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8 3/4" diff. How much can it hold ?

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Vermont Rock

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Rebuilding my 383, 4-speed Road Runner & will probably stretch its legs to about 425-450 hp. It has the original 8 3/4" rear end with 3.55 gears. I'm wondering if it will be OK with the added horsepower & torque. Any idea of the safe maximum is ? No drag racing, but it will see spirited street driving. Thanks for your help. Rocket
 
HP/TQ is not the issue. Hooking is and how hard?

If you are running radials. No worries.
 
A buddy and I ran a 4 speed road runner in Super Street in the early 80's and broke several. The index was 10.90 but the car could run 10.60's. I think the biggest issue with ours was that we launched it at 6000 rpm! :grin: Some don't have any problems with them running that fast but if you hook up well, do not side step the clutch. That's too brutal on the whole car. Most people that drag cars with a stick use some slip in the clutch setup (competition clutch) for a reason but if you don't, just lifting your foot should do fine with a high performance street clutch. We also used a line lock and would stage with just a tiny bit of drag on the clutch to take up any slack in the drive train. It doesn't take much and anymore than a tiny bit will just over heat it. An adjustable stop on the pedal could be helpful with that.....
 
My 4 speed Challenger still has it's original unrebuilt 8 3/4 3.55 with 110K miles. Car has been in the 12's. still going.
Doug
 
Thanks for the reassuring advice. You just saved me money that is better spent on aluminum heads. Rocket
 
My 8-3/4 is all stock other then the Detroit Locker in it and so far its holding up fine running 10's but it is an auto car. It hooks ok all the time as I come up on the converter to about 2000 rpm on launch. I do agree with a stick car and using slicks that hooks good will be harder on parts as I also launch stick cars very hard when I race them. Ron
 
If you're running an Auto trans, you'll be fine. 4 speed w/lotsa HP and slicks, good for awhile? That's why Mopar used a Dana behind all of the 4 speed cars. They used to run 8-3/4" rear endsin dragsters with a "Crowerglide" clutch system because of the "soft" hookup. The banging deflects the ring gear and after time destroys the unit.
 
Well it wasn't until 1966 that the Dana was introduced in Chrysler Big Block vehicles. So how many Stock and Super Stock records were made before that time with 8 3/4 rears? Think about it!
 
khryslerkid said:
Well it wasn't until 1966 that the Dana was introduced in Chrysler Big Block vehicles. So how many Stock and Super Stock records were made before that time with 8 3/4 rears? Think about it!
Click to expand...

And the tire technology sucked back then too. Those cars didn't really hook.... I think the OP will be fine. But I have broken a few 8 3/4 gear sets in my last race car before I put a 9 inch Ford rear in the car. But it was a race only car, slicks, running 10's and wheels up for about the first 45 feet out every pass too... it was punished.
 
khryslerkid said:
Well it wasn't until 1966 that the Dana was introduced in Chrysler Big Block vehicles. So how many Stock and Super Stock records were made before that time with 8 3/4 rears? Think about it!
Click to expand...

You make a good point but I was thinking about the speed and power back in those days. I was only able to find this from an article about the 1964 US Nationals. Times have changed.
64 Run times.JPG
 
I think the only important thing here is "street tires".
 
khryslerkid said:
Well it wasn't until 1966 that the Dana was introduced in Chrysler Big Block vehicles. So how many Stock and Super Stock records were made before that time with 8 3/4 rears? Think about it!
Click to expand...
Yup.....and I found out that low 11's launching at 6k was about all they would take back in the early 80's lol. IIRC, the car was around 3300 during that time and what we did to them was shaved teeth off of the ring gear. Never did break the bearing caps etc but I bet we were stressing them pretty good. The Dana was installed in the mid 60's cars for a reason........and it was because several of the SS and Stockers were having breakage issues. Don't really remember what the rules said but in some classes, iirc, you could not run rear ends from other brands of cars so the 8 3/4 was your only choice. The factory installing Dana 60's from the truck line and using semi floating axles changed all of that. After breaking 3 or 4 8 3/4's we finally installed a 60 and our breakage issue totally went away.

Shorthorse said:
You make a good point but I was thinking about the speed and power back in those days. I was only able to find this from an article about the 1964 US Nationals. Times have changed.
View attachment 286832
Click to expand...
 
I installed a Dana (456) in my 64 Polara. Had a mild built 383, stock converter, blew the rear universal joint on a "sticky" launch one time and it was the larger joint too. It even relocated the pinion. So I guess you can break anything that has its limitations and you have to know them...
 
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