Safe Limits For 8.75 Rear

[Road_Runner]

I apologize that this has been asked before, but I have a 'slightly' different question regarding 8.75 vs Dana 60 rears:

The 3 factors that seem to add up to whether or not an 8.75 will survive in a car are 1) Car weight where < 3000 lbs OK, >= 3800 lbs leans toward Dana 2) Stock or slightly modified HP <= 450-475 HP OK, High HP/TQ > 500 HP again leans toward Dana and 3) Automatic better for 8.75, 4 speed leans toward needing the Dana.  That's what I've been able to glean from a fairly extensive web search.

So of course I want to buck that trend with the Roadrunner because I'm seriously reconsidering how much of the original driveline to leave in the car.  So it probably weighs around 3800 lbs, Stroker build I'm looking at could be as much as 550 HP & 640 TQ (found a magazine build I may try to copy that resulted in these numbers), and want to swap in a 4/5 speed for the racing automatic in the car now.  Am I crazy?  Can the 8.75 be made to handle this combination?  I wouldn't want to weld on external bracing etc. like I've seen on some fabbed up or heavily modded 8.75 rears I've seen, looking mostly stock 70's correct is a primary design goal.

I know if I was thinking 700-800 HP it would be the Dana for sure, my combo seems to be borderline so the question is if the 8.75 can handle it then what do I need to swap out internally in the housing to make it safely handle the load?  I may contact some of the vendors to see what they say, but they could tell me the 8.75 will work with mega $$$ upgrades and it still not last in my car. 

Thanks in advance, Jim

[HP_Cuda]

The 8 3/4 is fine for motors up to 600 HP but if you want to hook up then it's game over.

[Road_Runner]

The 8 3/4 is fine for motors up to 600 HP but if you want to hook up then it's game over.

I don't have plans to run slicks but maybe an 11-12" wide set of the softer M/T Street Radials I think they're called.  Seem to get good reviews for decent traction especially vs TA Radials, etc.  Might do the occasional 1/8 or 1/4 pass, but won't be a regular car at the track.

Later, Jim

[johns cuda shop]

You may be surprised what a well set up 8.75 can handle. .It matters what you are trying to accomplish. Drag racing, Autocross, street.  Im sure its the torque # that matters instead of the HP.I'm using a 742 case with a 1350 yoke in a 3300 lb - 3400 lb car. I am already over 550 ft lbs of torque. I dont plan on using any soft tires it ,its only a street car ,I am confident that the rear can handle it ,and then some. Power shifting ,drag racing, + more weight maybe not

[Road_Runner]

You may be surprised what a well set up 8.75 can handle. .It matters what you are trying to accomplish. Drag racing, Autocross, street.  Im sure its the torque # that matters instead of the HP.I'm using a 742 case with a 1350 yoke in a 3300 lb - 3400 lb car. I am already over 550 ft lbs of torque. I dont plan on using any soft tires it ,its only a street car ,I am confident that the rear can handle it ,and then some. Power shifting ,drag racing, + more weight maybe not

I don't want to be afraid to punch it occasionally, even from a dead stop at times.  Not planning to ever mount slicks but the whole point of a good launch is to hook up as hard as possible so there may be the occasional hard launch as I get better getting it off the line.  The torque number will likely be 600+ based on a published build and although I'm going to change a few parts, I should see similar numbers.  I'm hoping the 8.75 will work, but it seems my build will be near (or over) the point where the Dana will be at least advisable if not mandatory.

Later, Jim

[vinb]

8 3/4 "  will be fine if your  NOT   1. Big Tire (slick) like a 11.5w or bigger
                                                     2. If your hooks really good
                                                      3. If your not going to racing regularly (everyweek)

Now,  if you have a stick shift car , that could be a problem if the car hooks everytime..

Make sure you are using the big u-joints and a good driveshaft...

Put the correct chip in the  rev limiter and go have some fun (and go rounds)...

 
                                             

[HP_Cuda]

I think based on what you already said you should go with the Dana.

You could make due with the 8 3/4 but you will probably end up at the same place eventually.

B

[Chryco Psycho]

seriously traction is the limiting factor , you can make 2000 Ft Lbs of torque but smoke will not kill the diff .
 I had a "mild" 440 that ate 5 - 8 3/4 center sections but I could run 1.45 60 ' at the track everytime out with the front wheels hanging .

[Road_Runner]

8 3/4 "  will be fine if your  NOT   1. Big Tire (slick) like a 11.5w or bigger
                                                     2. If your hooks really good
                                                      3. If your not going to racing regularly (everyweek)

Now,  if you have a stick shift car , that could be a problem if the car hooks everytime..

Make sure you are using the big u-joints and a good driveshaft...

Put the correct chip in the  rev limiter and go have some fun (and go rounds)...

 
                                             

I had a reasonably fast z28 back when Chrysler wasn't making anything close to a musclecar, and got pretty good getting it to hook up on launches.  I'm not sure but it seemed to pull one of the wheels off the ground.  The RR is going to have a lot more HP/TQ but it remains to see how good I can get on launches.  I have looked at the Dr Diff driveshafts, any others to look at?

Thanks, Jim

[Road_Runner]

I think based on what you already said you should go with the Dana.

You could make due with the 8 3/4 but you will probably end up at the same place eventually.

B

And that's what I'm trying to avoid.  I'd really like to keep the original rear, but if I have to back down the HP/TQ just to make it survive I don't think I'll keep it. 

Thanks, Jim

[Road_Runner]

seriously traction is the limiting factor , you can make 2000 Ft Lbs of torque but smoke will not kill the diff .
 I had a "mild" 440 that ate 5 - 8 3/4 center sections but I could run 1.45 60 ' at the track everytime out with the front wheels hanging .

Your example of eating 8 3/4's sure points to the Dana, not sure about track reference other than to say I really want to do as much of this car once and then enjoy it for the next few years with minimal maintenance.  That's going to be a goal for everything I do on the car, do it right once and hopefully it won't need as much tinkering, be dependable, etc.  Seems like there's no one answer for which works for my combo, was kind of hoping to hear the 8.75 would do just fine as long as I upgraded x, y & z. 

Thanks, Jim

[burdar]

I think you are over thinking this.  The 8 3/4 is a stronge rear and it would work just fine.  Since you seem pretty nervous about it, I'd spend the money and go with a Dana.  You'll be able to sleep at night even though it's probably overkill.

[johns cuda shop]

I think you are over thinking this.  The 8 3/4 is a stronge rear and it would work just fine.  Since you seem pretty nervous about it, I'd spend the money and go with a Dana.  You'll be able to sleep at night even though it's probably overkill.

yup

[Chryco Psycho]

I have virtually never had a dana but I sure have fixed a lot of broken 8 3/4 diffs , most ate the sure grip unit spiders etc , my lasy one chewed 37 teeth off the ring gear & all ten off the pinion the car barely moved !!
If you are thinking about spending $$ on the 8 3/4 DON'T I have seen billit caps break , I have seen blocks welded to the rear cover to support the caps , I have used spools to elimnate the sure grip distruction , they still break . Run the 8 3/4 until it breaks & place it with a Dana don't spend $$ on the 8 3/4 though .

[Road_Runner]

I have virtually never had a dana but I sure have fixed a lot of broken 8 3/4 diffs , most ate the sure grip unit spiders etc , my lasy one chewed 37 teeth off the ring gear & all ten off the pinion the car barely moved !!
If you are thinking about spending $$ on the 8 3/4 DON'T I have seen billit caps break , I have seen blocks welded to the rear cover to support the caps , I have used spools to elimnate the sure grip distruction , they still break . Run the 8 3/4 until it breaks & place it with a Dana don't spend $$ on the 8 3/4 though .

Thank you.  As to those who said I was over thinking this .... They're right.  I do it all the time, makes me good at my job, tough in other areas of my life.  But I keep hearing that an 8 3/4 can do what I want and then folks like Chryco's lifetime experiences say different, and I would be going against that advice at my own peril.  I got a lot of the same 'lifetime' examples on my web search but trust the opinions of those here over all the other sites combined.  Thanks to everybody for their feedback, I'm going with the Dana.  Now to overthink the engine build! 

Thanks again, Jim