8 3/4 Rear End

[Mateya]

Originally my 70 RR had a 383 which means that it has the 8 3/4 rear end. Now my car has a 440. I was wondering if anybody else has been in the same situation and what your approach was on your restoration. I did read through some of the other threads on here related to a 8 3/4. Here are a few of my initial questions:

1) I am assuming that the 8 3/4 is sufficient for a 440? I am planning on enjoying my car when finished. Nothing crazy, no drag racing, etc.

2) What is the general rule of thumb on rebuilding a 8 3/4 rear end when restoring? In other words what is the best approach to see if anything needs done to the rear end or not?

3) If I want a good all around setup meaning if I want to cruise down the highway at 70 mph but also have nice acceleration should I change anything with the setup?

If anybody has any additional information, thoughts, pictures, etc that would be great. Trying to learn as much as I can now before I get to this point in my restoration.

 

[RKNRLR]

I put a brace on mine when i rebuilt the engine. Got it from moser, about $70-80, welds right on. Stiffened it right up.
Gonna put some 4:10's in shortly too.

 

[Russ69Runner]

If you look back at some old thread's you can see. Have the 741 the lightest rear end in the production line. It had a 2to 74 ratio I think that is rite. It would be like dragging a weight down the road when leaving the stop sign. But bet it would top out at 150 on the highway. So went with the 3:55 gearing to get a little more ump off the line. Mine is going to be a driver not a race car but want a little something to show my friends I do have a muscle car. Now what differential do you have and bought my stuff from Mr Diff. He can steer you in the direction you want and is a good person to work with. Hope that in the future that I do not decide on putting a 5 speed transmission in. Only time will tell. I have the A833 four speed. Have heard different prows and cons on the subject. Just a few of my thought's on the subject. Russ.

 

[69hemibeep]

8 3/4 + 440 + auto + street tires =
8 3/4 + 440 + auto + slicks =
8 3/4 + 440 + 4spd + street tires =
8 3/4 + 440 + 4spd _ slicks +

 

[ACME A12]

Good summary. Nailed it.

 

[69hemibeep]

I like 323s, 355s will do what you want but a few more RPMs on the highway

 

[quikbird]

If it doesn't leak you can probably just clean it up and drive on down the road

 

[Russ69Runner]

Don't tell me that. Well that means a cam is going in to give me a few more RPM'S Then. Their goes that gas millage!!!!!!

 

[TOMRR]

I have in my 69RR 440hp 4-speed and 489 case 8 3/4 3:55 gears. 415 HP 475 Ft.lb TORQUE . NO PROBLEM. mine never seen the drag strip just street duty about 2000 miles a summer

 

[69hemibeep]

Tom that's why the smiling one is up there. You have the stronger gear set it depends on how you drive and if you rough it up some what kind of tires. BFGs for instance are so hard you will never hurt anything they just go up in smoke.

 

[ACME A12]

Don't tell me that. Well that means a cam is going in to give me a few more RPM'S Then. Their goes that gas millage!!!!!!

If you're driving a RR with a big block do you REALLY give two shits about the gas mileage? These things are toys these days. Daily driver status for these cars went out the door about the time that George H.W. Bush took office...if not years before in most cases. The only thing that should be measured these days is smiles per gallon...

 

[Russ69Runner]

Think this runner will be a week end car or just a night on the town. We have Old Town in Kissimmee Fla. all the old car's go their. Then some guy's go too a sport's bar up town and some meet up at the Mall. Their is a Mopar club in Brandon but that is 75 miles from home. Well that is to far into the future but think about it keep's me working on the car. Russ.

 

[Palmer Moe-par 68]

I would go on the safe side and just do a full rebuild. It's not that much trouble, and is not too costly either. The 8 3/4 can handle a 440 with no problem. I agree with 69hemibeep that the 323 is the way to go since it is still fast, but not too high winding. I dont know if I'm correct about this, but I believe the 323 was also the lowest ratio when it came to sure-grips. Anything lower would be an open diff.

 

[Russ69Runner]

I might be mad latter but I put the 3:55 in mine. Will see. My friend and I put 4:11 in a duster and it was not for driving around better for strip.

 

[dcala84141]

I’m thinking of going with 3.55 gears also with 295/50/15 tires to keep about same diameter as the F-70 tires which are on there now with 3.23 gears
Just hope the 295’s will fit without tire rub on my 68RR

 

[Russ69Runner]

If I am not wrong which someone will point out if I am. I put a set of 3.55 gear's in mine. The car will probably not reach 150 at top end. But still should get it to do 120 or so. Yes the RPM's will raise. Mine also had the 2.72 gearing and it would leave the stop sign like it had a weight tied to it's tail. Also wanted to have a little more power getting off the line. Have found pro and cons here in old forum's. Had 4.11 in a race car and could not drive it around town. It worked real well on the back streets dragging. I have an 741 open diff. In it now their telling me the 3.23 might be the way to go. Like to know where to get that RPM calculator. Doctor Diff could tell you if you ask him or Summit racing tech people are good also. I am sure Randy's also can set us straight.

 

[69hemibeep]

Well if you have Google in Florida like I think you do then enter RPM calculator and pick the one you like

 

[Russ69Runner]

I live so far in the country that people use to say I send smoke signals. Yes I have google down here. What do you figure is a comfortable RPM on the motor doing 70 MPH. On a 383. My buddy say's I don't have to worry about it because I am never going to get out of the garage. That's when I tell him he is a real smart A$$.

 

[quikbird]

he might be right

 

[Russ69Runner]

Not at all it will be on the road by 2021.