Hello everyone, now that spring is around the corner my wife wants me to get what parts I need off of my 1984 Trans Am parts car and get rid of it. My project car is a 1991 Firebird v6 base. I was wondering if it would be worth swapping the 84 rear into the 91. The 91 came with 3.23 gears and drum rear and the 84 came with 3.73 gears and disc rear. I'm reading that the rotors on the older cars are smaller and it's making me wonder if I'll find an appropriate brake proportioning valve. I've read that the proportioning valve in the 84 uses SAE fittings while the 91 uses metric, can I just get adapters and use the 84? Should I even go through this headache? In the end (years from now) the 91 will be getting either a 10 inch or a 12 bolt and the 454 that's in the shed. Thank you

 

IMO, no. You'd be putting in a rear with untold miles, no idea how bad it leaks, how much noise it makes, etc. It has the smaller (26 spline) axles. It has the crappy disc brakes that most of the time for most people don't perform even up to the poor standards (in 2022 especially) of drums.

Never seen any such "adapters" that would FIT THE CAR; sure, you can get ones that fit THE LINES, but will they move the brake lines to somewhere that they interfere with something, or whatever? Not what I'd consider a viable "solution".

84 might have metric fittings, anyway. Sometime in that year was when they changed over.

Apart from the threads though, will that old cast-iron valve, after MORE THAN 35 YEARS, be in anything resembling "peak" operating condition? Just an altogether BAD idea to move that over to a newer car (even though that one is also an ANTIQUE). You'd have to use the MC for those old crappy discs, as well; it's different from a drum one, in more ways than just the fittings.

IMO, no. Too much maze and not enough cheeeeze at the end.

If you just want the gear ratio, you'd be better off to just buy a new set and put them into your 91 rear, than to make this particular swap.

 

If you want 3.73 gears, do it. They're not going to be an ideal gear for highway driving, or the powerband of some engines, but they're perfectly drivable. And if you hate them, you could put the 3.23 rear back in.

Brake lines are fairly reasonably priced to buy reproduced these days, or bend yourself. However you "adapt," if needed, won't be a bank breaker.

If you bend up a line or line(s) to use the 84 rear in the 91, just plan on retrofitting the 89-92 disc brakes and lines onto it when you have time.

Why should you consider it? The housing flange is 4-bolt pattern on the disc rear. The drum rear is a 3-bolt pattern that needs modification to add discs.