Looks like I need a new diff. 3.91 is the way to go, right?

[jrhone]

Depends on your engine....

I have a 3.64 and a 5 speed and I wouldn't go with a 3.91.  I have zero issues with peppy acceleration and I LOVE how it feels on the freeway.  I also daily drive my car and I do put 15-20k miles on it and do road trips...so having a 5 speed with the 3.64 makes it nice.  2 motor builders that I spoke to said they would not do a 3.91 on their personal cars with 5 speeds.  I would call and talk to some and ask what they would suggest with your motor and how you expect to drive it.  If you keep the 4 speed and the 3.91 for any time you will be at super high revs at highway speeds...I like being comfortable at 85-90.  

[Preyupy]

The 245 OD box has a 0.86:1 5th gear.  If you are comparing it to a 4 speed (top gear is 1:1) with a 3.64 axle

4 speed with 3.64 final drive 23.2" tall tire (real speed NOT what your speedometer says you are going) 70mph=3692 RPM

OD 5th gear with the 3.64 final drive (calculates to an effective 3.13:1) 70mph=3175 RPM

OD 5th gear with the 3.90 final drive (calculates to an effective 3.35:1) 70mph=3358 RPM

OD 5th gear with the 4.10 Final drive (calculates to an effective 3.52:1) 70mph=3530 RPM

 

I was one of the very 1st people to install a 245/5 OD 5 speed in a 2002 back in March 1982.  I was autocrossing my car and wanted the 4.10 final drive but was tired of cruising down the freeway at 4000+ as well as sitting on 7500 rpm  on the main straight at the local race track on club play days. I just recently removed the 245/5 and installed a 245/10 CR transmission as well as a 3.45 final drive to go with a 8000 rev limit.  .   

[MattL]

This is all excellent info, input, and intelligence: just what I always get here. Lots to consider, now that I’m well-equipped. Thanks, everyone. 

Edited April 12, 2019 by MattL

[MattL]

Talk about timing! RussTii is selling a 3.64 LSD from an early E21 in the classifieds section. Does anybody know if that'll be a clean swap for a '76? 

[Son of Marty]

Yes it will go right in but, you'll need to change a bit on your half shafts to fit correctly, do a search on it as there are a couple of ways to do that. Oh you have a 76, you just need to use the output flanges from your old diff and you should be good to go.

Edited April 12, 2019 by Son of Marty

[MattL]

Thanks, SoM! 

 

(And great footer quote!)

[Conserv]

Let me interject one more viewpoint.

 

Full disclosure:

 

1. My October 1972 ‘73 tii presently has a 3.64 ‘02 LSD (dated 1972).  The car came to me with an e21 open differential (dated 1981) and my only desire was to install a date-correct ‘02 differential in the car.  But I accidentally bought an ‘02 LSD, the limited slip function being non-operable when I bought the differential. Give up an ‘02 LSD? No friggin’ way, not because I really need it, but because it’s so rare.

 

2. My April 1976 ‘76 2002 presently has a 3.64 e21 LSD (dated 1978). The car’s original 3.90 served well, but after 40 years and 100,000 miles I wanted to see if the...uh...mature me might prefer less than 4,000 rpms at 70 mph. The 3.64 puts me at 3,700 rpms at the same 70 mph.

 

So, yes, both ‘02’s presently have LSD’s.

 

But....for most of the ‘02 era, there were no e21 LSD’s from 320iS’s, 323’s, etc. Why? Because the e21 was not yet invented. And ‘02 LSD’s were rare everywhere, but especially in the U.S., as Hoffman chose not to order this expensive option. So 99% of us drove our ‘02’s without LSD’s, and....lived to tell the tale. I’m going to say that one more time: 99% of us drove our ‘02’s without LSD’s. Shocking? Well, yes, you could spin that inside rear wheel when accelerating through an off-camber turn, but even that was part of the fun of the ‘02! ?

 

Although there were times, back in the day, when I thought an LSD might have been an aid to my hoodlum or winter driving, I never owned an ‘02 with an LSD until I accidentally bought the unit for the ‘73 in 2014.

 

My conclusion: an LSD is not a necessity in an ‘02 for street driving, unless you use your ‘02 frequently in snow and ice. But, no, the ‘02 LSD in my ‘73 is not for sale....?

 

Regards,

 

Steve

 

 

Edited April 13, 2019 by Conserv