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3.91 Lsd in a '15 3.8 M/T Review

18K views 133 replies 19 participants last post by  Carlman 
#1 ·
As we know the M/T transmission Genesis 3.8's come with a 3.55 rear end gear which is a good compromise in performance and highway mpg. To most of us loosing an MPG or two is okay if the trade off is worth it or so I thought, my car is a base and didn't come with a LSD and as we know the car just isn't the same without a LSD. The car is unpredictable, pulling out of a street quickly sets off traction control too soon and sometimes can be dangerous because it cuts power HARD. So my goal get a LSD at a reasonable cost and also upgrade, I knew the car could benefit from a regear. After 2 months of research I decided to go with a 3.91 pumpkin out of a '14 2.0t, calculation after calculation was made to make sure I was happy before spending $800.00 on a diff. We called this in my mustang days the poor mans supercharger a whole lotta bang for the buck.

Expectations:
- expected to need new tires 3.91= more wheel spin
- first gear would be way too short
- worse mpg
- way better acceleration if I could hook
- afraid of shifting gears all the time

Reality:
- car has way more grip, no need to rev up and dump clutch to have an agressive launch thus reducing shock and wheel spin, car hooks hard and launches hard on factory tires, imagine grippy tires and an agressive launch.
- first gear goes to 35 mph, I shift up at 3k around town and never have to worry about merging. I'm usually 2-3 cars ahead when shifting to second yes car pulls that much harder.
- the way I drive I was averaging 20-21mpg no matter what, I now average 22 with a best of 24. Back roads 30-45mph with some stop and go when in the city.
- the car is a whole different animal, out of the hole it's an animal. How much of an animal? 0-100mph a 335i, s4, challenger r/t, sit can't keep up. The car just hooks and goes on factory tires yes I repeat factory tires
- last but not least you won't shift gears all the time, 4th gear is sufficient all the time. You'll actually shift less as you'll have to downshift way less.
- highway RPMs are up 400RPM which saves me from speeding all the time I cruise at 65 a lot more now haha and passing power is incredible.
- if you're stock you'll finish the quarter mile at the top of fourth.

I've had this done for a whole month now and commute 50 miles a day to work, I enjoy the car a whole lot more and recommend it if you're looking for a big change.
 
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#3 · (Edited)
You're welcome I figured I'd do one because when I was doing research I wish someone had made one of these threads glad I can help. I did, One of my buddies has a garage with a lift but it's totally doable on ramps if not easier as you're laying down haha. If you have a 13+ with a manual transmission it's wicked easy.

As easy as:
- remove 3 bolts from driveshaft
- remove 6 bolts from each cv axle (triple squares) most time consuming part
- remove 3 bolts holding diff in place
- above steps are for a manual 13+ with bolt on axles

If you have an auto tranny it might be more involved, I can't say how much as I don't have one.

You can swap the output shaft flanges in between all rear diffs, so if you buy one from an automatic use a slide hammer to pop out your flanges to install them into the new LSD.
 
#4 ·
What did you do with your old setup? I've been toying with the idea of swapping my ring and pinion gears in mine to give a different final drive ratio if I'm having an issue since the 3.8 turbo install. I have a 2013 Grand Touring with the 8AT, so my final ratio is 4.181. I'm gonna try it for a while, but if it can't get enough grip or seems to shift too often, I may want to go to a different setup. I'm not sure it'll be an issue yet, but I've heard from a couple of turbo'ed 3.8s and auto trans that it's almost too much power for our gearing to keep planted and the shifts are too close together. I may swap just to see how it goes.
 
#8 ·
It's sitting in my garage haha, 3.55s with your power should be more than plenty. Specially with the 8 speed see how it runs and decide might be too much gear for the power level. If you want to work something out we can I have a buddy who would love those 4.18s for his bk1 haha
 
#5 ·
I've been thinking of swapping my 2.0T base 8AT for an LSD as well. Saw one on eBay for $399, the entire diff, not just the gears.


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#6 ·
If you can get it at that price, well worth it!
 
#7 ·
This is it here.


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#10 ·
Was just about to click the buy now button, but noticed it was for an AT and based on previous post, it wouldn't work for me :(
 
#9 ·
Thank god I'm done with the engine work because this is my next mod for sure. Where did you source the diff, dealer?
 
#14 ·
I believe so, yes, again let me find the thread...

And no difference '13 vs '14 2.0T LSD but they'd both be manuals, there was no 2.0T Track in those years only RSpec, so either that listing is wrong about the A/T or the year, or both. Have them send you a picture of the sticker on the front cover of the diff and post it here. And all of our cars with LSD use the same Torsen unit, just different final drives.
 
#15 ·
they said there is no sticker. Anything else we can look for?
 
#20 ·
so what do i look for then because I called a place in KS and they said it was for an MT but the vin also shows an AT.
 
#21 ·
#23 ·
No auction, was another one the place in KS had. She just read me the vin.
 
#25 ·
whats on the sticker that i'm looking for? Anything stamped on the unit itself?
 
#27 ·
The guy from the auction listing called me back and said he found the sticker, it's a 4.18 STD, not 3.91 LSD.
 
#29 ·
thanks a lot for the info, I'd have ended up buying 10 of these if I hadn't known about that sticker.
 
#31 ·
when it comes to gearing you'll notice.
 
#32 · (Edited)
The splines on the axles are the same, but the 13+ M/T has bolt on axles so you would have to swap the flanges from your diff onto the auto diff. They are slide in clips. I sourced mine from a used parts place in Florida. I had the diff 3 days after I paid. Go on eBay an look up Genesis coupe LSD they have one listed for $749.00 it's a 3.727 gear LSD. I attached link below.

13 14 15 Hyundai Genesis Coupe 2 0T LSD Rear Differential Diff 6MT 3 7 | eBay
 
#34 · (Edited)
The part where they come with different output's is right and your thread proves that exactly haha, and they do come with bolt on flanges on the 13+ that is also true. If you're looking for a bolt on solution buy the right one if not swap output shaft flanges. There's enough confusion in the topic as is enough no need to complicate it more. Some people just feel more comfortable doing a few bolts and swapping diffs other than having to swap flanges damaging a seal and whatnot.

If your doing a lot of highway driving I'd recommend the 3.727 it'll be a mild change but deff noticeable.

Info updated 1st page
 
#35 ·
The part where they come with different output's is right and your thread proves that exactly haha, and they do come with bolt on flanges on the 13+ that is also true. If you're looking for a bolt on solution buy the right one if not swap output shaft flanges. There's enough confusion in the topic as is enough no need to complicate it more. Some people just feel more comfortable doing a few bolts and swapping diffs other than having to swap flanges damaging a seal and whatnot.
No confusion in this thread, just someone interested in the swap asking questions and getting answers, like almost every other thread here. And while you're right that some people prefer direct bolt on it's not your call to make, so to say it won't work for Titan is not accurate, he can take the info provided and make up his own mind, as will anyone else who reads this thread.
 
#37 ·
That eBay diff was not an LSD? Glad someone called! I bet that's why it's been there for 2 months.


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#38 ·
Nope, not LSD. I told them they should update it so someone doesn't buy it and get screwed but I doubt they will.

I am looking for the correct one, I will keep looking. I do prefer in this case a direct bolt-on swap, but if I can't find anything in an acceptable time frame (I get too impatient) then I'll just get whatever and do the swap. So knowing as much as possible will help me (and others) in the long run. I don't know enough yet and to prove that, I almost bought the wrong damn assembly, twice!

Appreciate all the info.
 
#39 ·
What about the Quaif?


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#40 ·
I was looking at aftermarket ones, but they are $1500 and I'm not sure if they will fit. Can anyone answer that? I assumed that if the 2.0T LSD fit the 3.8, and the aftermarket diffs fit the 2.0T, then they should work, right?
 
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