575 hp. 8"3/4 question

[68 charger guy]

I have a 575 hp. crate motor coming for my 68 charger. Will a 3.91 posi be strong enough or should i be looking for a dana 60 or some other rear end? i have heard 8"3/4 rear ends can take more abuse than one would think. thanks in advance for your opinions ,any other chassis set up considerations you have, let me know.

[DartGT66]

I have not had any problems with the 8 3/4" so far that I'm aware of. I haven't touched it for four years in my low 10 second street & strip car.

[Dart]

The 8 3/4 should be fine. If you are 4speed it may not last as long due to the shock on the drivetrain when you shift. PRO ran a ~550 horse (with NOS) 440 with 4sp and 3.91's. Only thing that happened was that he twisted an inner axle when running slicks. You may want to think about cutting down C-body axles for the 8 3/4 or buying the Moser street axles if you are going with a 4 speed. If you are automatic, I wouldn't worry about it.

[Blaine Peterson]

I can't speak from experience (I don't have anywhere near 575 horsepower) but I remember reading an article awhile back that the 8 3/4 489 was good for up to 600 horsepower.

[68 charger guy]

Thanks for the input, guess i'll give it a go as I am running an automatic.

[451Mopar]

In a 4-speed or trans-brake car, you should switch to the Dana-60. Other wise the 8-3/4" can take alot of abuse.
My engine puts out pretty good power, but I don't launch the car very hard (only 1.8 second 60' times) because anythine more than 1,500 RPM launches turn the tires (11.5"x26" M/T street ET's) into smoke.
Anyhow, my small 741 case 3.91:1 sure grip has been holding up fine under my 4,150 pound '71 Charger.

[kingjr]

I'll buy those 391's if you want to sell'em. Let me know, or anyone else with a good set. Thanks. <><

[Jack Z]

68 charger guy:

Based on my knowledge and experience, in order to make an 8 3/4" rear axle assy. live in a relatively heavy 575 h.p. application like you have that will get drag raced, it will need some work and even then it won't be as durable as a "stock" Dana 60. Specifically, you should do the following:

install a brace across the rear of the housing along the length of the axle tubes

weld small front to rear braces inside the housing around the circumference of the carrier opening (this is commonly called a "cage")

install steel caps on the carrier bearings (at least on the drivers side)

use a solid crush sleeve if you use the 489 gear set

It also doesn't matter if you use the 742 or 489 type gear sets. The 489 has a larger (stronger) pinion shaft but that's not where these gear sets usually break. The usual failure is tooth breakage and the main cause of that is improper meshing of the teeth which is caused by flexing of the carrier and/or housing. That's why the strengthening items I mentioned above are done.

I can also tell you that I tried an 8 3/4" assembly in my race only car (3600#, 550 hp, A/T, 9" tires, 1.40 60ft.) which was strengthened like I described and it lasted about 90 runs. I then went back to the Dana 60 that I had in the car previously and it's still fine after about 400 runs. I can also say that there was no performance difference between the two (after adjusting for the 30 less pounds of the 8 3/4").

After all this, I guess what I'm trying to illustrate is that, in my opinion, you are much better off with a Dana 60 and aside from the 30 or so extra pounds, there is really no "down side" unless you already have an 8 3/4" available. You should also keep in mind that when the 8 3/4" fails (and it will if you race it long enough with that much hp and the car hooks well), it will probably be at launch from the starting line and it may cause damage to the transmission and engine when the driveline freewheels instantaneously due to the breakage.

[Leigh]

JackZ, that was a very accurate description of the two units durablility. Very well put and 100% truthful with no embellishments. Its also why I too traded my trusty 8 3/4 for a Dana when 10's started to appear on the time slips.

[451Mopar]

KingJr, The 3.91's are not for sale, but I do have a 4.88:1 center chunk with sure-grip that is all new parts.