Diff Gone, Time for Something Better

Drunkenturtles · Sep 6, 2022

I'm looking into getting a new diff for my 14 5.0 making 600whp, the one I had was the standard 3.31 ratio. I put more importance on acceleration and less on top end, so should I get a new diff with the same ratio or one with a slightly lower ratio? I still have the standard mt82, but plan on swapping a tremec into it, so the tremec ratio is something I have to consider, too.

What diff would be recommended? What diff and trans ratios do you guys recommend if I want good accel but not have 1st and 2nd be unusable gears?

Hand-filer · Sep 6, 2022

If you're switching to a Tremac, 3:73 could work. 3:90 would be better. Not sure how the differential went out before the MT-82? Stock differential should have no problem with 600HP.
FWIW I've been at 600+HP for 9 years with the MT-82 and with 3:55 gears. The perfect ratio gear set for this trans IMHO.

Juice · Sep 6, 2022

Yea, how did it fail? The 8.8 diffs are very close in strength to a 9 inch diff.

There are 2 ratio sets available on the Magnum, consider that when choosing a final drive ratio.

I have the 2.97 first gear magnum, and HPDE track it. I'm going back to the 3.73s. (tried 3.90, and 3.55) And you will lose cruise control in a few gears if you don't get the transmission ratios corrected in the tune. Your speedo will read 1/3rd of actual speed without a tune adjustment or a converter box going to the magnum.

Drunkenturtles · Sep 6, 2022

Juice said:
Yea, how did it fail? The 8.8 diffs are very close in strength to a 9 inch diff.

I wish I could tell you... somehow I ate up the pinion gear. I'm equally as amazed I still have my third gear on a stock mt82, I fully expected it to grenade before my diff, but such is life.

What was wrong with the 3.55 and 3.90 final ratio?

Drunkenturtles · Sep 6, 2022

Hand-filer said:
If you're switching to a Tremac, 3:73 could work. 3:90 would be better. Not sure how the differential went out before the MT-82? Stock differential should have no problem with 600HP.
FWIW I've been at 600+HP for 9 years with the MT-82 and with 3:55 gears. The perfect ratio gear set for this trans IMHO.

From what I've seen in the comment bellow, 2.97 magnum with 3.73 final seems to be a good spot for 600hp. What are your thoughts on a 3.55 with a 2.97 1st gear magnum?

Juice · Sep 6, 2022

Drunkenturtles said:
I wish I could tell you... somehow I ate up the pinion gear. I'm equally as amazed I still have my third gear on a stock mt82, I fully expected it to grenade before my diff, but such is life.

What was wrong with the 3.55 and 3.90 final ratio?

3.90 too short, 3.55 too tall for what Im doing with the car.

RED09GT · Sep 6, 2022

I broke my spider gears to bits in my 8.8 with under 500hp and 450 ft/lbs of torque so they certainly can break. The stock tremec 3650 survived just fine.

If you are going to a Magnum XL with the 2.97 first gear, you will need 4.10 gears to get to the same overall ratios you currently have.

I have a TR6060 and use 4.10's with a turbo setup in the 700 rwhp neighborhood and it works great for me.

Juice · Sep 6, 2022

MT82 only has one overdrive gear. The magnum/6060 have two overdrive gears. I wish they had a magnum with the MT82 ratios and one overdrive. 5th, which is overdrive in mine cannot be used on track. (well it can, but it lugs. too much torque loss) Even the 3.55's were too tall for WGI backstretch and 4th gear. I used 5th with the 3.90s, but 4th pulls way better with 3.73s.
This has been my experience.
3.55's are good for all around street use.

skwerl · Sep 6, 2022

My 2014 5.0 6 speed had 3:55 gears. I would have preferred 3:73 when I bought it but the 3:55 ended up being great. Also go with the Eaton Tru Track limited slip. Had that in both my 2014 and my 2009 Bullitt.

Pentalab · Sep 7, 2022

The Eaton tru-trac comes in 2 versions. The heavier version has 4 x pinion gears on each side..and a heavier casing. The HD version requires 33 spline axles..... so only Moser..or Strange Engineering 33 spline axles.

Standard version uses oem 31 spline axles..and has 3 pinion gears per side. It's ample for most cars. Apparently, if your 60' times are down to 1.4 sec, then the HD version is preferred.

The HD version is abt $150.00 more.

Anything is better than the oem LSD. The GT-500 version is no better...with it's carbon fiber discs...which melt.

The tru-trac is transparent. The bias ratio is 3.5:1 which is ample. The tru-trac is a bargain vs the Torsen.

Drunkenturtles · Sep 7, 2022

skwerl said:
My 2014 5.0 6 speed had 3:55 gears. I would have preferred 3:73 when I bought it but the 3:55 ended up being great. Also go with the Eaton Tru Track limited slip. Had that in both my 2014 and my 2009 Bullitt.

I was looking into getting a 3.55 as this car isn't for serious competition, just a fun street build. It seems to be a good spot for both my mt82 and most tremec ratios. I've heard great things about the Eaton tru trac lsd, so I'll put that at the top of my research list. Thanks!

Mustang Terlingua · Sep 7, 2022

I put a TruTrac in my '08 Bullitt. The stock carbon fiber clutches in the LSD just disintegrated at 14,000 miles. I installed the Diff myself the pinion bearings were Koyos & already pitted & worn. The carrier bearings (Koyos)weren't much better. So I installed Timken Made in USA bearings, set the pinion depth & installed Strange 31 spline axles the left axle splines were badly worn. Had to install new exciter ring (one time use) on the axles & used the Strang allen bolts to clear the exciter rings cuz the flange is thicker studs wouldn't clear. It started out I'll pull the cover to check the clutch chatter & it turned into a project. At least I have the tools & 45 yrs experience to do it. Oh yea I put the Ford Racing Diff cover & welded up the axle tube vent hole so that won't seep oil in the future. Don't Y'all just love all the One Time Use Bolts !

Juice · Sep 7, 2022

Im at a loss why all thes clutch failures in the LSDs.
The only way I see it happening is if you are doing one leggers, and a lot. Going around turns, there is just not enough rear wheel speed difference between sides to burn up the clutches in the diff.
Now if it is from staying on the gas with just one wheel spinning, thats a different story.....
Just sayin'

RED09GT · Sep 7, 2022

Drunkenturtles said:
I was looking into getting a 3.55 as this car isn't for serious competition, just a fun street build. It seems to be a good spot for both my mt82 and most tremec ratios. I've heard great things about the Eaton tru trac lsd, so I'll put that at the top of my research list. Thanks!

What power adder do you have? If it is a centrifugal, you'll probably be disappointed with the 3.55's and a tremec, if it is a PD blower, you'll be fine.

Drunkenturtles · Sep 7, 2022

RED09GT said:
What power adder do you have? If it is a centrifugal, you'll probably be disappointed with the 3.55's and a tremec, if it is a PD blower, you'll be fine.

VMP blower on a larger pulley. I know I can make way more than 600, but 600 is what I'm happy with.

Drunkenturtles · Sep 7, 2022

Mustang Terlingua said:
I put a TruTrac in my '08 Bullitt. The stock carbon fiber clutches in the LSD just disintegrated at 14,000 miles. I installed the Diff myself the pinion bearings were Koyos & already pitted & worn. The carrier bearings (Koyos)weren't much better. So I installed Timken Made in USA bearings, set the pinion depth & installed Strange 31 spline axles the left axle splines were badly worn. Had to install new exciter ring (one time use) on the axles & used the Strang allen bolts to clear the exciter rings cuz the flange is thicker studs wouldn't clear. It started out I'll pull the cover to check the clutch chatter & it turned into a project. At least I have the tools & 45 yrs experience to do it. Oh yea I put the Ford Racing Diff cover & welded up the axle tube vent hole so that won't seep oil in the future. Don't Y'all just love all the One Time Use Bolts !

since you've been through all of that, what website or place do you recommend to get my parts for a whole rear-end rebuild? Preferably it came as a whole kit.

Midlife Crises · Sep 7, 2022

My 2010 GT came with a 3650 5 speed and 3.73 gears. It was a very usable combination. I switched to a T56 mag with a 2.66 first gear and the 3.73s were still great for cruising around. Just not ideal for drag racing at my power level. Installing 4.30 gears and the 31 spline Tru-Trac was a much better match for my purpose. After many hard launches on slicks I inspected the Tru-Trac internally and gave it a good cleaning. Judging by the rub marks on the helical gears, the drive gears and axle splines it was obvious there is deflection and a little binding going on inside the carrier. Most certainly do to dropping the hammer hard, again and again. Some day I will buy another Tru-Trac and it will definitely be the 33 spline 4 helical gear per side model. :driver:

Diff Gone, Time for Something Better

Drunkenturtles

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Juice

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