Third member questions

[drew43a]

I have a 70 cuda 340 4 speed car that has a 489 case rear end with a 373 or 391 gear Posi.  I am looking to change it to 355 gears and possibly just buy a reman. unit from one of the many venders.  but i dont know what i have as far as Differencial and Pinion yoke. can anyone help with that info? and recommend a vendor that sells these units rebuilt?

[RacerX]

Many folks here swear by Dr. Diff...

https://www.doctordiff.com/8-3-4-489-case-third-member.html

[headejm]

 

[7212Mopar]

70 Cuda with 489 case meant it is not original to the car. You need to check what U joints you have before ordering a new center section, assuming you want to keep the driveshaft you have. Dr Diff needs that info when you order. He can walk you through this when you call him.

[Solarguy]

70 Cuda with 489 case meant it is not original to the car. .

Why would that be?

My 70 that has what I believe to have its original rearend had a 489 case

[JS29]

  I have two 1970 E-Body's with 489 centers. 

[7212Mopar]

Well, according to this that it is possible, so I was not correct.

http://www.mymopar.com/index.php?pid=62

[Mopar5]

489 is the best carrier

[7212Mopar]

I thought Dana 60 is the way to go if you have the means.

[RacerX]

I thought Dana 60 is the way to go if you have the means.

Dr Diff will build you one to e-body specs...    at around $2300 for one with a standard suregrip I guess might be worth it if you want a Dana and can't find/don't want to spend for an original...

[Chryco Psycho]

You have a lot of options , , a 340 will never hurt a 8 3/4 diff  , 489 is normal for 70 , 69 was 741/742 , I actually prefer the 742 as it has no crush sleeve which will commonly fail , but you can use a spacer & shims in the 489 making it equal to the 742 .
You could build a Dana 60 but a new center section should be cheaper as you only need a center carrier , you can aslo mount new gears in the carrier you  have with new brgs & a recommended spacer instead of a crush sleeve .
You can also choose the type of sure grip , most cone style are worn out after 50 years & are a poor design anyway , so you can use the 4 pinion clutch type sure grip or the even better Eaton Tru Trac .
A local drivetrain shop can set up your carrier with new gears & brgs .

[drew43a]

Doctor Diff has these differential options 
6-helical gear "Sure-Trac II" (30 spline) (+$150.00)
Clutch type "Powr-lok" differential
30 spline spool (-$270.00)
35 spline spool (-$270.00)

my buddy said use the clutch type.

any thoughts on these options?

[7212Mopar]

Depends if you do any competition driving or not. Clutch type wears out, requires modified lubricants but rebuildable. You can change type and quantity of the friction discs to change the locking characteristic and tune to the track like road racing. Torsen type like the Suretrac has a fixed ratio for lock up so not easy to tune, but it is maintenance free and use regular gear oil. Spool is not for the streets unless you do not make turns. BTW, I have the Suretrac from Dr. Diff and it has been flawless, no funny noises and leaves behind two black stripes when I floored it.

See this video for in depth explanation.

[Mrbill426]