Sooo i just picked up my very first firebird


its an 89 305 tbi 700r4 automatic with what used to be the stock posi rear end.


i picked the car up from the guy today and was well aware the rear end did some humming, but wasnt exactly aware of the extent untill i got it on the highway it was absolutly horendous i figured it would last at least untill i got home but the moment i pulled outta my first stop sign there was a huge loud clunk and then i had nothing...


i droped the car off today to have the rear end redone now my question is, what kind of room is there for upgrading these rear ends? and what ratio is most commonly used when the usual mods are put on... ie headers, true duals,heads cam ect ect, ive got the rear end broken now i figure why not start there right?

 

Is it the 10 bolt or the Borg Warner 9 bolt? If it's the 10 bolt the first thing I would do is trash the Auburn differential and put in a new Eaton. If it's the 9 bolt then you are stuck with the Borg Warner differential, which isn't bad, except that when it wears out it isn't easily repaired.

If you have the 10 bolt then you could have 26 or 28 spline axles. If you have 26 then an upgrade to the 28 spline axles would be good, although a 305 woth an automatic probably will never hurt a set of 26 spline axles.

The 3.42 or 3.73 are pretty popular with overdrive transmissions.

The best advice that I can give is to buy QUALITY parts. Bearings made in China are not worth the box they come in. Cheap gears are just that, CHEAP. Buy good stuff the first time and there shouldn't be a second time. Also, gear set up is VERY important. If the pinion depth, backlash and bearing preload are not set right then even the best parts will cause problems.

 

Thanks a bunch thats a ton of info that i will absolutely keep in mind this repair however im leaving to a trusted speed shop, i simply just dont have the time to tear into this thing and the car needs to be back running again within the week soo im kinda stick, i did ask him to upgrade 2 the 3.73's and he said thatd be no problem.

 

If its original, then its likely a 10-bolt in that year/car. Also it is 28 splines.
You need to look at the RPO decal to see what gear ratio you have. G__codes. If you have a G80, then its a limited slip unit. If not its an open diffy. Most likely an open diff. Do you have a 120 or 140 speedo or lower? Typically a 140 speedo would indicate a higher (lower numerically) gear ratio. Typically but not absolutely.

 

Probably cheaper & faster just to get a 4th gen rear end & swap it in. Other than the e-brake cables, it simply bolts right in. The gears may or may not be exact, but that's not a tough issue to tackle either, or just be aware of howfar off your speedo becomes.

You get a better posi, bigger disc brakes (or upgrade from drum to discs)....Just all around, a nice upgrade.

 

look into 3.73 gears, my transmission instructor orders most of his parts from randys ring and pinion .com which is a pretty cool place. you should be able to pick up another rear end at the junkyard pretty easy, just spin the rotors to make sure its a posi, but if u want to upgrade to something better id say upgrade to an eaton clutch style posi, the posi on 4th gen camaros are okay for daily driving but the torsen gear style posi isnt the best for high performance.