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  #1  
Old 11-18-2004, 01:30 AM
turbotim23 turbotim23 is offline
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Default what is the most h.p. a 8 3/4 rearend can handle ?

Hi i'm building a 671 supercharged big block. My goal is 550-600 horse power. It's going to be underdriven,trying to keep the motor and drivetrain together.I worry the most about the rearend. It's a 355 gear but a 741 case 8 3/4. Anyone know what kind of power they can handle? Also What about other case types. I've heard you can buy special braces to strenghten the peg[sold by Mark Willams]. Are these worth while or too costly and better off with a Dana? This is just a street machine so I might have to dial the boost back rather than sink huge money in it. Thanks Tim
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  #2  
Old 11-18-2004, 02:29 AM
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pishta pishta is offline
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741 is the weakest of the bunch with the smallest pinion diameter. Supercharged big block-go Dana. Mind you, the 741 is the weakest of the 8 3/4 family, but it is still pretty stout.
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  #3  
Old 11-18-2004, 08:45 AM
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I agree the 741 casting is going to be your weak link here. My brother just blew his out with a locker and 4.10 gears in his belvadere 440 naturally asp. He's only cranking about 450 hp. I would go to 9 1/4 or Dana 60 or 80.
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Old 11-18-2004, 08:49 AM
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Biggrin

8 3/4 will take 600 hp easy but I would install a Moser back brace and like Pishta said, that case is the weakest of all of them. Randy's has a nice aluminum center that would work for you. Check him out at www.ringpinion.com .
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Old 11-18-2004, 10:04 AM
DartGT66 DartGT66 is offline
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I have been thinking about this too. Unless your car launches extremely violently, you should have no problems even with the 741 case. If a 8 3/4 breaks, where does it break from? Would a back brace help in that kind of situation? Or bigger pinion axle diameter and bigger bearings there? That's the difference between the various cases, the pinion diameter and bearings. And the smallest od the bunch, 741, has bigger pinion bearings and stem diameter than a ford 9".

We have broken a few 8 3/4's and it's the gears that have broken. Backbrace may help with the housing twisting alhough with our performance it hasn't actually been a problem , but is it straight in the first place? At least our 8 3/4's were not straight; no use to backbrace without a jig and correcting the straightness first if needed. Unless the intention is to just spend money.

Pretty popular belief is that among the first upgrades in a 8 3/4" are aftermarket bearing caps and a solid spacer instead of a crush sleeve in cases so equipped. But I have begun to think that even that is a waste of money; if you need upgrades spend your money in to a stronger rear axle, Dana 60, in the first place. A ford 9" can be built tough too, but it tends to cost more than building a Dana 60.
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  #6  
Old 11-18-2004, 10:24 AM
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MoparMarcIdaho MoparMarcIdaho is offline
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Biggrin

The only 8 3/4 rears Ive experienced difficulties with were wore out to begin with.Spat the cross shaft right through the housing and into the gas tank once but I knew it was shot and kept beating it anyway.Danas are pretty much bullet proof if you dont run them dry too many times.
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  #7  
Old 11-18-2004, 10:55 AM
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You can also have the housing modified to install the cap support bolts which will help keep the caps from distorting and breaking.
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  #8  
Old 11-18-2004, 12:00 PM
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I'm using the '742 case under my 69 Runner without problems. The engine puts out somewhere around 480-500 HP. You can't really go wrong with the Dana 60. Just about bullit proof, but it is much more costly than the 8 3/4 and it weighs more, also. Either the '742 or '489 case should be able to handle that much HP.
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  #9  
Old 11-18-2004, 12:21 PM
Dennis Jokela Dennis Jokela is offline
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In the past there have been many alcohol dragsters, and funny cars that ran the 83/4 rear with very few failures. I have see a couple that are still using the Chrysler rear end.

Dennis Jokela
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  #10  
Old 11-18-2004, 02:12 PM
DartGT66 DartGT66 is offline
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Are you sure about that? I know they used the very similar looking 50's Olds rear axle, but wasn't aware of them using a 8 3/4". Those things are way lighter though than a street car. Anyway, my point is that if you think you may break a 8 3/4, it isn't very cost worthy to try to make it stronger and put a lot of cash in to it. If it still breaks, you have paid that for nothing, it's better idea to go with a Dana 60 in the first place. We have used 8 3/4's in DOT tired 9 second street cars making close to 800 hp, and not until lately the other has started to break the rear. Both have been backbraced , have steel axles but are otherwise stock with sure grips etc. The other one also has those cap preload bolts and it has been OK. But the other has recently broken a couple of ring gears. We will still give it one more try.
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  #11  
Old 11-18-2004, 05:51 PM
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Turbo, You must consider what the car weighs,And what you intend to do with the car.The heavier the car the more stress on the rearend and its componets.This is almost as important as the H.P. Are you going to put slicks on it and launch it from a sticky starting line? If its street duty,street tires,the 8-3/4 should live a long time.BUT before I sunk a bunch of money into a 8-3/4.I'd go ahead and bite the bullet and get the DANA.I've had em both.-Good luck!
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  #12  
Old 11-18-2004, 06:44 PM
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I think the question I have is how is this car going to be driven??? If it's going to be a street car with DOT tires you won't hook hard enough to break the 8 3/4... My car has +/- 750 horse and can't hook for crap... I doubt I'll ever have rearend problems....

On the other hand my buddy has +/- 600 horse and he has busted gears, bent axle tubes, twisted springs... but he has 14" slicks and hooks HARD.

Even with DOT slicks, I doubt you could tear one up on the street.... The road surface won't bite hard enough to worry about. You just can't get that much HP on the ground... Unless of course you have cement highways and launch there all the time... and even then I still doubt it....
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  #13  
Old 11-18-2004, 07:21 PM
Stus300 Stus300 is offline
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I am glad to see this popup as I myself am in somewhat the same boat.

I will be superchargin my built 383 and be anywhere between 500-600 hp with the 8 3/4 rear end in my 64 chrysler 300.

I was thinking I would have to go posi and tub the rear end to be able to hook on the street (it will be 90% street driven and "maybe" a few times to see what the 1/4 time is at the track (I know, track and street run different 1/4 times)).

So, as long as I never run slicks I should be fine, is that what I am seeing correctly?

I know already I am getting a overhaul kit for my 727tf trani to handle the extra hp..but thought the rear would be at least ok for street application.
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  #14  
Old 11-18-2004, 07:37 PM
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Wow... those are some pretty low expectation from a 671 blower. You can get that kind of horsepower from a stroked 440 running on pump gas.

As far as the rear goes, updrade to a 742, make sure that it is a clutched SG unit (not the cone style), and then weld a brace (Mancini sells them) to it.
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  #15  
Old 11-18-2004, 07:46 PM
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I broke my 8 3/4 this summer. I drove it to the track, made 3- 13.00 passes @105mph and it twisted an axle. I drove it home that way. I didn't think twice and put a Dana in it. It added 55 pounds to the car. It was going to cost a bit more to get a Dana than rebuilding the 8 3/4 which had a 741 case also. I just wanted something I wouldn't break after putting money into it.
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  #16  
Old 11-19-2004, 12:55 AM
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MOPARMANJAMES MOPARMANJAMES is offline
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Default 8.75 and Fuelers

Quote:
Originally Posted by Dennis Jokela
In the past there have been many alcohol dragsters, and funny cars that ran the 83/4 rear with very few failures. I have see a couple that are still using the Chrysler rear end.

Dennis Jokela
Funny you should mention that, I was at a Stupid Chevy show a couple years ago(my chebbie friend made me go!), any way, imagine my surprise when I saw Big Daddy Don there!
He was just hanging out at a booth and he had one of his Swamp Rats there with a 8.75 in it. I said "is that just for show?"
He said no, that's the rear I was running.
We talked for about 30 minutes and he told me more than my burnt brain can remember but I'm sure he said they should be ok up to 600hp in a heavy street car.
I've been running around 400-500 with two different ones, a 4.30 742, and a 3.23 489. My car weighs 3700# with my lard ass and so far so good with a variety of passes from 13.6 to 11.9 over 4 years.
I don't have a brace yet but I intend to get one soon.
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  #17  
Old 11-19-2004, 02:23 AM
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Quote:
Originally Posted by pishta
741 is the weakest of the bunch with the smallest pinion diameter. Supercharged big block-go Dana. Mind you, the 741 is the weakest of the 8 3/4 family, but it is still pretty stout.
This is true .
But the real (?) should be about both weight and HP.
100,000 horse power is what an 8 3/4 can handle.
If it has no load on it.
Weight And load is what decides the answer to you (?)
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  #18  
Old 11-19-2004, 03:11 AM
BDS 871 Cuda BDS 871 Cuda is offline
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Default Poor mans Dana 60

Go find a 3/4 or 1 ton ford or dodge with a Dana 60. Stay away from that Chevy 3/4 ton 14 bolt pile of s#!t. For my 65 prostreet fury, I found a 3/4 ton dodge truck with a Dana 60 for it. I had moser cut the truck ends off and put mopar car ends on it. Then I bought the moser 35 spline harden axels for it. Next I bought a used locker from a guy in the 4x4 club. I then had it rebuilt. The whole works with all of mosers parts & labor came to less than $800. I had run the car on the street and strip for 4 years with around 60 to 70 hard passes at the drag strip. (65 Fury Pro street 4000lbs, 440 with 590 H.P. pushed the car to 11.16 @ 117mph.)
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  #19  
Old 11-19-2004, 08:27 AM
dusterrcr dusterrcr is offline
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We have a 8-3/4 in our Duster.440 weighs 3030 w/o driver.we have broke a couple of axles,broke a couple of sure-grip units,a couple of ring and pinions.Had the housing straightened and braced.I have spent enough money on it to buy a Dana.But its been spread out over about 10 yrs so it aint to bad.But if I was building a car,I would go with the Dana.
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  #20  
Old 11-19-2004, 08:48 AM
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Quote:
Originally Posted by robboski
I agree the 741 casting is going to be your weak link here. My brother just blew his out with a locker and 4.10 gears in his belvadere 440 naturally asp. He's only cranking about 450 hp. I would go to 9 1/4 or Dana 60 or 80.
As luck would have it, yesterday my bro actually showed me the damages in his 8 3/4. He blew the spider gears in the locker, didn't break the housing. But did hurt the carrier bearing bores ( not the fault of the housing obviously)
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  #21  
Old 11-19-2004, 04:28 PM
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Just to beat the horse a little longer.... I will not deny the Dana 60 is the best choice for a bulletproof solution... but is it needed in a street machine??? I don't think so... On the other hand the 8 3/4 is strong enough for just about anything on the street AND you get the flexability of changing gears easily...

I know.. I know.. There are a few of you out there that say they can change gears in a Dana in an hour, but the average person can't come close to that... But anyone can change an 8 3/4 in an hour... I've been known to do it in less than 1/2 hour (With proper tools).

I have 3 chunks I use... 3.23 for highway 3.91 for crusing 4.56 for the strip.. Granted the 3.91s are in 90% of the time but I still like the ability to change at a moments notice. I just keep a couple of gaskets and a couple bottles of friction modifier around and I'm good to go.

But if you truly believe (I'm smiling when I am writing this) you have a machine so bad a$$ that can hook pavement harder than ANYTHING I have seen... Then I guess you better get a Dana and have a talk with Moser and sleep better at night.

This is from personal observation... I have no documentation to support these claims.... and I don't mean to upset anyone... but I have 8 3/4s in my blowen Hemi Challenger... In my 67 Pro Street GTX... and my 70 Pro Street Dart... I have ran the last 2 cars as hard as anyone could (with warmed up 440s) with the only problem being a shelled out yoke. That is 4 years of steady abuse.

With that being said.... I wouldn't put one in out of a 65 Newport with 300,000 miles on it without first going through it and giving it a refresh. Wore out is wore out.... wore out stuff breaks... I don't put used up engines in my cars... I try not to put used up trannys and rearends in either. Heck, I've busted 9" Fords with less horse than any of these cars, and talk has it... their stronger than any 8 3/4...

The point I'm trying to make (I think) is ... everything breaks... if you abuse it long enough... but a decient 8 3/4 is strong enough for the street for at least 99% of the cars I've seen.

Just my thoughts on the subject....
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  #22  
Old 11-19-2004, 10:21 PM
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I miss my 8 3/4. I changed gears all the time. I used 3.23, 3.55, 3.91 and 4.30's. I still have everything. When I detune the car, or if it is street only the 8 3/4 can go back in. I race my car around 10 times a year and it has to live the non spinning starts at the track.
I agree a street only car will not break one, unless you do break stands. My brother broke a pinion doing one on the street.

I put the Dana in so I should not have to worry about it breaking. Better to put in better parts than you need than to put in parts that are borderline.

My car is a true street car, drives 150km one way to the track, gets there in 1 hour 45min puts down 11.90 passes then drives home. It corners extremely well for a 440 F body.

The Dana 60 added 55 pounds and slowed the car down .05 and .3mph.
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  #23  
Old 11-20-2004, 02:32 AM
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Default is a 489 casting better?

I recently bot a 68 satillite with a 742 casing, but there is a 489 posi in the trunk. The guy i bot it from was going to install but never got around to it.
Stephen
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  #24  
Old 11-20-2004, 11:19 AM
turbotim23 turbotim23 is offline
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Default what is the most h.p.a 8 3/4 rearend can handle?

Well I have all 741,742,489 pegs and a ford peg and 9" housing also. They all need to be rebuilt but would give a good start. I'm not sure about using the ford as some might think the car is worth less,not having a Mopar rearend. I also don't think I'd use the 489. While it's stronger,there was something about rebuilding it that made it very hard. Does anyone know what that was?
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  #25  
Old 11-20-2004, 12:41 PM
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489 uses a crush sleeve in the bearing preload as opposed the the other 2 that use shims. The only thing difficult in the process is that the crush sleeve must be crushed to determine if the pinion depth shims that are used are the correct thickness. You must put it all together, crush a sleeve and then mark the teeth to determine if the pinion depth is correct, if not you have to take it apart and start over again with a fresh crush sleeve. Some use the old to mock it up and then use a fresh one for final assembly. Not that much different than the other 8 3/4 and it has the largest pinion shaft.
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  #26  
Old 11-20-2004, 01:51 PM
turbotim23 turbotim23 is offline
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Now I remember Thanks. Wasn't there a way to completely replace the crush sleave or cone with some kind of solid spacer also? How good is that option?
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  #27  
Old 11-20-2004, 02:51 PM
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The claim is that the larger pinion on the 489 makes it a better choice. The fact is, the 489 pinion tapers to the same size as the 742 taper. I would rather have a straight pinion, instead of a tapered pinion.

Also, in stock trim, the 742 used the clutched SG unit and the 489 used the coned SG unit. The clutched unit is superior.
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  #28  
Old 11-22-2004, 08:09 PM
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The 83/4" mopar rear was used by don garlitist until the early 70's at over 2500 hp.But that was propelling a 1700-1800lb dragster with tires that dont hook like todays tires,front engine dragster at that.A heavy b-body that hooks good will kill a lot of rears,but a well built 742 or 489 should hold up well.Now if you can rotate and get good air on the front end,well now thats a lot of torque.Also the ford 9" has a support bearing at the nose that prevents the pinion from walking outward under high torque loads,i.e the pin does not need to be as stout to prevent flex.peace out,waynebo
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  #29  
Old 11-26-2004, 04:43 PM
George G. Leverette George G. Leverette is offline
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What has the pinion size got to do with strength? Anyone ever see a pinion fail? Why did chrysler go with the 742 case for the alunium case if the 489 is stronger? What is the actual weakest part of the 8 3\4" rear ends? Most failures I have witnessed are the teeth on the gears, are they the same on all cases? Is there a link where actual data can read to determine the actual differences between the Mopar 8 3\4" rear end vs The Ford Dana 60 truck rear end?
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  #30  
Old 11-26-2004, 04:58 PM
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crush sleves are pre crushed when you buy a rearend kit you will never crush one when seting the preload on the pinion bearing
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