Hello

I have a -88 Iroc with the 9-bolt, 2.77, discs, LS and I am considering a swap to a 1989 Trans Am GTA rear.
The GTA is also a 9-bolt with gear ratio 3.27, disc brakes, limited slip etc.
From what I read here there is no problem switching the rear ends, everything should fit. (Correct?)

My concern is if changing from 2.77 to 3.27 is a big enough step?
Will it give noticable performance gains? Will I really feel it?

I have a newly built motor with flattops, CC XFI268, Brodix IK180, ported base and plenum, AS&M runners, #24 injectors, Corvette TC etc.
It feels a bit slow off the line.

Thanks

 

Everything will "fit".

However, you'll be upgrading from the crappy cast-iron Saginaw calipers, to the superior PBR ones. You'll need to change the master cyl and prop valve, because the hydraulic requirements of the 2 systems are CONSIDERABLY different.

That's a fairly substantial step in gear ratio. About 18% change in ratio; or to look at it in a practical sense, an 18% increase in engine RPM at any vehicle speed. You will DEFINITELY feel it.

A single step, like 2.77 to 3.08 or 3.08 to 3.27, maybe not; but 2 steps in ratio, you'll notice it, for sure.

Don't forget about your speedometer; you'll need new speedo gears as well. www.transmissioncenter.net Rather than fiddle around with a bunch of "calculators", just look at the driven gear you have now; and get a new driven gear with 18% more teeth. If that's not possible, look at both your drive gear and your driven gear, and calculate their existing ratio; and get a new pair of gears with a ratio 18% higher (existing ratio times 1.18). For instance, you might have, say, 19 teeth on the drive gear and 40 on the driven, which is a ratio of 2.105; you'll need a ratio of 2.105 x 1.18, or about 2.484. Real-world gears that you can actually buy might give you a combo of 18 and 45, that will be very close to that ratio (45/18 = 2.500); and will restore the speedo to correct operation.

 

i hate there site =)

i'll have to do some surfing casue i am about to do this swap also and this would help me out , casue i never even though of that

 

Thanks for the detailed answer!

However, will the stock Saginaw calipers fit? Can I just move them from my old rear to the new?

It seems like a better idea right now rather than upgrading the brakes. Changing brakes and all that comes with that I can do in the wintertime.

The speedo gears will need to be changed, but that is also a rather small problem I hope.

Thanks

 

-I just did the swap yesterday. I have an '88 Iroc like you. Out came the Disc 2:77 Posi and in went a 3:23 10 bolt Posi with the '89 1LE PBR setup. 1st off, it's a nice change. Of course it pulls stronger. At 70 I used to turn about 1750r's. Now it turns about 2250, more into the power band. At lower speeds around town the mtr doesn't lug as bad as before, seeing how the converter would lock up right around 45mph @1050r's. I did not change the proportion valve. The pedal is firmer and braking power has improved, though I have not jumped on the brakes seeing how the pads are just seating on new rotors. I will be doing the LS1 upgrade on the fronts after the holiday. Those are just over 12" rotor size. I still don't anticipate the need to change the proportion valve then either. Here's some photos. Thanks go to Dean for the axle build, fresh bearings and installing the brakes and his brother Chris @ TooKoolKustoms for the install.

 

Did you change the master cylinder?

Anyone know if the stock calipers will fit instead of PBR calipers?

BR

EDIT: Nice pictures, thanks!

 

I did the same swap on my car awhile back. It was much nicer. Car actually got better gas mileage after the swap. It was always hunting between 4th and 3rd on the highway with hills.
You will definitely notice a difference.

 

If you will be using your existing brakes with the new rear end and the new was a pbr brake setup just swap the backing plates and any brake lines necessary and you should be good to go. You must remove the axles to change backing plates. For axle removal I suggest a battery post wrench, I find it a better tool for removing the pin for axle removal. Also don't worry about finding a gasket, use rtv from the local auto parts store.

The two rearends are the same in configuration with the exception of the brake hardware you choose to use. It is just an in and out, but remember the spedo info from above...I have had both ratios in mine and you will notice a difference.

 

-Widarsson, what is on your new rear now? The '89 rear w/ disc came with the PBR's. Is it stripped to the axles? are there any backing plates and hardware on it? I have a set of backing plates for the '89 rear. They came w/ some calipers I had bought for cores. If you can measure the the centers of the mounting holes on your cast iron calipers I can measure the spread on these backing plates. Unfortunately my 2:77 rear is still at the shop and I won't be going there till after the holidays. IMHO if you're going to do the axle swap now, then do the PBR setup now while its out. No sense going in there twice right? Either that or take your time and put the brakes on it, get it all set, then do the swap. Kinda like spend the $ now or save the $ then build it and put it in. Another thing, you'll have to order new handbrake cables for the PBR calipers. They do not match at the caliper and of course the r/s is too short anyway. I ordered mine yesterday at my local Chevy dealer, $29 each and a week for delivery.

 

I haven´t actually bought it yet, I have it on hold for me.
I thought it would be good to gather some info before I go ahead and buy it.

I have just found out that it has all the hardware except the calipers , so I´m stuck with my stock ones for now. Therefore it is even more important to get it to fit with the stock calipers.

If I understand your answers correct this is the way to go about it:
Change backing plates to the stock ones instead of the GTA.
Change speedo gears.
New brakelines etc.
Everything else should fit.

What about that pin for axle removal? I thought the axles were bolted in place with plates from the outside? Is this pin located inside the housing?

Would it be possible to move the entire axles (with backing plates, discs etc) from my old rear to the new?

Lots of questions, please be patient with me.
Your answers are very much appreciated!
BR

 

Using your brakes will be no problem as all except the backing plates are installed and functioning now. I replaced my rearend and added pbr brakes just to have the rearend fail due to the pinion nut not being tight and then replaced it with a 9-bolt 2.77 until after my engine swap....reason for saying this is that I do not recall some of the specifics on the retaning pin I mentioned, I think it is in both the 9-bolt and 10-bolt...in either case use the battery terminal wrench to remove and reinstall.

Look at your existing and the replacement backing plates and be sure they are different if they are the same then removal of the axles is not necessary. Axle removal is not difficult and the remove and replace is not difficult just big wrenches, heavy stuff and an opportunity to get pretty dirty. I have swaped mine a couple of times now without help and it is not hard it just takes a little time.