Just looking for some input on good rear end options

[84cutspreme]

Okay...I am interested in swapping out the weak *ss 7.5 stock rear in my 84 cutlass that was originally behind the stock 231 3.8L Buick v6 and the 200c metric trans. I have put in a mildly upgraded chevy 350 (old school mid 70s block) probably between 320-350 hp, so nothing crazy. I have also installed a 700r-4 trans with a few upgrades as well. Shift kit and some beefed up hard parts basically. I was originally looking to swap in an 8.5 10 bolt. I am just trying to find out if this is best option for me. I have been led to believe that a 12-bolt is just a pain and overkill for what I have. With the availability of aftermarket rears now, Is a Currie or Moser 9" a direction I should look. Again cost is a big issue to me, but I want to upgrade to some 3.73 gears and upgrade the axles...but really how much is too much. I have axles as low as 27 splines and as high as 35 spline. I had been leaning toward upgrading to a 30 spline, any thoughts on it? Again just looking for some opinions or experiences, Thanks in advance.

 

[rebelgtp]

One of the biggest pains with a 12 bolt is finding one the right length 60.5 inches or under. It can be done for less then buying a Currie or Moser rear buy far especially because many of the parts you would buy to make sure a stock 12 bolt won't bind you would want for the new rear to make sure you are getting the most out of your new rear. I picked up my 12 bolt for $150, has a high gear in it but new gears and a posi will still cost me less then a complete Currie or Moser setup. The other thing you will need to do is get new rear wheels with one more inch on the back spacing.

Other options are you can find a 9 bolt and convert that yourself, or find a Ford 8.8 rear and do the same (can be found with disk brakes and posi easily), hold out for an 8.5 or rebuild your 7.5.

We also have a fairly extensive thread about this topic going on right now over here viewtopic.php?f=14&t=16896

 

[Bernhard]

They now make a new 12 bolt that bolts right in. With ford ends so that if a axle brakes it will be retained. Housing start at $1000. I thought I might buy one and use used gears and posi that I have. I will have to by axles I think you can buy street axles for around $445. The stock brakes do not bolt up. They were selling ford brake kits. I know a lot upgrade there rear brakes anyway. So if you could get a used posi and gears in good shape for a 12 bolt cheep. I think this is one option.I find the 8.5 442/hurst/GN diffs are hard to come by in my area. When they do come up for sale they want $500 to $1000. The axles are still not retained C clip just like the stock 12 bolt. Chances are I would have to rebuild the diff so I would be looking at new bearings, rebuild the posi , bushings for the upper control arms, sand blast the housing, paint and I still would not have my axles retained so I would have to have ford or olds ends welded on and the cost of axels at $445. I think the cost would be very close to a new 12 bolt g body diff. One thing I know it allways cost a lot more than you budget for. Bernhard

 

[84cutspreme]

A question to add...What companies make a posi unit for the 7.5 stock rear?

 

[rebelgtp]

Eaton makes one specifically for the 7.5 or you can slap a F body Zexel Torsen unit in with the GNX 28 spline axles.

 

[Bernhard]

I talked to a few rear end supplyers they said one of the week links in the 7.5 diff was the small ring gear. With a heavy G body car 3750 to 3500 lb weight. That there was a very good chance if the car hooks that I would bust the ring gear. I was told that there was no fix for this. They told me not to waste my time and money with axles, diff cover, because if I was going to the track with it that it would be just am matter of time before I broke it. Bernhard

 

[RITTER]

About the only thing you can over do is the gear. With an overdrive, 3.73 should be just fine in a street car though

 

[84cutspreme]

Yea I'm really thinking of just grabbing a posi unit and a set of 3.73 gears and throwing them into the 7.5, the car won't see the track anytime soon, just a mild street car. Like I said mild 350 setup, 700r-4 overdrive...so I think as long as I don't go being stupid and neutral dropping it, the 7.5 should be fine right? The only thing is that eaton posi unit is a little pricey at $500+. I was really looking to either save up and spend a good $1000-1500 on new rear or throw a couple hundred on beefing mine up. I just really need to get rid of the 2.63 or whatever horrible ratio is in there now. I see other options though with detroit lockers and powertrax. I haven't researched them that much at all, anyone care to shed any light on that situation? Just remember car is mild street setup for a daily driver, Will see parkway/highway and around town use really.

 

[custom442]

the 7.5 will hold up to less than 350 street driven hp. Just don't be beating on it from a stop or around a corner until you can get stronger axles, ...no locker/spool. I also vote for an old zexel torsen, find one in a junkyard or buy one off car-part.com for less hunting, they last forever. The zexel unit is a worm gear driven limited slip. It acts as an open diff until activated, there is a split second of lag compared to a true posi, but not enough to worry about or notice at your power level. Theres also no maintenance needed.

...I don't think your car weighs 3500-3750 lbs, more like 3200-3500 lbs

 

[84cutspreme]

I am led to believe that the gm 7.5 and 7.625 are identical...I don't know that much about rears but will the zexel unit go in with no modifications other than 28 spline axles?