Third-Gens are plentiful in...
Third-Gens are plentiful in the boneyard but they don't last forever. This GTA was ripe for picking and a customer is already liberating parts from the IROC next to it.
Though junkyard pickin's for muscle-era Pontiacs are almost non-existent these days and Fourth-Gen Birds are a little too new to be relegated to the recyclers, there is still a good supply of Third-Gen Firebirds, T/As, GTAs and Formulas from which to choose. Hobbyist interest and aftermarket support for Third-Gens continues to grow, albeit slowly, as many love these cars for their great value, hearty performance-when properly modified and tuned-and great budget upgrade potential. Although junkyards and specialist recyclers offer up a banquet of excellent parts, how does one know which to harvest and which to let the reaper till back under? To that end, we did a bit of footwork in an effort to provide a sampling of practical upgrades to benefit your project.
The Third-Gens were produced in a variety of guises from 1982 through 1992. During those years, although many details and components were upgraded, the basic body structure remained the same. It's also shared with the Camaro, so parts availability and interchange between the two brands is excellent.
An "Australian 9-bolt" can...
An "Australian 9-bolt" can be easily identified by counting the rear cover bolts and noting that it has a middle bolt on the bottom only. The gear ratio tag is circled in red and inset. The tag boxed in purple specifying "limited slip gear oil only" verifies a factory installed limited-slip or "posi" differential.
To upgrade your Third-Gen Firebird, many desirable performance, style and comfort features can be harvested from GTAs, which were built from '87 through '92. Although most of these parts were Trans Am options as well, they were standard equipment on the GTAs. Additionally, the fact that GTAs were produced in significant numbers makes the hunting much easier.
Desirable Rear Ends
One of the most effective performance upgrades to a Pontiac with highway gears is swapping to a limited-slip differential with steeper gears. The first order of business is to determine what gear ratio you already have, then you can decide on what you want.
Even the GTAs have the standard...
Even the GTAs have the standard 10.5-inch brake rotors and single-piston calipers (right), which are adequate in stock form. Only the rare '89 Turbo T/As and the 1LE (factory road race) cars were upgraded to the 11.85-inch front rotors and twin-piston calipers. With the rarity of these parts, we suggest investigating new component upgrades from Hawks Third Gen or the C4 Corvette adapter kit offered by Ed Miller (left).
GTAs, and the corresponding Chevrolet IROCs, often came with a Borg-Warner "Australian 9-bolt" rear end, which will bolt right into any Third-Gen Firebird. Though the 9-bolt was a bit stronger than the 10-bolt and considered a good upgrade back in the day, there are very few parts available for it today and the 10-bolt can be built stronger. The lack of parts commonality with the 10-bolt means the 9-bolt's gears and limited-slip carrier will not swap into your 10-bolt. So if you desire the 9-bolt rear simply to upgrade the gear ratio from whatever you have, make sure the entire rear is already in good working order. Both 9- and 10-bolt designs are available with different series limited-slip or open (nonlimited-slip) third members and gear ratios, and either will bolt into your Third-Gen.
The steepest ratios are for manual transmissions, which are very rare in a GTA. (It required ordering a 305 in place of the 350.) Most rear ends will have a metal tag-held on by a differential cover bolt-showing the gear ratio. If your tag is missing, you can still quickly determine your gear ratio. (See sidebar.)
As the number value gets higher, the gear ratio allows quicker acceleration, but results in higher engine rpm at any given road speed, when compared to numerically lower gears. For example, a 2.73:1 ratio will provide poorer acceleration off idle, around town and for drag racing, but will provide a higher top speed than a 3.42:1 ratio. Conversely, the 3.42:1 will offer higher rpm at any given road speed for better acceleration, yet reduce top speed when compared to the 2.73:1 ratio.