I recently bought a 9-bolt from a mid eighties IROC to replace the 9-bolt in my '89 Formula. My Formula has rear discs with aluminum calipers and thin rotors, and the IROC rear has steel calipers on the back with thicker, vented rotors. Will this swap require any modification to the master cylinder or proporsioning valve to accomodate the larger brakes? Thanks.

 

Wrong.

The 89 aluminum brakes are the good ones with the 12" rotors, that actually work; the early cast-iron-caliper ones are the crap that don't with the much smaller rotors. Don't downgrade your car that way.

Why are you making this swap? Can you not put the old guts in the 89 rear? That's by far the smarter thing to do.

 

Mid- Eighties rear disc brake cars used the poor designed 10.5 dia rotors with a iron caliper. Big complaint is a non-working parking brake.
1989 9-bolt was the BEST rear disc car using 11.655 dia rotor with a aluminum PBR caliper. And most important a parking brake that worked. I would not think of using the earlier design.

 

The reason Im doing the swap is because my current diff. is shot. Will the aluminum calipers bolt up to the mid-eighties housing?

 

No.

What is "shot" about it? Again, it may be possible to take the guts out of the older one and put them in the newer housing, which would preserve the vastly superior brakes on your car.

What are the gear ratios in the 2 rears?

 

since both the rears are 9-bolts..... the swap is quiet easy. remove all the old stuff from the "shot" rear including... backing plates, calipers, hardlines, flex lines, e-barke cables, and rotors. then transfer them over to the new rear. it requires removing the 4 nuts that are exposed when you remove the disc. after removing those 4 nuts, you can then pull out the axle to remove the backing plate. this is if you don't want to pull the "guts" out. as most don't want to anyways. it's very easy and should take more than an hour.

 

One minor point-26 & 28 spline axle shafts on this swap.

 

The backing plates are not the same. The crap stuff will not bolt to the late-model backing plates or vice-versa.

It's a 9-bolt so the spline count (26 vs. 28) doesn't apply.

It would be nice to know what's wrong with the rear that's in teh car; it might be fairly easy to make that one like new again with the other one's parts, and maybe a couple of bearings or something. I know, if I had a PBR caliper rear that was ailing and a Saginaw caliper rear that worked, I would be looking for ways to sacrifice the Saginaw-equipped one in order to revive the other. But what do I know about cars anyway, especially these cars....

 

that's why i suggested that he swap the backing plates as well.

i have done this very swap on a 87' rear -------->89' setup. as long the the pre-89 rear has disc brakes, it should work out. drums have a different bolt pattern on the diff flange as well as slightly different axle flange(larger).

 

To change the backing plates he has to mostly disassemble the rear anyway.....

 

Thanks for all the information guys. The problem with the diff. is it shudders and whines during coasting, and it has started to make an audible clunk during shifts. (Its not the trans. mount BTW) Im guessing it started out as excessive backlash due to posi cone wear (or poor factory tolerances Ive heard), but it may have progressed to damaged teeth by now judging by the noises its making. The posi is damaged as well, I can hold one wheel and spin the other freely.

Just out of curiosity, what does PBR stand for?

 

PBR is the company that makes the aluminum calipers.... they're out of Australia.

Really sounds like the thing to do would be to put the guts out of the one into the other.

A worn posi will not affect the gear backlash. Worn pinion bearing however will.

 

DO NOT TOUCH THE INTERNALS. Replacing the internals require tools and experience that most people don't have. In fact it took me forever to find a shop that would just look at mine.

I've done the same swap - 88 9bolt from formula to 89 IROC. The 89 and up have the better rotors and calipers. All you have to do is change the backing plates for the brakes - 4 bolts on each side where the axle goes into the tube. Really no big deal. Then you can carry everything over.

::EDIT::
Oh yeah you will have to remove the axles to get the backing plates off. To remove the axles, take a block of wood and place it on the flange, then hit it with a hammer, axle should just pop out. Don't forget the posi additive