im off to the wreckers fairly soon. next couple of days anyways.
im making a list of things im going to look for. and one of them is the
4th gen rear and the 4th gen alum. driveshaft with the balencer.
my question is.
how much power can the rearend take?
im making an engine that will make about 500ish hp. will the rearend hold up to that kind of power? i was planning on getting a 12 bolt or 9" rearend. but there way to expensive for me at the moment.

 

Until it breaks.

That one will take just barely less than however much broke it.

I broke my first 7½" 10-bolt with a 305.

I kept another one alive for years and years behind a 400.

Go figure.

Luck of the draw.

 

Fourth gen diff is just as strong as a third gen diff. It uses larger 28 spline axles but so does the 90-92 third gen and the older ones can be converted from 26 to 28 spline axles. Third and fourth use the same 7-1/2, 7-5/8" ring gear so they are equally weak.

The factory aluminum driveshaft isn't very strong. It's a very thin walled tube.

I pushed a beefed up 10 bolt into the low 11's before switching to a 9".

 

4thgen rears, for all intents and purposes, are the same as 3rdgen rear ends. They are both weak. The only real difference is that the 4thgens have slightly longer axles and tubes, and usually disc brakes on the V8 cars (that might be worth the upgrade if you have drums now).

 

yep they are pretty weak but some break with low power, some break with lots of power


i think the driveshaft is weaker than the rear itself tho. i know alot of ppl breaking those before the rear itself

 

oh ok thanmks for the info.
ill have to see what my rearend has a 28 spline axle since my car is a 1990.
if they are the exact same i might "beef" up the rear end and see where that takes me. i could probably try and find and see if i can find a 12 bolt or a 9" at the junkyard. but i dont want to have to modify the axle it fits. it might cost me just as much as buying a brand new one with the gears and everything that i want.

right now my rearend on my car is 3.08 gearing non posi. im looking somewhere near the 3.78 or whatever that gearing is i cant remember or a 4.11 gearing.

i was reading that the driveshafts from the 4th gens were just as strong 3rd gen stuff.
im starting to disagree now with what all you guys are saying.

well insted of looking for a 4th rear. should i look for a 12bolt or 9" while im at the yard? would it be worth the hassle or just save up the money afew more months and buy one already built made for are cars?

im just trying to figure out what my options are for the budget i have.
i was thinking about $1000ish for a rearend. cheaper the better. if i really have to spend an extra $1000 ill do it. but i rather not lol more moeny for other parts for the car

 

Unless you have good fabrication skills, a junkyard 12 bolt or 9" will probably cost more than a bolt in version because there's no easy way to attach the torque arm unless you plan on eliminating it with some other type of rear suspension. The key to finding a donor differential that you plan on adapting to fit is to find one the same width as the third gen diff. Trucks are wider. 12 bolt truck diffs are actually weaker than the older 12 bolt car diffs. They use a smaller diameter pinion. Trying to find a 9" from a late 60's early 70's pony car that's roughly the same width is difficult. Trying to find a 12 bolt from the same era f-body is even harder.

Buy the aftermarket 9" housing/axle package for around $1000. Get it with 31 spline axles then look for a center section from the junkyard that uses 31 spline axles. If you're lucky, you can find one with a posi and deep gears from something like an early Bronco or a maybe van. You can probably install the whole thing for less than $1500 worth of parts.

At least you're trying to buy expensive parts when our dollar is just about at par with the US dollar. When I was buying all my stuff, I'd look at US prices and add half that to calculate a Canadian price. That $1000 diff package would have cost $1500 5+ years ago.

 

my fabrication skills are very poor right now. im acturally in the practice mode right now. i would have to take it in and get them to do it for me and that would probably be big bucks to be done.

i found a complete package and has rear disk brakes and gears, the whole package and there about 2grand. im probably going to haveto go that way. its all ready to be bolted up. not really looking to spend that much money. ill have to do some shopping around. ill check ebay. they seem to have some good deals on.

i checked spohn and they have lots of stuff for rearends it seems. not aton but enough.

 

i just checked out the Spohn site and looked at the rearends. i found a moser 9" for $985 i have a question before ordering.
the Axle Splines. they have
28, 31, 33, 35, 40. with the 40 spline. it will cost you $100 extra.
you were saying to get a 31 spline? would a 35 spline be better?

i think i can find a used 9" gears or shop around for one for fairly cheap. you never know what you can come across!

so the only difference with the 4th gen rear is that there only 2" wider then stock 3rd gens? why are so many people swaping them out if they are the exact same as the 3rd gens? is it because its the posi? or they want to have a 2" wider rearend?

 

Because they have better brakes, a better posi, and (usually) fewer miles.

 

oh ok. it all makes sence now.

 

The 4th gen axles aren't great but they're not horrible either. It really depends on what year car they came out of, posi or non and how many miles they have on them. 2 years ago I bought a complete axle with ebrake cables and everything out of a wrecked 98SS. The car had less than 10k miles on if before being t-boned. Since it was a 6 speed car the axle has 3.42s in it so it was a nice switch from my previous 2.73 drum setup. I've made about 40 track passes and probably 8k miles give or take with it under the car. I have a mildly built 355 in the car and a T5. Which is currently good for 13.5s at just over 100mph on street tires. That being said I've seen these axles give up the ghost with out much power infront of them. If you go the 4th gen route try and find one out of a 98 or up LS car as they have 12" brakes with the drum style ebrake and a better posi. For added insurance I would recomend an aftermarket diff cover to better stabilize the main bearings.

 

Right: as far as junkyard rears, that exact one (98-up V8/T-56) is probably the best one there is to get. Good posi, good brakes, great gear, fairly new; all in one easy-to-install package.

The axles are identical to 90-92 3rd gen ones, just 1-5/16" longer on each side, no real "upgrade" there.