I've got a 84 Z/28 H.O. with a 383 stroker and 700R4 and I believe 308 geared open dif that I had a mini spool installed into. I hate the minispool and am going to replace the rear with one from my local pick a part. I was gonna buy the rear end and have it built with a posi, while I'm still running the old one in the car.

I'm not possitive it's a 308. I guy I know supposedly looked it up and according to him a 308 was what was run in the H.O. version of the Z/28 in 84. I know it's the original rearend in the car.

My question is this 1. Is it really a 308?
2. If it is what is the next step up to get a little lower highway cruising RPMs? I'm running between 2500 and 3000 RPMs at 60 or so. I'm willing to sacrifice a little off the line grunt ( I've got plenty to spare)to get better cruising on the interstate. Any advise you guys can give would be great. Thanks!!

 

If the rear is a 3.08, it's not original to the car. The HO (vin G) came with 3.73's. IIRC, 84 was the only year the 700R4 fronted the 3.73's. Also, the RPMs you're seeing are much too high for a 3.08 at 60 mph--more like 3.73's. Furthermore, a 3.08 wouldn't give you any "grunt to spare" off the line--more like 3.73's. Notice a pattern here ? Drop the cover and take a look. Numbers are stamped on the ring and pinion:

41 : 15 = 2.73
41 : 14 = 2.93
40 : 13 = 3.08
42 : 13 = 3.23
41 : 12 = 3.42
41 : 11 = 3.73

By the way, if you go from 3.73's to, say, 3.23's you'll achieve what you wish for cruising, but you'll lose a barrel of fun.

JamesC

 

i got 3.23..........i like em, gives you a good mix of off the line and highway

 

I'm positive that the rear end is the original. Most of my off the line grunt is lost in tire smoke. LOL. So going to a slightly higher gear probably wouldn't hurt my take off. Mine burns the tires all the way through 1st and well into second. So a slightly higher gear might help my take off. You think 3.23s would be a good step?

The main reason for doing this is gas mileage. I'm using a tank of gas every 4 days driving to work and burning almost a 1/4 tank for every half mile race. In my streetracing group around here we're running half mile runs. Don't know why. My trip to work is almost all interstate driving. There's not much I can do about the racing but highway I'm hopeing to improve. 3.23s you think would work? If I smoke the tires that much with the 3.73s will the 3.23s really hurt my take off?

 

its going to hurt your take off some, but i think you will find that the lower cruising RPM may be worth the trade

 

Are you thinking of swapping the complete rear end or just the gears? Do you have drum or disc brakes on the rear?