I have the opportunity to purchase a relatively low mile 9 bolt rear end out of a 91 Z28 with the 3.23 gears and disc brakes for $350. Right now I have 2.73 gears and drum brakes. I've been thinking about doing a gear swap since I bought the car but I'm not sure if its worth it. How hard would it be to swap this out? Would I feel a noticeable seat of the pants difference? Also I do most of my driving at 60 mph and get a pretty good 24 mpg right now. How much should I expect that to drop if I do this swap? Anything else I should know?

Thanks!

 

I could be wrong, but I thought '89 was the last year on the 9 bolt rears.
I'm pretty sure the '91 came with a 7.625" 10 bolt.

Also if it's a 9 bolt it would be a 3.27 gear, not a 3.23 gear. 10 bolt would be a 3.23.

Yes the change from 2.73 to 3.2X will wake the car up and feel like a whole different animal.

The swap is easy really. Going from drum brake to disk brakes you will need a disk brake prop valve and E brake cables.
It's all bolt in swap. Heck the rear end is only held into the car with around 7 or 8 bolts, brake line, and 4 bolts on driveshaft.

I have air tools and done it a few times but from start to finish I can swap a rear end in these cars in under 3 hours... That's jacking it up, putting stands under it, removing/lowering exhaust, taking rear out, putting another one in, hooking everything up and bleeding the brakes.

 

There's not a huge difference between 2.7 and 3.2. Back when I was in Maryland I had an 89 with 350 tpi and 2.77 rear, a buddy of mine had practically same car with 3.27. I would run 14.7-14.9 he would run 14.4-14.6.

That price seems kind of high to me, I would pay that price in Germany, but not in the states.

 

Yep. Check the 10-Bolt FAQ sticky for info about combination valves, parking brake cables, master cylinders and so forth.

JamesC

 

swap is very easy. stared at 8am and was done by lunch. and did not use air tools.

 

I lost about 3 mpg going from 2.73 to 3.73 gears. I had figured it would be more dramatic but it wasn't.

 

Thanks for the replies, if it turns out its a 10 bolt would it still swap in fine?

dmccain- I'm pretty surprised it wasn't more of a drop, that makes me wonder if I should hold out for a 3.42 instead.

 

I wouldn't pay more than $200 tops. The LB9 / 2.73 combo should do way better than 24 MPG, unless you're running 80 MPH all the time, in which case MPG isn't important. I had a near-stock carb'd 350 with a 700R-4, 3.08:1, and 235/60R15s on 8s do 25 MPG at 65 MPH in a second gen with the centrif. advance not working. It had the potential for 26 MPG had I figured out the dizzy sooner.

 

3.23 is a pretty good gear for a TPI motor.

If it actually came new in a 91, it will be a 10-bolt. Last year of the 9-bolt was 89.

Plus, that 91 rear will have the good PBR discs, and 28 spline axles.

I'd swap it. Personally I'd be willing to pay $350 for it if I was in your situation, not a bad price at all for that rear, really. About right IMO.

Your odds of finding an otherwise identical one with 3.42 are so tiny as to not be worth even thinking about. There never were many to begin with, and for the most part, people who have them aren't selling.

 

Well I had the owner send me some pics and it is actually a 9 bolt. He either doesn't know what year it came out of or it had been swapped into that 91 at some point. Is there an easy way to confirm what gear is in this thing?

 

Appears to be an '89 9 bolt due to PBR brakes...gonna be 2.77 or 3.27, pull the cover is only way to be accurate.