8 3/4 (489 casting) gear problems

I have a question for all the Mopar GURU's, I am running a narrowed mopar 8 3/4 (489 casting) rearend in a 3000# car making roughly 725HP at the flywheel, 3 speed auto with a brake. I have strange axles and spool, and have the case back-braced, my trouble is I only get about 40-60 passes on a set of ring gears before there are signs of fractureing, or accually the top of the teeth bust off, I have used every type of gear manufacters and currently running richmonds with redline rear lube. I have tried several different settings ( deep, shallow, tight backlash, loose backlash, and dead on) and all combos are giving the same result. Any help or advice in this situation would be greatly appecicated.

-Michael

is the gear survace on your spool true?

Yes

It does the same thing with a posi unit (just doesn't last too long)  

You may just have to much power for an 8 3/4" you should go to a Dana 60 or Ford 9".  

an 8 3/4 properly sey up can take that much heat

Ok, What do you suggest? Is there a secret to setting the 8 3/4 up that is not documented anywhere? Any suggestions or pointers would be greatful. I am just running out of options for this rearend and really not looking forward to swapping it out.  

well how about posting up a list of all the parts you're using and we can check them out. is there anyway you can put up pics of the gear wear or broken parts so we can check them out. Also any specs you are using to set this rear up would help that way they could be compared to another set and we'll see what we can do.

we can all through out suggestions to you all day long but i'd like to have some detailed info from you so i can give you a feasable response. Please list all and anything you can think of, including noises, tire size, power, torque...etc

I am thankful for all your guys help, But looks like I will have no choice but to build a 9"...Had a small mishap at the track, 4th pass of the season the sidecaps and one bearing cap exploded...it's a train wreck in there, It is just unfortunate that Crysler couldn't make a product to handle any type of power.

Thanx again for all your help

P.S. If anyone is looking for a 8 3/4 spool for 35 spline axles let me know  

Ma Mopar has two rears that can take it. the 9 1/4" and the Dana 60. Both will work with your set up how ever the Dana is the best.  

seriously man, not to be a jerk, but dont knock it. its a proven fact that chrysler made the stringest drivetrains put under a passenger vehicle. i seriously doubt it was the 8 3/4's fault.

If you're using a brake, that may be the cause of your problem. The initial shock along with the 725 horsepower is causing the third member to flex which in turn causes a misalignment between the pinion and the ring gear which you won't see out on the work bench.  The solutions are two. Either  live with it or swap it out for a dana 60.

There is one thing that wasn't mentioned. Is the housing braced? If not, you might try that.  

Well with closer investigation, you are correct about the Third Member flexing, I am sure there are a lot of things that made this rear-end fail. 1.) The weight of the car 3205# with driver. 2.) Turbo 400 with a Transbrake 3.) weakening third member with each ring gear failure. I am not knocking the 8 3/4 rear-end, it’s a good rear-end for a light car, or even a heavy car with a powerglide. Everything in our power was used to make this rear-end compatible with our use, but unfortunately the fate of that rear-end came to its end early in the season of '04. The damage it sustained was far from repairable. 85% of the right-hand side of the third member was blown apart during a ring gear failure, the impact was so severe it literally pushed the right axle tube out of the carrier (axle tube were firmly welded to carrier) thus destroying 95% of the internal components.

Through later research we discovered there is an aluminum third member for this rear-end, I highly recommend this option if you are having similar failures.

FYI If anyone has given up on the 8 3/4 rear-end and is thinking about building a stronger rear-end, I recommend building a Ford 9", Not only do you get a strong rear-end, but shorten the new rear-end 3/4" on each side and under cut one axle just behind the splines (enough to slide the axle in about 1/2" more) and the old axles fit like a dream, the 8 3/4 and Ford 9" have almost the same offset.

Please don’t take this comment as I am trying to bash or discredit this rear-end, But to just inform people with more information than just “Well it works fine for me. You must be setting it up wrong”

My 69 Charger weights in at 3322 lbs. with driver and has a Ray Barton built 496 that makes 665 hp and 646 lb/ft of torque at the crank. At the wheels its got 537hp and 616 lb/ft. I put steel main caps in and a back brace and have never broken the rear. I also use Mark Williams axles with 5/8 studs. It also has the 1350 u-joint and the drive shaft came out of a 66 New Yorker.  

did you check for proper ring gear to pinion depth tooth engagement with dye? I run a 742 case with spool and moser axles and a non back braced 8 3/4 with a trans brake and 4500 converter ,I normally launch at 2500 rpm to be on the safe side as I worry as you do of blowing the pinion out of the case. the car,a 71 dart with weight around 3300 with me in it runs consistant 10.50s at 2500 launch,you need to step up to a dana60. Man, at 725 or 600hp at the rear wheels your really pushing the 8 3/4  

with my current engine combo im pushing over 650 hp but with the 8 3/4 ya just have to launch at a lower rpm or face the consequences.