GT, V6 and EB. The housings are the same, the only difference is between the manual and auto, being cast iron (MT) or cast aluminum (Auto) and possibly the coupler, although from the pic even the couplers look the same. Should be a direct swap. Also all use 34 spline half shafts and 30 spline pinions, the coupler (Ford terminology, may be what you call a yoke) mates to the pinion (the cone looking thingy in the second pic).

 

[TheLion]

yeah now that I'm thinking about it
my comment of the axle shafts being interchangeable and not the driveshaft is probably related to the trans yoke and DS length (MT-82 vs A6), not on the rear
sorry for the confusion

Exactly, the only incompatability would be trying to possibly use an auto rear with a manual trans...not a manual to manual. If it's an MT-82 it doesn't matter. Pick your gearing, pick your diff or buy a complete assembly (if you want the Torsen without paying 1900 for a new assembly or 1k for the Torsen itself, you'll need to find a salvage GT PP rear as it will have the 3.73 and Torsen).

 

[TheLion]

Rear Axel Ratio 3.73 Speed Engine RPM
1st Gear Ratio 4.236 36.00 MPH 7000
2nd Gear Ratio 2.538 60.08 MPH Tire Height (in.)
3rd Gear Ratio 1.665 91.58 MPH 27.3
4th Gear Ratio 1.238 123.17 MPH
5th Gear Ratio 1.000 152.48 MPH
6th Gear Ratio 0.704 216.59 MPH

Rear Axel Ratio 3.55 Speed Engine RPM
1st Gear Ratio 4.236 37.82 MPH 7000
2nd Gear Ratio 2.538 63.13 MPH Tire Height (in.)
3rd Gear Ratio 1.665 96.22 MPH 27.3
4th Gear Ratio 1.238 129.41 MPH
5th Gear Ratio 1.000 160.21 MPH
6th Gear Ratio 0.704 227.57 MPH

Rear Axel Ratio 3.31 Speed Engine RPM
1st Gear Ratio 4.236 40.56 MPH 7000
2nd Gear Ratio 2.538 67.70 MPH Tire Height (in.)
3rd Gear Ratio 1.665 103.20 MPH 27.3
4th Gear Ratio 1.238 138.79 MPH
5th Gear Ratio 1.000 171.83 MPH
6th Gear Ratio 0.704 244.07 MPH

Again, here's the gearing tables. 3.73 isn't as short as you think. Compared to 3.31 it is, but not dropping from 3.55 to 3.73. For performance the 3.73 is going to outdo a 3.55 with stock EB's and bolt on EB's. 3.55 and 3.31 are probably more appropriate for big turbo or other power adder category mods or a few stock turbo's running water / meth and full bolt ons etc. But even then some people are reporting turbo lag going into 3rd, which I definitely have unless I shift just perfect with the 3.31.

StangTV did a 3.55 to 3.73 swap and also put in a T2. They reported it was very noticeably improved, their stang is bolt on, having a tune and IC and a few other changes. They were using it mostly for auto cross and daily driving and loved both changes. It makes sense, we're taking about a turbo 4 with a 30lb fly wheel...shorter gearing is going to help reduce "overload" and dumping the fly wheel energy and tanking the engine (requiring re-spool), which you feel as lag.

That's probably not the case with NA V6 or V8, but obviously both would be faster for most applications with the 3.73 and the V8 GT PP actually comes with 3.73 as a factory option. Now the only question is, will this diff I bought have any issues, I'm going to bore scope it and if I don't find anything wrong inside from what I can see, I'll put it in and test it out :-).

Also bought the Steeda diff bushing kit and IRS cradle bushing kit (the aluminum lockouts). Combine that with the T2 and even on all seasons it should be a massive improvement. Right now I can't break loose the rear end in 2nd gear no matter what, only on launch (which doesn't take much if you let the clutch out too quick even with 3.31's). But 3.31's are pretty darn tall for the manual.

Auto's have a torque converter and usually unless they have a lockout you won't exceed 90% (10% slip). So that's why auto's run shorter gearing (numerically lower ratios aka more speed). Also autos can brake boost on launch and the torque converter can double torque to the trans depending on how it's designed. Most are 1.5x to 2x effective reduction at lower rpm, so in a way it's a non-linear gearing which is very different from a manual with fixed ratios.

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