I have a 1985 Iroc z camaro and it has a 3.42 gear ratio in the rearend. I was wondering why it doesn't have a posi? It is a one wheel wonder. Does anyone know if it was rare to have this option without a posi? It has the 305 tpi engine with 700r4 automatic. I want to install a posi, which is the best one to get?

 

Eaton is best for a street/strip car. If you want to auto cross or road race then the Torsen or Truetrac would be better.

 

BGH is assuming you have a 10 bolt. Best to make sure before ordering.

 

Yes, with the 3.42 gear and lack of a limited slip differential I assumed it was the 10 bolt.

 

Time for an upgrade to 28-spline axles:

https://www.thirdgen.org/forums/tran...-10-bolts.html

JamesC

 

With the 305 the 26 spline axles will probably be all right, but if any power upgrades are planned then the 28 spline upgrade is a very good idea.

 

You could also you a lock right then you do not have to mess with rear end set up.It replace the spiders in the carrier and works like a posi

 

The reason it doesn't have a posi is it's not your car's original axle. an '85 IROC TPI could only have a 3.23:1, and you couldn't even special-order any other axle ratio with that engine.

 

It is the car's original rearend. I bought it with 55,000 miles on it. There also was an option of 3.42 gears in that year on the Iroc. I saw it on the bottom of the page when I clicked on (1985 Camaro '85). I also installed a 1989 corvette L98 engine in it. I think I should get 28 spline axles and a good posi for it. I have never had problems with the 26 spline axles and have spun through three sets of tires and I used to do alot of street racing.

 

G92 is the 3.42 gear option on automatic IROC's in 1985.

 

None actually built.

 

you could get a 3.42:1 with the carbureted IROC-Z, but this one is either not an original TPI car, or it's not the original axle. GM never emissions-certified the '85 LB9 with any ratios except 3.23:1 and 3.27:1. Therefore, none were built.

 

MPFI V-6 LB8 motor introduced to replace carbureted LC1 V-6. It Had similar port injection as LB9 TPI V-8. All manual transmissions now 5 Speeds. 7.5 inch rear end dropped and replaced with a new 7.625 (7 5/8) inch rear end for all Camaros. G92 (performance axle ratio) only available on IROC-Z for 1985.



1985 Chevrolet Camaro Drivetrain List

YrModelTransEngine
code/typeCRDispl.HPTorqueInduc-
tionManual
std.Manual
opt.
(G92)Auto
std.Auto
opt.
(G92)85Sport CoupeM5,A4LQ9 L49.0:12.5 (151)88@4400132@2800TBI3.73----3.73----85Sport Coupe
BerlinettaM5,A4LB8 V68.9:12.8 (173)135@5100165@3900MFI3.42----3.42 ----85Sport CoupeA4LG4 V89.5:15.0 (305)155@4200245@20004bbl--------3.08----85BerlinettaA4LG4 V89.5:15.0 (305)155@4200245@20004bbl--------3.08----85Z28M5,A4LG4 V89.5:15.0 (305)155@4200245@20004bbl3.23----3.08----85Z28A4LB9 V89.5:15.0 (305)215@4400275@3200TPI--------3.23----85IROC-ZM5,A4LG4 V89.5:15.0 (305)155@4200245@20004bbl3.23----3.23----85IROC-ZM5L69 V89.5:15.0 (305)190@4800240@32004bbl HO3.73------------85IROC-ZA4LB9 V89.5:15.0 (305)215@4400275@3200TPI--------3.233.42

If none were built mine must be a one of a kind.

 

Does anyone else have a 1985 tpi iroc with 3.42 axle and no posi

 

Mine was a original 1985 Iroc Tuned Port Injection car and I am sure this is the original axle. Disc brakes on the rear and I changed oil in rearend when I bought it and put synthetic 75w90 in it. I am sure it had never been apart before.

 

May I ask how you determined that the ratio was 3.42?

JamesC

 

I counted the teeth on ring gear and pinion and divided ring gear number by pinion number.

 

Humm, well here's my story. I purchased my 85 IROC (LG4) with 54,000. Someone had swapped out the 3.23's for a 2.77 or perhaps 3.08 with drums--even the 16" IROC wheels were missing. I've never discovered why, though I suspect the Delco-Moraine discs were a problem. At any rate, who knows what, where, and why may have occurred along the line--because the diff fluid had never been changed doesn't mean the rear end is original to the car. Anyway, good luck with your quest.

JamesC

 

They are fairly rare with 205 made. https://www.thirdgen.org/forums/hist...re-asking.html but they were made.

 

Thanks mr369 that is awesome information. I new mine had 3.42 gears from factory.

 

Read this post there was a shortage of LSD's in 1985. https://www.thirdgen.org/forums/hist...akes-open.html