8 3/4 strength before breaking ?

[Mike Gaines]

8 3/4 strength before breaking ??
I am right now building a new motor for my Coronet. Engine Bay is empty waiting for the new build to go in. QUESTION...The motor is a 512" and will make right at 650 HP. Now everyone is saying the 8 3/4 will absolutly break. My car weights about 3650lb. I have 11.25 wide slicks and caltracs. 2 questions....(1) do any of you guys have similiar "long term" experience with a 8 3/4 without it breaking (or breaking) and (2) if I keep the 8 3/4, which i have to because of money, what can I do to make the 8 3/4 stronger on a budget of about $1000 max. ( I know a Dana will be closer to $2300 which is too much right now. Thanks, Mike

 

[super-bee_ski]

Great question!

I do not know the answer...but I do know that Hemi automatic cars came with the 8 3/4" Rear.
I would suggest a DANA when getting into that territory.
Why?
I'm sure the 8 3/4" will withstand the rigorous demand of your 521 but: after months of abuse, it would be considered a "weak-Link" in your drive train.

 

[bobtile]

Street or strip? I would change the axles and use Mark Williams billet bearing adjusters.

 

[Mopardaddy6969]

:angry4: i have a mustang with 1300HP with a 8.8 rear and everyone said that i need to put in a 9in ford rear its not true. If you beef up the components in your rear it will take it.

 

[Longroof]

Is this a dedicated race car? If so this will significantly shorten it's life. I have broken my share of 8 3/4 rear gears In my 65 Valiant drag car that was runnings 10.70's at 5 he time with a small block...

 

[W8avenger]

There's a guy on here with a 67 Plymouth going high 9's with a 8-3/4 but I'm pretty sure it's not stock and has some back bracing
35 spline axles.
He might have broken it a few times also but I'm really not sure.

 

[Budnicks]

I'd say pro-gears {softer take shock better, especially if it's a racecar}
& aftermarket/hardened axles, spool
& welded on backbrace to begin with
& good aftermarket yoke
& probably green bearings too

check out this thread kind of spells it out, mine was post #5
http://www.forbbodiesonly.com/moparforum/showthread.php?89505-8-3-4-Info-needed

 

[lewtot184]

i've broke 8 3/4's with a 383 4spd. i think you'll grind it up in short order.

 

[actionange1]

There's a guy on here with a 67 Plymouth going high 9's with a 8-3/4 but I'm pretty sure it's not stock and has some back bracing
35 spline axles.
He might have broken it a few times also but I'm really not sure.

Actually started as a street/strip car then dedicated race only. Cleaned the teeth off a ring gear once on a center section purchased of unknown origin. Switched to Mark Williams 35 spline axles and spool and never looked back. I've back braced the housing, use Caltracs and 10 inch Hoosier slicks. Been running the car like this for 10 years...

 

[actionange1]

This pic was taken a few years ago at Beaver Springs Dragway
Race weight is 3750 and the 540 engine has taken it to 9.88@ 136+

 

[66Satellite47]

This pic was taken a few years ago at Beaver Springs Dragway
Race weight is 3750 and the 540 engine has taken it to 9.88@ 136+

Curious, with that combination of torque, power & weight, how many runs do you get on the ring & pinion? Which gear ratio? 1350 U joints?

 

[actionange1]

Curious, with that combination of torque, power & weight, how many runs do you get on the ring & pinion? Which gear ratio? 1350 U joints?

This has been in car for 4 seasons approx 65 passes avg. per season. Going to start 5th season in 2015.
3.91 gear ratio Richmond gear not Pro gear.
Yes 1350 yoke.
I do not leave the line at more than 2200 RPM.
Lenny at Ultimate Converter built me a 9 inch that works extremely well at harnessing the torque, allowing 10 inch tires to hook and run 1.37 60 fts as well as great MPH.

 

[theman440]

My 3650lb '66 Coronet goes mid to high 10's @125, lifting the tires with an 8 3/4 - 489 case, aftermarket axles, spool, 7290 u-joints, green bearings and an un-modified 49 year old greasy housing. It can be done...

 

[actionange1]

My 3650lb '66 Coronet goes mid to high 10's @125, lifting the tires with an 8 3/4 - 489 case, aftermarket axles, spool, 7290 u-joints, green bearings and an un-modified 49 year old greasy housing. It can be done...

I like your setup! I ran stock driveshaft for a long time with a conversion yoke then upgraded after twisting the first housing.
No issues after bracing the next one.

 

[W8avenger]

This has been in car for 4 seasons approx 65 passes avg. per season. Going to start 5th season in 2015.
3.91 gear ratio Richmond gear not Pro gear.
Yes 1350 yoke.
I do not leave the line at more than 2200 RPM.
Lenny at Ultimate Converter built me a 9 inch that works extremely well at harnessing the torque, allowing 10 inch tires to hook and run 1.37 60 fts as well as great MPH.

Which center section do you have? Aluminum or stock? Great looking car my man!

 

[Mike Gaines]

Street or strip? I would change the axles and use Mark Williams billet bearing adjusters.

I forgot to say...I already have Moser Axles. Thanks...
Tell me more about "Mark Willilams billet bearing adjusters" please. thanks, mike

- - - Updated - - -

My 3650lb '66 Coronet goes mid to high 10's @125, lifting the tires with an 8 3/4 - 489 case, aftermarket axles, spool, 7290 u-joints, green bearings and an un-modified 49 year old greasy housing. It can be done...

So...If I already have good axles...then I need a Spool rather than my Detroit Locker Posi Unit, a 489 case (Or 742 case ?,,,I dont know the difference on which is the good one,,someone please tell me...and Green Bearings ...and 1350 yoke ... and Maybe NOT have to weld on a Axle Housing Brace....let me know comments please. thanks, mike

 

[theman440]

The 489 uses a bigger pinion shaft, for the street I'd use the locker. You shouldn't have an issue if you are running an automatic. If you put a 7290 next to a 1350 there is not much of a difference.

 

[dvw]

High tens at 3800 foot brake seems to be around the limit. Some will live below that, some won't. Pro gears are available in only a few ratios. The stock drive shaft and u_joints don't seem to be an issue either. A 12.50 stick car can put a hurt on them as can a trans brake. I'd sell the 8 3/4 and build a Dana. If the axles are 35 spline you can reuse them if the Dana is narrowed 1" narrower than the 8 3/4.You'll save money in the long run.
Doug

 

[bobtile]

I forgot to say...I already have Moser Axles. Thanks...
Tell me more about "Mark Willilams billet bearing adjusters" please. thanks, mike

- - - Updated - - -



So...If I already have good axles...then I need a Spool rather than my Detroit Locker Posi Unit, a 489 case (Or 742 case ?,,,I dont know the difference on which is the good one,,someone please tell me...and Green Bearings ...and 1350 yoke ... and Maybe NOT have to weld on a Axle Housing Brace....let me know comments please. thanks, mike

I was told by the guys that set up my rear that the main cap area is a weak spot in the 8-3/4. They said I was alright with my HP level but recommended the billet cap and adjuster for higher Hp or a stick. I have a locker and 1350 joints in mine as well.

 

[66Satellite47]

This has been in car for 4 seasons approx 65 passes avg. per season. Going to start 5th season in 2015.
3.91 gear ratio Richmond gear not Pro gear.
Yes 1350 yoke.
I do not leave the line at more than 2200 RPM.
Lenny at Ultimate Converter built me a 9 inch that works extremely well at harnessing the torque, allowing 10 inch tires to hook and run 1.37 60 fts as well as great MPH.

Great looking and RUNNING car! The durability you're seeing is very impressive. I wouldn't have expected nearly 250 runs with that stout combination.