Getting closer to getting my build done in time for autocross soon.
First ive decoded my rear end, and yes I took off the over and checked.
I have 1987 iroc-Z with a 10 bolt 2.73 ratio rear end, limited slip posi.
RPO codes G80 and GU2. Currently its a drum brake rear end.

Later on today I plan on going to the junk yard in hopes of finding a better rear end. Before I stripped my car down I drove it around a little, and its lacking off the line speed. I want to do some 'semi' competitive autocrossing, but mainly the car will be for street use. I dont want to go too low on the ratio to suffer the street use, but I was want it low enough for good autocrossing. Willing to compromise.

Im looking into either a 3.08 or 3.42 rear end...maybe even a 3.27 if you guys suggest..any suggestions????...im hoping for a disk brake (if i can find one). I know that I need the brake lines and maybe the e-brake too) and the proportioning valve...Also if im lucky enough to find the rear end with disks ..will they clear my wheels? They are the factory 16" IROCs. Thanks much guys.

 

Also corretc me if im wrong, but I do have a 26 spline now...and compatibility issue if I go with a 28 spline....as far bolting in the rear end, and clearance of the disk brakes.

 

yes 26 splines now.

If you take a complete rear from a mid '89 year, to '92, then it'll bolt in fine. The 28 splines are only a consideration once you start swapping parts from the innards of your 26 spline and a 28 spline one.

ie, your posi that you have now will only work with up to 3.23 gears, in a 26 spline rear, so a car mid '89 and older. I'd recommend 3.23 or 3.42 for your application.
Remember, V6 cars had steeper rear gear sets, where the v8 cars had the lamer 2.73's. Parts on V6 cars are usually cheaper as they are "low perf". Nifty eh?

Clearance with your 16" wheels should be a non-issue.

I'm not sure if your choice of posi is related to the fact it's an autocross app. Not my area of expertise there. that is, if you were considering buying a new different posi.

 

one thing you might want to consider if auto crossing is your goal is finding something that has rear disks on it the axels on these cars are different so dont forget to take that in considersation 3:42s be best prolly cause most auto crossing is done in parking lots. auburn pro series because thats what i run plus i work there an ive run both eaton an auburns i think the auburn locks up better true you cant rebuild but they have a new swap programs that keeps cost down being my car is set up for autocrossing im happy i am running 3:73 gears though an only auto crossed once strip a few more times tryin to get buggs out of knew motor an really havin fun maybe some more trips to local auto cross events next year

 

If you talk to national level autocrossers the general consensus is that the Torsen T2-R is the best differential for F-bodies. Sam Strano Jr. at Stranoparts.com is a good source for a T2-Rs as well as car setup tips. He's won the E-Street Prepared National Championship twice.

I'm running 3.42 gears with my T-5. I wouldn't want to run any higher a ratio, because it would cause me to have to shift to 3rd on some long stretches and that would be wasting time. I've already been know to push 7000 rpms on a few courses.

I hope you understand that changing axle shafts or differential will not be legal for stock class.

 

a quick ? about stock class you said if you use a limited slip and axals you cant run stock class why is this alot of third gens came with these units in them right off the floor i know the rules are very strict to keep a level playin field but guys that had these units stock cant run stock class either

 

The stock class says that it needs to be any stock part or an optional part that was offered. So I should be able to put any other 1987 Iroc rear end, and be ok with the rules. So what do u guys think will be my best option. Mainly a driver, and some autocrossing....Im think anywhere from 3.08 to 3.42 depending on what i find what was offered on my model. thanks.

 

Most of the suggestions that have been made in this thread about diffs and axle shafts with a different spline count will be illegal in stock class.

The stock class rules say that the car has to be run the way it was delivered from the factory, with a few allowed exceptions. The only way that you can change a part is if that part was an option on that year and model of car. Additionally, if the upgrade part was only available in conjunction with other parts, such as a performance package, all of the parts included in the performance package must be changed.

Since rv1890's car came with 26 spline axles and since 28 spline were not available that year, he has to keep running GM 26 spline axles to stay in stock class.

 

I'm pretty sure you can update/backdate within the same body style and generation. So I'm pretty sure you can use any diff originally available in a 3rd gen, but you can't use 4th gen parts. I'll have to double check the rule book.

 

just wanted to say the rule book is great exspecially if you want to compete i should of got one when i was building because when i went out i was in a class that i really should not have been in but never the less if you look at it as challenging yourself an inproving driving skills its alot of fun an i diffenatly know what my car is doing an needs. great event an a few more tuning sessions at the strip a few minor adjustments on suspension geometry an ill be back there testing my skills great fun great people the knowledge out there ok enough great experience

 

Update/backdate is a Street Prepared rule. You can't do that in Stock.

 

3:42