8-3/4 Rear Performance Limits

[padam]

Last Sunday I broke my rear end on the starting line at Magnolia Drag Strip.
I figured the gears broke, but I got it tore down today, and there's so much damage it's hard to tell what went first.
1 bearing cap is completely blown apart, the other is broken.
Missing a few ring gear teeth.
Specs below for reference.
489 case
Richmond 4.56 gears
Dr Diff 35 spline spool and axles
73 Charger appx 3700 with driver.
413 engine, fresh rebuild, hft cam
Holley 750 vac secondary, headers etc
4 SPEED TRANS 23 spline
Slicks 28 x 10.5 /15
4000 rpm launch
1/8 mile best 7.71 ET, best 90 MPH
This thing only probably had 25 hard launches where it hooked good.

I was about 1/2 second slower for years, and running a 742 case with sure grip, and the only failure I had was a broken stock axle. So I'm thinking I may have been right at the limit there.

 

[happyjackdaddy54]

you'd better check your tranny. Chances are,it's hurt too

 

[33 IMP]

If I had to guess, I think you probably busted a tooth off, and it got between the ring and pinion, BOOM!
I go back to what Monte Smith said about the 8 3/4. He said that the problem was the case deflecting, no matter whether the housing was braced and good axles used or not. Stripped teeth, exploded housings.
I have gotten good use from my 8 3/4, in spite of still using original Mopar axles, in my low eleven car, cause of the 727, and the s#$tty 60 ft. But I know im on borrowed time. I think I will go to a Dana before I break the 8 3/4 while it still has some value.
With a stick, I would for sure go to a dana.(or a nine inch, sacrilege I know!)

 

[440 PHIXX]

There’s a reason Chrysler had the Dana 60 standard on the high torque engines. Especially with 3700lbs. In your case. And... the tires made today are a little sticky,er.

 

[ruffcut]

Too much flex in the housing I was told. ruffcut

 

[padam]

you'd better check your tranny. Chances are,it's hurt too

What would hurt the trans?

 

[Fran Blacker]

Think he didn't see you have a 4 speed.

 

[Photon440]

Even a 383 with a four speed has limits, as shown by this factory field bulletin. Basically, a 3.91 ratio in an 8.75 rear end was barely strong enough, and if it broke under warranty, the solution was to install a 4.10 Dana. (1969)

easier to read version

 

[70chall440]

I have a lot of Mopars pretty much all which had 8 3/4 rear ends and never spit one, these were 440 cars, 383, cars, etc. Crap happens with everything and I do not think this condemns the breed. But if you are racing the car perhaps a Dana or 9" is worth the piece of mind.

 

[1 Wild R/T]

Last Sunday I broke my rear end on the starting line at Magnolia Drag Strip.
I figured the gears broke, but I got it tore down today, and there's so much damage it's hard to tell what went first.
1 bearing cap is completely blown apart, the other is broken.
Missing a few ring gear teeth.
Specs below for reference.
489 case
Richmond 4.56 gears
Dr Diff 35 spline spool and axles
73 Charger appx 3700 with driver.
413 engine, fresh rebuild, hft cam
Holley 750 vac secondary, headers etc
4 SPEED TRANS 23 spline
Slicks 28 x 10.5 /15
4000 rpm launch
1/8 mile best 7.71 ET, best 90 MPH
This thing only probably had 25 hard launches where it hooked good.

Remove any one highlighted item & you'd be fine... Put them all together & your on borrowed time...

 

[383man]

With a 4 speed and sticky tires the 8-3/4 has its limits. I used to bracket race a 340 Dart 4-speed car and over 4 years I broke two rear ends and two 4-speeds. The fastest the car ran was 11.90's @ 113 mph. But I ran it hard and put the go pedal to the floor when I let the clutch out.
Now I have my 63 Sport Fury with a 727 auto thats run 10.70's @ 125 and I race it once or twice a year since 2011. Its never broke anything in the stock 8-3/4 rear other then broke an axle gear tooth once but I still drove it home. The auto is so much easier on the 8-3/4 since it dont shock the rear like the stick car does. And at your weight its hard on that rear with a stick. If I were you and you want to race alot I would look into a Dana or Ford 9" rear with the stick car at your weight. Ron

 

[440 PHIXX]

This thread reminds me of the past. Corvettes with the 427/435HP engine are getting 1/4 mil $ these days. Supposedly it was one of the quickest cars you could buy back then, other than the 427 Cobra. In the late 70’s as a kid, I don’t remember seeing many Corvettes at the drags. The few that you did... had a Dana or a 9”.

 

[Outlawd]

A buddy of mine always said you build your rear end based on your tires, not your motor. I've owned many high hp mopars and never broke a 8 3/4 racing on street tires or even street legal drag radials but with drag slicks you will find your weak spot, somethings gotta give.

 

[padam]

Apparently it wanted outa there.

 

[GrabberOrange69]

I know it’s not cheap to swap, but at least today you have so many options, including aftermarket Dana’s (S60), and the 9” that are bolt in swaps. All you need is a credit card, no junkyard crawling.

 

[padam]

Here's my problem;
If I'm going to get a Dana, might as well narrow it, maybe move the springs in, or ladder bar suspension?
Have to stretch the wheel wells to make it worthwhile, and do a roll bar/cage while I'm at it.
Tranny will probably be the next weekest link, so I gotta figure that out.
And there's that stroker motor I've been dreaming about.
Anybody have a race car for sale cheap?

 

[weedburner]

I didn't see any mention of what clutch you are using?

A mis-matched clutch can send far more torque to the input shaft than an engine actually produces. For example if you stick a 850ftlb clutch behind a 550ftlb engine, when you dump that clutch the input shaft is going to see 850ftlbs if it doesn't break something or spin the tires first. Doesn't matter that the engine only makes 550ftlbs, that 850ftlb clutch is going to draw an additional 300ftlbs of inertia out of the engine's rotating assy before it reaches it's slip threshold.

If you currently have too much clutch, an alternative to swapping it out is adding an external clutch hit controller like my ClutchTamer to control it's hit. The 'tamer can make an 850ftlb clutch draw inertia like a 550ftlb clutch during launch, that extra reserve capacity then gives you some room to grow later before needing a clutch upgrade.

After you discover that it's possible to control the rate that additional inertia energy is fed into the drivetrain, you will then soon realize that there's not much keeping you from raising launch rpm to make more inertia energy available during launch. Raising launch rpm allows you to take advantage of a much longer lasting inertia surge without drawing the engine down below it's torque peak.

Grant

 

[GrabberOrange69]

Here's my problem;
If I'm going to get a Dana, might as well narrow it, maybe move the springs in, or ladder bar suspension?
Have to stretch the wheel wells to make it worthwhile, and do a roll bar/cage while I'm at it.
Tranny will probably be the next weekest link, so I gotta figure that out.
And there's that stroker motor I've been dreaming about.
Anybody have a race car for sale cheap?

Lol, it’s called Project Creep. Go stock length.

 

[Fran Blacker]

Lol, it’s called Project Creep. Go stock length.

Agree with the idea of project creep. It doesn't mean it all needs to be done at once. Pick out what you need to do in steps that your budget and time allows. The time part is don't take on a large project where you miss a season of racing. MO.

 

[33 IMP]

Here's my problem;
If I'm going to get a Dana, might as well narrow it, maybe move the springs in, or ladder bar suspension?
Have to stretch the wheel wells to make it worthwhile, and do a roll bar/cage while I'm at it.
Tranny will probably be the next weekest link, so I gotta figure that out.
And there's that stroker motor I've been dreaming about.
Anybody have a race car for sale cheap?

It is often easier and cheaper to buy an ex racer as a roller to go fast, rather than try to make a racer out of your street car. Plus, that way you get a fast race car and still have a fun street car too.
I did it that way, ended up with a nine second car for under ten grand.