Alright,

So I just rebuilt the 305 and it now has an estimated 300 hp, (comp XE268-HR) with a rebuilt 700r4 (with a shift kit). This will be a daily driver. I believe it is a 26 spline since it's an '86 abd I would really like to have positrac. I hate the 1 line burnout.

Any help is greatly appreciated

Thanks,
Matt

It is an Iroc without posi?? have you pulled the diff cover off yet?

I recommend the Eaton Posi. It's the best clutch type differential available. If you want to save a little money then the Yukon Dura Grip is a good choice. If you do a 28 spline upgrade then a used Torsen would be nice. Just avoid the Auburn and the Governor Lock. If you don't already have 3.42 gears then I would recommend changing them too.

To: M.Leblanc

I'm 100% sure it's not a posi. I don't have the luxury of changing it to a 28 and had to move to another state. I had to leave it with a family friend so he can work on the wiring that needs to be done. I'm going over there at the beginning of next month to bring it back and i can only take a couple of days off work.

I just figured that since we are going to be opening the diff and changing the oil, I could save time and money by ordering what i need and have it shipped over there so when i get there all we have to do is install the parts.

Thanks y'all,
Matt

All you have to do is install the parts ?
Its not a tune up. Its a differential rebuild.
Here this will give you a idea on what you your going to need.

1.3.42 ring and pinion =$ 200.00 (highly recommended)
2.master bearing kit = $ 90.00
3.axle bearings and seals = $ 30.00
4.26 spline Eaton posi. Unit = $ 450.00 ( highly recommended )
5.installation/labor = $ 300.00 est.
You have to have someone with the proper tools and know how to use them perfectly.

I have already been through this process.

Rear end work is not like changing a cam or intake manifold. It's not just remove parts and stick new parts in there. If you change the gears you MUST set the pinion depth, bearing preload and backlash to get the correct contact pattern. If you just change the differential you MUST check the backlash BEFORE removing the original differential and you MUST reset the backlash and bearing preload when you install the new differential.

There are 2 different series differentials that are used in the 7.5 rear end. There is a 2 series for 2.14 to 3.08 gears and a 3 series for 3.23 to 4.56 gears. You must get the correct series for the gear ratio that is in the rear end that you have.

Dumb question here, our cars are 10 bolt, correct?

Since you have the overdrive you could run 3.73s and it'd be manageable, and give you great performance.

Around '85 to '90 they used the 9 bolt Borg Warner rear end in some of them. Mostly T/As and IROCs.

3.73 = Nice! That's pushing it, but I like the suggestion. I'll look into it.

Thanks guys!

Yeah, I have a TH350 and that would be too much gear for me, I've heard they're manageable with overdrive though, at highway speeds.

Did you figure out if you have a 9 bolt or a 10 bolt? (it's as easy as counting the bolts lol)

That will determine your gear options (because the 9-bolt has different available ratios than the 10-bolt, etc).

Yeah. I hear you on that.

Thanks everyone for your input

I have 3.73's in my 84. I upgraded to 28 spline axles and run a Zexel Torsen unit. I love it!

I have a 700R4 and 3.73's with 26" tall tires (Stock for 84 15" wheel) runs me 2600rpm at 70mph on the freeway. I can't imagine running anything higher (Numerically lower) like 3.42's or 3.23.