DirtySpleen

Okay, I know that this question must be asked a million times a day, but here it goes. There are a few roads I could go with my rear. I plan on swapping the almost 180k mile L03 for the GM 290 horse 350 at some point possibly within the next year or two. It would most likely remain rather stock minus a few bolt ons just for kicks. I am very big on autocross and drift racing (at the speedway we have open drift weekends every 2 weeks) and as such, the peg leg needs to go.

Option 1: LSD the 10 bolt. My thought process? Costly but reliable as a regular daily driver.

Option 2: Save that money towards a new stronger 12 bolt or 9". My thoughts on this? Very costly, might be overkill on a non built 350, would have to get new brakes which means more cash, but also will not have to worry about any failure whatsoever.

Option 3: Lock the 10 bolt, save cash for option 2 when the swap happens. My thoughts? cheapest option, not dumping money into the 10 bolt, better performance, not 100% sure on highway manners, eats tires a bit faster.

Im leaning towards option 3 due to the cost efficiency, and I am pretty sure the 305 won't be snapping my 28 spline axles in my 91 RS. What have I missed, What are your thoughts, experiences, 2 cents on all this?

Thanks!
~Spleen

big gear head

Forget option 3. VERY BAD IDEA!

If you can get by with the extra 4 inches then a '98 to '02 rear end swap would be the cheapest best option. The Torsen differential in these rear ends is much better than any of the older ones, and you get better brakes on it too. You are not going to hurt it with the engine you are planning.

DirtySpleen

To clear this up, what exactly is the difference in strength between my 91 rs' 7.5" 28 spline 10 bolt vs a 4th gens which is the exact same? I understand some non rs' are 26 and thats understandable but what is the difference for me? Minus the posi which would probably need a rebuild and the brakes and that extra 4"

big gear head

The '98 to '02 has the Torsen differential as it's LSD. All others had the Auburn, or in some rare cases, the Gov Lock as the LSD. Lowest cost option is to swap a '98 to '02 rear end with the Torsen differential into your car. You can get these pretty cheap, but they are 4 inches wider. You also get a good set of brakes with it. If you know how to set the backlash and bearing preload then you could just swap the Torsen into the rear end in your car. Still pretty cheap. If you want to go with a new differential then the Eaton Truetrac would be your best option. You don't need the extra strength or weight of the 12 bolt or 9 inch, and welding the differential or using a mini spool is just an extremely bad idea.

DirtySpleen

Well the local shop said that A TrueTrac install would be $1500 on the lowest end.. holy crap!

big gear head

Go somewhere else. That's crazy. The Truetrac is about $400 and installation should be around $200. I've got a thread that tells how to install it yourself if you want to give it a try.

Ron U.S.M.C.

A compleat Eaton Posi and gear ratio change with support cover cost me =
https://www.thirdgen.org/forums/tran...de-photos.html

supr_fly03

iTrader: (2)

My rear end is a bastard built out of multiple vehicles on the cheap. It is the original 1983 housing with 90 f-body 28 spline axles, an auburn limited slip from a 96 transam, 4.10 gears from a 4cyl S-10, and brakes from a 01 2wd blazer with modified blazer e-brake cables. All pulled from the junk yard and now have been under my Z28 for more then 7,000 miles without an issue. I did it myself and it is all in this forum somewhere detailed enough to replicate.

Performance you ask? Well with the T56 and LT1 it is a whole lot of fun to drive and easily becomes tail happy if you want. Hind sight I would go with 3.73's instead but given what you are looking for 4.10's would probably suit you best.

1991sleeper

iTrader: (-3)

Will ls1 rear brakes bolt right up to the 91' rear in my bird ? N how much of an upgrade is it really ? What about front brakes? bolt in?

91' bird 305 TBI T-5, trick flow heads, 4.10 gears, full UMI setup, track times coming soon!

supr_fly03

iTrader: (2)

Well I went from drums to disk and LS1 front brakes at the same time. I run a Wilwood prop valve and a hydroboost. Did it make a difference? Yeah big time!

As far as making a drum to disk swap, the modifications between Blazer brake backing plates and LS1 is the same. If I wanted to I could bolt LS1 stuff right no but I would only gain a 1/4" in rotor size plus I like the drum style parking brake setup over the caliper parking brake of the LS1 setup.

Front LS1 brakes are much more work and require a bracket. I had a buddy with a CNC plasma table and he cut the brackets for me. I also scoured the junk yard for the spindles with the extra meat around the backing plate holes that need modification.

I dont know if you would gain much by swapping the rear brakes if you already have disks on the back. I am sure there are plenty of threads to read over on that subject.

Sorry for the thread hijack.

DirtySpleen

Another shop quoted me $600 for JUST the install. better, but for those prices im nearing just getting a darn 9"!

InfernalVortex

iTrader: (4)

NO! That's a HORRIBLE engine! It's a smogger 350 with horrid compression that they threw a big cam into. Lazy and slow and nowhere near 290horsepower. You want the Vortec L31 crate:

http://www.jegs.com/i/Chevrolet-Perf...rentProductId=

This is BY FAR teh better engine and is WAY faster. WAAAAAY faster and WAY better on gas mileage and long term reliability too. Im not sure what the price difference is, but I assure you it's worth it. Dont pay attention to the GM power numbers, they're not measured the same.

I also agree with the guys that the numbers you're getting for labor to install these parts is absolutely insane. There's NO WAY it should cost that much. Keep trying.

Also, welding the diff is great for drifting, not for anything else. You will HATE autocrossing with a welded diff. All understeer, all the time. If you weld the axle for drifting you may want to be ready to routinely uninstall and reinstall it for drift events.

It really is a tough thing... for drift you want something that locks as eagerly and hard as possible, but for autocross you want something that allows the car to rotate as much as possible without allowing on throttle tire spin. This is what the Torsen is so good at, but I would imagine the Torsen isnt nearly as well suited for drift. It will work for it, though, better than a spool/welded diff will work for autocross.

big gear head

$600 labor is too much. These guys must be paying a lot of over head, trying to get rich off of you or just trying to tell you that they don't really want to do it.

supr_fly03

iTrader: (2)

I agree with the cost of hte gear install. I charge $250 to do a basket case axle that comes in a box with nothing in the housing. It is a lot more work that way to set the axle up since you have to guess on the shims unless you have a pinion depth tool which I dont have.

cason152

I have an 84 trans am H.O. i want to swap out the stock 3.23 gears, and i believe a 10 bolt, 7.5 (havent seen the car in a minute). Im just trying to really figure out what spline the pinion is going to be. Ive counted the splines once before, and i got 27. but it was kinda hard to see, and i may have missed one or etc. so im wondering if anyone knows for sure what spline they are stock.

big gear head

27 spline pinion. You will have 26 spline axles.

cason152

Thanks. Appreciate it.

mjw2993

If I was to swap out my rear end from a 10 bolt drum brake to a 12 bolt disc. What all would i have to swap out? i know i have to take one of the valves out of the master cylinder but which one? And what gear ratio should I go with in the 12 bolt, I have a remanufactured 350 and a rebuilt 700R4 trans with a shift kit.

James Sutton

iTrader: (2)

Useful site for repair estimates:

http://www.automd.com/repaircost/