Building a strong 8 3/4 rearend.

Hi all. Looking for some ideas, tech and info for the 8 3/4 rearend. I was thinking the race section would be a good section to ask than the Pro-Tour section as this question is more for strength and durability then handling. I have a 1970 Barracuda and will eventually be putting an Indy headed 440 I traded for as a swap engine. It's speced for high horse power and figured the rearend in it will not survive due to my 'aggressive' driving style (also making very bad noises already). Though my car will be mostly a street car, I will try for a few times at a straight line drag and eventually Willow Springs Raceway. Currently I have a 1969 b-body housing I plan to use with 'Cass' Doctor Diff inboard kit with Hotchkis springs and Viking shocks. I don't have a welder to make a back brace, but I was thinking of improving the internals. Doctor Diff offers a new 3rd member case with his Helical Gear Sure-Grip ("Sure-Trac") that would work well and looks like the strongest option without going to a spool. Looking for axles, bearing, gears and WILWOOD brake options for this. I do not want to go to a DANA 60 due to weight, noise and price (out the door prices have put them at over $3.k before shipping).Rear tires will be larger than the 295/50R15 Cooper Cobra's I currently have on it, but like the profile to be low for driving and looking for a radio around 3:23/3:55. Any help/info would be appreciated.

There is nothing you can do to make an 8.75 live with weight and horsepower like you are talking about. Nothing.
Been there, done that. 3400 pounds and 10.30’s took one out i had on the 4th pass. The ring and pinion just wont survive.
Light and lower power, sure. Weight and power kill them in tandem

Here we go ........

9” or Dana 60 , or even Ford Explorer 8.8 , all better rear ends than 8 3/4

This will go on for days now ..... lmao

Yes, there will be guys come forward who have ran 8's at 5000 pounds with a stick and a stock 8 3/4 and never had an issue.

My experience mirrors Don's, the second pass my then new 408 hooked up it shucked the teeth on a pretty fresh 4.30 geared 8 3/4. It ran high 6.50's at 3260#.

I have never seen anyone slow down from adding a Dana. I have seen a lot of people slow down from shelling out an 8-3/4.

The best you can do isn't cheap , and won't be good enough.
Billet cap on the driver's side for sure.
Mark Williams sells them.
If you don't have access to a mill and measuring equipment , you are paying to have it fitted.
Some guys do both caps , but I don't know that it is necessary.
Good gears.
Excel by Richmond are made by the people who brought you Covid 19.
Regular Richmond are made in Italy.
US Gear are made here , but were recently acquired by an investment group , and I am having trouble getting the set I ordered through Strange. ( 4.30's for a 9" Ford )
A back brace is mandatory , and you cannot just fire up sparky and cook it on there.
The housing will warp.
Stitch weld , and take a long , long time.
You should move the leafs inboard if you stick with them , so you are moving the perches.
And I found out the hard way that the stock shock location and a back brace on an early A body will require some creativity.
I am not sure on the E body.
I don't bother with the 8 3/4's any more.
90 % of the racers on the planet seem to like the 9 " , and so do I.

You are already buying
Axles
ring and pinion
bearings
carrier (sure grip)
Buy all the same parts just for a Dana and get a Dana housing
You can buy all this stuff for a 83/4 then in a year or two buy it all again for a Dana in my experience.
The issue is the housing flexes and it breaks gears.
I had every part made to upgrade a 8 3.4 and it still broke the ring gear after 2 years of racing a 3600# car that ran 10 teens.
Take every ones advise and spend the money once

On your deal a used pickup core Dana 60 would be my choice to use based on me breaking many 8 3/4 rear ends bracket racing
I bent a 8 3/4 housing bad enough in my old pump gas street race Duster to make it break the welded on ladder bar rear end brackets off of the housing
Luckily it happened on the starting line and not going around a corner on the street
That car pushed a bunch easier after swapping to the cut down Dana 60 pick up housing, a bunch easier
No noise and maybe twenty Lb. heavier with the thicker pickup axle tubes with the 35 spline race axles
I'm a dedicated Mopar racer, Dana 60 came stock in the hi po muscle cars with stick tranny after 1965 for a reason

7 very informative post..... but still waiting for that “one” guy!! ....btw that one guy isn’t me. Stopped the bleeding and went Strange S60.

For your application buy a Ford 9 inch. Doctor Diff can build one for you that will drop in and be ready to run.

Yeah I run a 8 3/4 in my race car going high 8s but the car is light.. for the OP I would either
set up a Dana or a 9".. I dont think a Dana will fit without denting in the fuel tank

For what you want to do sell everything you have and go dana 60, you will never be sorry. I would ditch the indyhead motor and go g3 hemi too. If you are worried about pro touring and weight I think a good 200-300# off the front would do way more than worring about 50 in the rear. By the time you backbrace and add 35 spline axles, it might only be 25 pounds. The indy motor will never be as fun as a g3, unless you like to work on things. As far as noise, a properly set dana is silent.

I bought my 64 with a Stock 440 8/3/4 lasted 10 passes put the Dana in 1994 still there and cover has not been off since. You put them in and forget them for years, literally. By the way, when done you’ll have the same money in both, fords are more $ but they do have better gear selection
When I built the Dana I used the 8-3/4 35 spline axels, housing ends, bought used 4.88 gears and spool and truck housing. When done I might’ve had $300 in it. A friend bought an S60 for his Duster, right at $2000 from Dr Diff, setup and ready to install or it you scronge for parts the housing is around $600 from Strange and you fill in the blanks.

Unless you're a glutton for punishment, sell your 8.75 and move on to a stronger rearend that will actually take some abuse. If you make decent power and the car isn't a featherweight, you will fill the bottom of the housing w/ teeth.

You CAN build a Dana for your car from a truck housing for a LOT less than what a new rear costs if you're willing to put forth the effort.

You will not go slower due to the weight of a Dana...that's been proven over and over, yet people refuse to accept it.

A 9" IS A BETTER OPTION, but do the math on it and see what you're willing to spend. You have to realize a good 9" won't use any factory parts other than maybe the rear housing (not the center section). That rear housing also needs a backbrace.

I have under a $1000 in a cut down Dana 60 we pulled from a truck. I had way more than that in the 8 3/4 that was all new and died within the first year.

I ran identical times (11.41) back to back weekends with both rearends.

Weight-HP... I had an 8 3/4 in my slingshot in the 80's. The 340 maybe made 450-500HP. About 500+ passes before I sold it.

They had 8 3/4s in TF cars many years ago !

That old gEeZeR from BendOver Oregon mentioned this me thimks !

I forgot to mention that you should buy and use a steel cap on the drivers side of any 8 3/4 that will get abused by spinning tires that hook up quickly while spinning
8 3/4 ring gears get cracks in the outer edge of the teeth that eventually break off and fall into the rear end ending up getting jammed between the ring gear and pinion gear breaking both gears in a major way, which can lock up the rear end momentarily, NOT GOOD
Good luck on your car and decision

a stoopid question, [i'm sure] but those that have back-braced an 8 3/4 and moved the springs while running the brace to the very ends of the tubes, have you inserted tubes into the brace for the U bolts to pass through ?
if so, how did that work out ?

Before you weld on the brace just cut the holes in the brace for the bolts.. I did that years ago
when I ran leafs.. since that time I ran either a ladder bar or a 4-link.. the last 2 are easier
to set up.. the ladder bar needs one more brace for the street for cornering

Who here with a Dana has passed a 8.75" at willow Springs with similar hp/driver?

thank you mr. P.
i have narrowed several rears over the years, using an 1 3/4" or 2" bar, plus machined pucks for the ends and differential case.
some for leafs, some coil springs, and some coil overs.
these were followed by straightening the housings using a 12" piece of very heavy I beam to clamp the housing to, [it's what i had laying around] plus heat and a jack or press as needed. also to make sure the bearing ends were parallel with each other in both the vertical and horizontal planes, approximately 18-20" out from center line.
i just never back braced a housing before.

thanks again mr. P.

I didn't feel running the brace to the ends was really necessary. However the shock interference issue is real, and you can see in the link the slight "kink I put in my brace to miss the angled shocks. The rear end housing outside the springs is being thrust forward, the housing inside the springs is being bent backwards, and all torsion/twist is only acted on the housing between the springs, We are excluding any braking forces as, they are much less and "softer" IMO. I don't feel a typical back brace is very efficient at resisting torsional loads.

https://board.moparts.org/ubbthread...ay-protour-practice-car.html#Post1553866

Here is the 8 3/4 I had in my wife's 1968 Barracuda, it has Mark Williams caps on both sides (dad told them to put it on the wrong side the first time..lol), 35 splined alxes, spool, 4.30 gears, back braced and notched for shocks, boxed in spring perches, 5/8 suds, Strange billet 1350 yoke and then dual caliper Strange disc brakes.

Never broke on me running at 3,600lbs with 700hp motor running 1.43 sixty foots but I knew I was on borrowed time and called up Doc Diff and ordered a Strange S-60 that bolted right in, so now this sits on the shelf for the next project.

Probably shouldn’t throw this out there but..
I was told this long ago by a racer-engineer that this will make the 8-3/4 street gear last longer. He said they were hard and brittle to last a long time in a regular car but not good for racing, this process (in oven) will soften them up so I wrote it down. Never tried it but?..

I believe the concept is valid, but not sure if the numbers are spot on.

I think I have taken the 8 3/4 as far as you can. I cured all the issues with durability up to the point where I started shearing off pinion shafts. Everyone beefs up the wrong parts in my opinion. A stock case needs support mainly in the caps. The housing must be reinforced to support the caps not only from the rear, but also (and mainly) from the sides. The biggest problem with high power going through the 8 3/4 is that the case spreads. I doesn’t take much at all to flex the caps away from each other. You can easily confirm this by measuring the caps from outside to outside before pre-loading the spool bearings, and then after.
Putting supports in the case in both directions eliminates the gear chipping problems, even with stock caps. My car was 3700lbs, trans brake, mid 1.3x 60ft. Sub 9.50 times with stock case and gears. Started feeding it power and it began shearing pinions. At that point there is nothing that can be done to make it live. Went to a 35 spline pinion 9” ultracase from strange. Never had to worry about it again.

Yes, I would agree, once pinions start to shear, that is the end of the line in upgrades. But curious, which pinion, 742 or 489's?

Its been a while since I performed this heat task. Only worked for a specific gear series by Richmond, if I remember correctly. Did 3 sets for a 426 wedge car that I had making 500+ HP @ 3600 lbs. 3 years with many passe with a J converter and 4.88 gears. Sold the chunk and did another with same amount of time with lots of street racing too. I would not try this today. That formula was for a specific steel and treatment of the time. Most 8 3/4 gear are either softer ar brittle.

all tese stories have me nervous taking my 8.75 to the track not running that much power only 520 hp 560 torque at the crank in a 3150 lb car people tell me if my tires hook the dif will break , guess the Dana is the only way to go for reliability . like you say py me now or pay more later . maybe i'll sell my 2 8.75 A body difs and have enough for the Dana .
interesting thread

I don’t think this thread has addressed traction enough and it’s affect on 8.75 strength. On a street car that may see some track time, but still shod with DOT rubber, the 8.75 has proven capable over the years.

With a 4 speed and slicks, and a decent amount of torque, things change.

Has the OP even stated if the car is an auto or stick?

Probably the best bang for the buck out there is the ford explorer (2000- 2006?) 8.8 rear. You can usually buy one from a junkyard for 100.00 to 300.00 . They usually have 4.10 gears with a somewhat decent posi unit, and factory disc brakes. My son bought one from a junkyard for 150.00. We put new perches on it, bolted it in and ran it......Stock axles and all, even left the junkyard fluid in it ! ! ! It literally has hundreds of passes on it , 11.50's to 10.90s in a 3200lb duster. Cant get much better than that. THen when you upgrade your power its not that big a deal to upgrade the 8.8 and there are a ton of aftermarket good parts for it, and its light and strong enough to handle 7 second HP.

I completely agree a dana 60 or 9" when starting from scratch is a better option. I wouldn't be so nervous on 8 3/4 for occasional runs
8 3/4 in wife's car normally 60' in the 1.40 zone would be little better if we took the small wheelie out of it. Car weights #3200 with her #3300 with me. Probably 120+ runs now and a dragweek trip.
Typical run is 10.1 @ 133 best is high 9's one 9.8 with small shot of spray
Ladder bar car with back brace, gears installed by me, only third set of gears i installed so not much experience.
I do know i'm on borrowed time and agreed the dana or 9" is better..

i completely forgot about the shocks ! [CRS and "oldtimers" i guess. ]
i also have a sacrificial center section cut like yours, although it is a 489 case.
i have checked several rears before shortening, and have found some to be off almost 1/4" from straight. these were rears i know for sure weren't beat on. i did a pontiac rear one time that was off a tad more than 3/8" ! this was going in my buddy's 55 chevy, and needed to be cut 1 7/8" per side. we cut it at the spring mounting pad, then boxed it in around the tubes. a little heat and the bottle jack straightened it right out so the pucks would glide into the bearing bores with just a flick of the finger.
the reason for the 1 7/8" per side cut allowed stock oldsmobile axles from the "short" side to be used. i forget the application, however.

Anyone ever tried the Moser fab housing 8.75?

I never found the need for them.. where a failure accures is the carrier.. thats why I went to
the alum MP carrier its much stronger with thu bolt for the caps that carrier dosent let the ring
and pinion separate like the stock one

I think if I were doing a back brace I would dimple die it too. Those do make metal quite a bit stiffer.

Thanks , I'll lok around for an old Explorer diff , does it make a difference 4x4 or 2 wheel drive ? I had an old explorer i think the carrier was offset if I remember correctly

I don't think it matters 2 or 4wd, yes one Axle is longer than the other. On my sons we just cut the old mounts off, centered it under the car and welded on new perches.

This has to be the best junkyard rear ever...4 series gearing, posi trac and disc brakes

I will never run anywhere close to 10 sec , I'll be lucky to be in the 12's so guess I'm safe besides under 11 sec I would need a full roll cage that is not going to happen, daily driver in summer . plus insurance is hard to get and expensive once you put in a full cage . feel better about the 8.75 now . thanks

You’ll just have a fancy looking 8.75.... that’ll still break

.... I’ve got picts on my phone that says otherwise! At least when the aluminum one lets go you get style points!

Well consider, 11g is a bit tough to dimple die, and IMO the housing is resisting two primary forces in the back brace area, torsion/twist, not sure how much dimple die would add to resist that, and that calculation is above my pay grade, and bending, from the forward thrust forces, which bends the center span of the housing rearward, or the opposite of the tire side of the spring pad(?) which is bending forward, meaning the back brace is mainly under tension, and adding dimple dies adds little stiffness/strength. Now if the brace was in compression, I agree, your suggestion would likely be beneficial.

Thank you to everyone for the info. It was helpful and informative. Kinda sucks as it’s friggin RIP to the wallet as funds are starting to get tighter (the grocery store chain I work for stopped ‘hero pay’ though we are now the most infected state and have higher work demands required for us). So from what I read, if it was abused in anyway, 8 3/4 would go to ‘rearend heaven’ and would just be better to upgrade to a different rear all together. What makes this even more RIP is that I can no longer use the rear swaybar I had for the rear and it was a modified factory unit and it won’t clear the DANA 60 (if I go that route) and may have to rethink leaf springs with Dr Diff relocation kit I had bought for my assembly. Gonna have to check my HP numbers and see if it’s still a ‘do or die’ with the rearend.

Friend of mine put one of these 8.8" Explorer rearends under a Chevy S-10 Blazer that ran mid 5s in the 1/8 w/ a 496" BB Chevy on nitrous. Still had all the stock internals as far as I know. Never hurt it in several years of racing before he sold it.

What's the stock width on the 8.8? Is it close to 68 B-body?
Thanks

im pretty sure its 54" It fits in an A body pretty good, so it should fit a B body very nice

Thank you

The width is different depending on the vehicle it came out of

Guess I didn't specify but was referring to the Explorer axles mentioned in this thread. Need to find one cheep or it wont matter anyway.

Do any of you guys that bad 8 3/4 rears still have any of the parts? I am looking to see if I can save this 741 clutch type sure grip w/o getting a whole new unit. I need one side of the unit that keeps the pin from spinning, as it is starting to happen with the unit I have. I am trying to save it as it has the gears I want and it is the same 741 as my car is. Here is a picture of what is going on with mine.
Thanks

Sorry but the 2 units I have are both in use.. your unit is getting hard to find.. good luck
EDIT
I should have said the 2 units I run are the 742

That’s a cone type, and it’s toast. Start over with a clutch type powr lok.

You are right, Pac, it is a cone type... all this time I thought it was a clutch type and I went back at looked at it, cone