86 Z28 IROC Z. I have an RPO code GU5, which means 3:23 gears, right? The G92 option would have an RPO code of G92, correct? Oh, and I don't have posi, I'm pretty sure. There's no G80 RPO on my sticker. I'm considering an Auburn unit and possibly 3:73's. Just got my car a few days ago, finally.

GU5 = 3.23 gears
G92 = Performance option

If you don't have G80, you don't have a factory installed posi.

Many cars came with a base code for a gear option. Some were offered a performance option.

Are you sure your car is an IROC-Z or just a Z28? Options vary depending on a specific model, engine and transmission. In 1986, the Z28 with an LB9 305 TPI and A4 had a standard gear option of 2.73. With the G92 option, it got 3.23 gears. The IROC-Z with the same LB9 engine and A4 tranny had a standard gear option of 2.73 and the G92 option gave it 3.42 gears.

If your RPO list says GU5 then you have 3.23 gears which means you don't have an IROC-Z but just a regular Z28. Your car will have 26 spline axles if you decide to buy a new posi. I'd upgrade to 28 spline axles the same time you upgrade the gears with the new posi.

It's weird because it says IROC Z on the dash. I wonder if someone put that decal on the dash or just put in a whole new dash that says IROC Z.

Not hard to do. They could have also installed a center console out of a Z28. Does the VIN number on the RPO decal match the VIN number on the cowl? Do you have RPO B4Z?

How is the center console in a Z28 different? Do you mean overhead console?
Dash badges are easy to swap. I have the 5.0 H.O. badge, but only because I found it in a JY and it took about 5 seconds to swap with the original Z28 one.

Edit: nevermind, I think I answered my own Q, you mean they could have swapped the console to get a different RPO sticker...

It's possible the car really is an IROC-Z. If the center console was pulled from a Z28, the VIN number on the RPO decal won't match the VIN on the cowl. It's easier to just change the emblem.

I don't know if the 86 models got the IROC-Z decal on the side of the car but again, that can easily be added.

Check the door placard to see if it calls for 16" wheels, which an 86 IROC would.

JamesC

I recommend an Eaton Posi over the Auburn. You will like the Eaton much better, especially if you get an Auburn and it wears out in about a year. The Eaton can be rebuilt and tuned. The Auburn can not.