looking to find out what rear ends came with posi and what ratio gears. going to see if i can find anything in the junkyard.
also is there any way possible to determine what gear ratio it is without taking the cover off?
would be appreciate i performed a search didnt find much
thanks

 

If you can get the vin number on the car you're pulling it from you could get the info...but assuming you're just looking at diff's I dunno how you'd tell. I believe all the LT1 cars came with posi or locking diffs.

 

thats great if they did , its alot easier to lift a hood. still wonderin about gear ratios though, is there any website i can go to enetere the vin and figure out what model and features it came with ??

 

There are codes stamped on the front of the pass side axle, right inside of the sway bar bushing - but they are rarely easy to see, given that the axle rusts - but these codes will tell you posi or not and gearing - and the decoder is in the tech articles here. But like I said - very hard to see and read.

The only REAL way to tell 100% on gearing is to pop the diff cover. You might could assume that all disc rears are posi, but I'm not even sure on that. Removing covers only guaranteed way - but sticking to disc rears would be a starting point at least. On a disc rear, make sure you get emergency brake lines as well so you have them for the rear swap. And you'll want the proportioning valve from under hood as well.

 

thanks alot appreciate it i figured there had to be some coding some where . does anyone know if rear discs means posi ? also has anyone found anything "interesting" when swapping rear ends ? i rather not have to run back to the yard and pick up some lower contorl arms because mine ar a little short

 

I just swapped a 4th gen rear into my 1992. Nothing fancy - just a swap - except for the brakes. 2 things on brakes.

1) Emergency brake - like I said - get the e-brake cables in tact from the 4th gen rear. If it's an LS1 rear, they will be too short - but you can cut the old cable ends from your 3rd gen rear and attach with cable clamps to make up the length - all else will fall into place on the E-brake. See my post this thread:

https://www.thirdgen.org/techbb2/sho...4&goto=newpost

2) Brake line pressures - the disc rear needs (as I understand it) less line pressure than a drum rear. Either a) snag the proportioning valve from the disc donor car (the easy solution) or b) get an adjustable brake pressure valve from Summit, etc. and attach in-line between your proportioning valve and the rear brake line. I say a) is the easy solution because I'm having a hard time finding the right fittings to add an adjustable valve inline - it can take days of running back and forth to the store to get the right combo of fittings sometimes.

One other note - you change gearing, you'll need need speedo gears for the speedo to read correctly - there is an article on this in tech section, telling what gears to get for the speedo. You'll have to remove tailhousing on tranny to swap speedo gearing. Gears can be gotten at 700R4.com.

Some other considerations when swapping rears:

1) Fuel Pump - your 1/2 the way to replacing a fuel pump if you remove the rear end - your pump is how old? Think it over. The exhaust has to come out as well for the fuel pump. See below for a fuel pump link.

2) might as well get those poly sway bar bushings and end link bushings while your there.

3) shocks - they have to get disconnected - might as well put in new

4) coil springs - you can grab these from the 4th gen when your there - when you swap rears they'll fall out - the 4th gen springs will be stiffer (less age sag) since they are xx years newer, right?

5) rear tranny seal - see if it's leaking while under there - they are cheap, and the rear is jacked up, so the tranny fluid won't come out - remove drive shaft and replace

6) new tranny mount - you'll be removing tailshaft housing for speedo gear swap - might as well do this.

7) torque arm mount - again - with the tranny tailhousing coming off for speedo gears - might as well get this replaced too

8) If you do the fuel pump, exhaust has to come down (including I-pipe). Nows the time to get a new higher flow muffler, and cat, since you have to remove 1/2 the exhaust anyways for the fuel pump. I've got a breakdown on fuel pump at:

https://www.thirdgen.org/techbb2/sho...4&goto=newpost

Good luck - and if you get a 4th gen rear, feel free to email me with any questions - I'm (hopefully) almost done with my swap.

 

thanks for all the info great help. im planning on doing it after i get my k member from pa racing.

i have another question? will our sway bar fit the 4th rearend , cause i might end be picking up the front and rear sways from spohn.

no need to waste money
appreciate it

 

Yes, sway bars the same. The LS1 rear I got had a sway bar attached - didn't change it out (yet), as that's easy to do later (when cash avail).

 

So to clear this up for a dummy like me, the 4th gen rear diffs (any year?) will be a direct fit on a 85 Z28? All the control arms, panhard, springs, etc. will be the same?

What are the rear diffs on the 4th gens anyhow? The same 9-bolt aussie diffs from the late 3rd gens? If not, are they strong compared to my 10-bolt posi from the 85 Z28?

Thanks

RV

 

The 4th gen rear is the same as the 10-bolts in the 90-up 3rd gens; and not too different from the earlier ones (# of axle splines).

The 9-bolt was discontinued after 89; although there are reports of a few 90 cars, presumably early production, that still got them.

The 4th gen 10-bolts are about 1-3/4" longer on each end. Which means that if you put a 4th gen rear in a car and put 3rd gen wheels on it, you get that stupid silly rĄcer "skateboard" look. However, a 4th gen rear with 4th gen wheels, is exactly the same width as a 3rd gen rear with 3rd gen wheels; so they're directly interchangeable, as a set. You can also use spacers on 4th gen wheels to make them work on a 3rd gen rear. However, unless you know of a way to build a spacer that makes things thinner instead of making them thicker, there's NO WAY to use 3rd gen wheels on a 4th gen rear and have it come out any way but looking stupid.

There is no particular strength difference in a 3rd gen 7.5" 10-bolt and a 4th gen one. The strength of the parts does not respond to the sheet metal wrinkles.

The posi units are a different matter. Depending on which posi your 85 has, the 4th gen could either be somewhat better, or ALOT better. And of course more than likely would have 10 years or so less wear. But without changing your axles to the 90-92 type, you can't use a 4th gen posi in a 85 10-bolt.

 

Thanks for the info.

I would be changing the entire diff assembly, not just the posi carrier. But if the overall diff isn't much stronger than my current 10-bolt, I could just spend the money on a aftermarket posi carrier.

 

There is an easy way. Put a mark on one of the axles (or tires, if they're attached). Make sure both wheels or axles can turn freely. Now, slowly turn the driveshaft (or pinion yoke if the rearend is out of the car), counting the number of revolutions while a helper watches the axle (or tire) turn. Have them stop you after it has made one full revolution.

What you're doing is checking the ratio. If you have a 4.10 gearset, for example, that means you have a rearend ratio of 4.10:1. The ring gear (and axles and tires) will turn 1 time for every 4.1 times the pinion (and the yoke and driveshaft) turns. So, if you have a 4.10 gear, the driveshaft or pinion would turn 4 times and and a nudge for every time the wheel or axle turns. If you have a 3.42 gear, the pinion yoke or driveshaft would turn just under 3 1/2 times (3.42) for evey time the axle or wheel turns; if you had a 2.73 gear, you'd get about 2 and 3/4 revolutions (2.73) of the yoke or driveshaft for every turn of the wheel or axle.

 

Given that it's only 1.75 inches longer, the look really isn't that bad - I just swapped a 4th gen rear, and the 3rd gen tires still tuck under the fender on my 1992.

In other words the 3rd gen wheels will stick out on the 4th gen rear, but as I said, mine still tuck just under the fender lip - it's only an extra 1.75 inches each side. 4th gen wheels on a 4th gen rear will sit the same as 3rd gen wheels on a 3rd gen rear.

BUT - spacers must be purchased for the FRONT if 4th gen wheels are used on the front - otherwise they are too far IN the well and won't clear.

I picked up a complete 4th gen posi LS1 rear with LS1 disc brakes, AND the complete set of 4th gen wheels for $225 locally - much cheaper than gear sets and installation. For the savings, I can live with the 1.75 inches, and since I got the wheels too, I can later decide to buy the front spacers and switch to 4th gen wheels all around - something to consider if your on a budget.

It's an opinion thing - some say the extra 1.75 inches is ghetto, some like it. You can search and find some pics with the 3rd gen wheels on a 4th gen rear, and see the stance. For me it's all about the cash at the moment - I take what I can get cheap! LOL