• Hello there guest and Welcome to The #1 Classic Mustang forum!
    To gain full access you must Register. Registration is free and it takes only a few moments to complete.
    Already a member? Login here then!

soooo... how much power can an 8" rear really handle?

steveh326

Active Member
my main planned upgrade for 2019 include a new posi with better gears, and probably rear disk brakes cause, well. as long as I am working in the area...

how much can an 8" rear end handle? at some point I will need to build another motor, but nothing radical or exotic. I know a 9" would be better, but it'll add a grand to the project. but it would add a lot more to have to do it twice...

thanks for opinions and advice.
 
Unless you get really crazy in both the power you put through it and how you abuse it I highly doubt you'd "break" it. The stock axles are one of the weaker parts that you might want to replace. Stay away from slicks and the race track and you shouldn't have any worries.
 
thanks everyone. just running a mostly stock 289 now with a couple bolt ons, my concern is for the future. once my budget recovers, my plan is to build a stroker for it. Maybe get close to 400hp in my dreams. I may look at the extra cost of adding 31 spline axles during the changes and call it a day.

thanks for the input.
 
thanks everyone. just running a mostly stock 289 now with a couple bolt ons, my concern is for the future. once my budget recovers, my plan is to build a stroker for it. Maybe get close to 400hp in my dreams. I may look at the extra cost of adding 31 spline axles during the changes and call it a day.

thanks for the input.
For that application the 8" should be fine. I ran a similar drivetrain in a 65 fb and beat on it with zero issues.

Mel

Sent from my Pixel 2 using Tapatalk
 
I've grenaded (and I mean GRENADED) several 8 inch center sections with my 306. The motor last dyno'd 345 rwhp with my old cylinder heads and intake (I upgraded to a 9" when I started making more power). The type of tire along with auto vs manual also play a huge part in longevity.

The weakness is in the pinion pilot bearing support- once that fails, it gets ugly.
 
I've grenaded (and I mean GRENADED) several 8 inch center sections with my 306. The motor last dyno'd 345 rwhp with my old cylinder heads and intake (I upgraded to a 9" when I started making more power). The type of tire along with auto vs manual also play a huge part in longevity.

The weakness is in the pinion pilot bearing support- once that fails, it gets ugly.
You should have it straightening and welded...
31826

31828

31824

31825
 

Attachments

  • Tubeswelded1_zps8cc255ab.jpg
    Tubeswelded1_zps8cc255ab.jpg
    94 KB · Views: 6
I would think the 8 welded should handle a 306 with 345rwhp easy

The axle tubes really are not the achilles heel for an 8" (one could argue that it is on earlier models with tapered tubes). The pilot bearing pinion support is usually what breaks first as they are very weak. The problem is exacerbated when using anything other than a solid spacer on the pinion gear. Using a crush spacer allows movement of the pinion which in turn causes more stress on the pinion bearing support. Once broken, the pinion moves freely and begins chewing up the ring gear. The 9" has a thicker casting in this area to help prevent this. Combined with a beefier Daytona style pinion support and solid spacer, the problem can be almost completely eliminated.

The 8.8" design is completely different as it does not utilize a removable center section to hold the gears/differential.

Image result for ford 8 inch pinion pilot bearing
 
Ok, let me chime in. The 8" should be able to take 350 hp or maybe more.

The key is setting up the internals correctly and installing it at the proper driveline angles. I don't remember them off hand.

Lastly you want to make sure your suspension is set up so it doesn't move all over the place and change those angles.

What is described in posts above is more for abusive driving not street with occasional hard power. What I mean by abusive is hard off the line and burnouts and doughnuts and such.

DNE for how you and I drive our 67s with the power our SBFs are putting out you will be fine.

The above posts are not wrong just for your application and mine not a total representation.

Mel

Sent from my Pixel 2 using Tapatalk
 
Clarification, torque and not horsepower breaks things. The distinction matters.

You can do burnouts until your tires pop and you won't break a rear end. You can pop the clutch all day when the light turns green and you won't break your rear end...on the street, typically. Things break when your tires hook. And I mean HOOK. With the typical small block power output on street tires you'll be fine. Now if you stroke that motor which really cause the torque numbers to explode things change. BUT you still need to plant it to worry. It is really hard to hook that well on street tires on the street.

Couple other things to note. If you don't hook but have severe wheel hop you WILL risk breaking things. If your pinion angle gets all out of whack you can also risk breaking things. That angle is usually in the 2-3 degree range. On a stock set-up the leaf spring saddles welded to the rear end set that angle.
 
Now if you stroke that motor

How do you stroke a motor? Do you put a different armature in it or do you use more wrapping?

It's not a motor, just sayin'

It's an engine.

Mel

Sent from my Pixel 2 using Tapatalk
 
Clarification, torque and not horsepower breaks things. The distinction matters.

You can do burnouts until your tires pop and you won't break a rear end. You can pop the clutch all day when the light turns green and you won't break your rear end...on the street, typically. Things break when your tires hook. And I mean HOOK. With the typical small block power output on street tires you'll be fine. Now if you stroke that motor which really cause the torque numbers to explode things change. BUT you still need to plant it to worry. It is really hard to hook that well on street tires on the street.

Couple other things to note. If you don't hook but have severe wheel hop you WILL risk breaking things. If your pinion angle gets all out of whack you can also risk breaking things. That angle is usually in the 2-3 degree range. On a stock set-up the leaf spring saddles welded to the rear end set that angle.
What you say is all correct. I was not ever disputing that.

What you did is scare the daylights out of someone who is running a street car.

BTW I have genaded some pretty beefy rear ends launching on street tire. I think the first one was a Dana behind a 427 four speed. I also know that generally speaking you'll loose a Ujoint before the rear end on most small block applications.

I used to blow them all the time on a 8" with a 289 and four speed in my younger years. It also always was due to abusive driving by a teenager doing things that should not be done.

Mel

Sent from my Pixel 2 using Tapatalk
 
Car enthusiast pages kill me sometimes.

The focus is on mega-horsepower, extreme duty powertrains, and "elite" finishes. Oh, and aftermarket options up the wazoo.

Despite very few of these cars being used for anything other than sitting in the garage, or idling to the drive-in.

Some of the best cars I've ever driven were modestly equipped, and built for "light".

Nothing wrong with a 300 hp or so 289/302 feeding through C4 or Toploader (or T5, if you must) into an 8" with 3.25 gears. Lots of people raced that combination back in the day, and those were cars you could drive hard, without fear of killing yourself.
 
What you did is scare the daylights out of someone who is running a street car.
Mel I wasn't arguing with you or anything. I was just expounding on the subject. And how did I scare anyone. My very first post in this thread said the 8" would be just fine and I haven't changed that stance here since. Me thinks you confuse me with another poster

And I would bet money that all those 8" being written about here being broken are not because of some 300 hp small block putting out too much power. It's going to be because of how those suspensions were putting down...or I should say, NOT putting down that power. Wheel hop is a real killer and that's what I'm sure was really to blame.

And come on. Really? Engine/motor stuff? We all know better but use the two interchangeably all the time. Get on the grammar police squad with me and let's go after all the young guys using made up words like "prolly"! That's a much better use of energy. :D
 
Mel I wasn't arguing with you or anything. I was just expounding on the subject. And how did I scare anyone. My very first post in this thread said the 8" would be just fine and I haven't changed that stance here since. Me thinks you confuse me with another poster

And I would bet money that all those 8" being written about here being broken are not because of some 300 hp small block putting out too much power. It's going to be because of how those suspensions were putting down...or I should say, NOT putting down that power. Wheel hop is a real killer and that's what I'm sure was really to blame.

And come on. Really? Engine/motor stuff? We all know better but use the two interchangeably all the time. Get on the grammar police squad with me and let's go after all the young guys using made up words like "prolly"! That's a much better use of energy. :D
I chuckled out loud reading your post. Clearly we are on the same page.

The motor/engine thing is just a huge pet peeve.

Mel.



Sent from my Pixel 2 using Tapatalk
 
IME, back in the 70's-80's before tricky and expensive aftermarket parts emerged, we'd build ball bearing pinion supports and run mini-spools and the 8" did pretty good in stock form, albeit with aftermarket axles for safety. Most prevalent was in the sportsman classes where the OEM rear end type was required. A number of guys I raced with had small blocks and automatics deep into the 12's with 8" rears and 9" slicks. I'd already graduated to a 9" and a four link at that point and was running low 11's. I didn't know anyone in that realm running a toploader and a 8", not at the drag strip anyway. Of course materials and practices have improved over the decades.

My recollection of the most common problem at the track was spitting the pinion especially the low tooth count high numerical ones. Case couldn't handle the torque/force. For a modest build on street tires IMO no worries. Ran similar for years in the 70's without issue. A fellow racer ran 4.62's and a toploader in his Ranchero with good success on the street and mid 13's at the track on street tires.
 
Back
Top