Need some schooling on 8 inch rears.

spirockp · Jul 19, 2008

First off I'm interested in learning because I want to convert my rear to a POSI. (what else is new? lol) I don't know the difference between an 8 and a 8.8 rear. Which did a stock V8 66 Mustang have? Also what is the part to convert, TrueTrac?

Thanks for the help.

Peter S.

zookeeper · Jul 19, 2008

Stock '66's should have an 8 inch rear axle in nearly all cases. The quickest and easiest way to a limited slip rear is through one of these Powertrax 92-0680-2800 - Powertrax No-Slip Traction Systems - summitracing.com
I know it sounds spendy, and granted, nearly $500 is nothing to sneeze at, but you'd spend a lot more than that on a 9 inch that fits your car. There are also lots of people who'd like you to beleive the 8 inch Ford rear axle is so weak it's like it's made of glass. Not true. They are a good stout rear axle as long as you're using street tires. If you're using slicks, you should step up to at least a 31 spline rear axle anyway. I say get a Power Trax and be happy.

spirockp · Jul 19, 2008

ZooKeeper, you answered my next question, that was what spline is it. 28, I got it.

Now what about the rears in 67 thru 70ish that are 2 inches longer? Are they also 8 inch and can I use that same part number to install. My Dad has a 65 with one of these rears in it.

Again Thanks.

BarnStang · Jul 19, 2008

The difference is the size of the ring gear. Here is a link to learn a bit about it:
Ultrastang - Steve Ainsworth

zookeeper · Jul 19, 2008

spirockp said:
ZooKeeper, you answered my next question, that was what spline is it. 28, I got it.

Now what about the rears in 67 thru 70ish that are 2 inches longer? Are they also 8 inch and can I use that same part number to install. My Dad has a 65 with one of these rears in it.

Again Thanks.

As far as I know ALL 8 inch gear carriers are interchangeable. That includes Falcons, Mustangs, Mustang II's, Pinto's, Fairlanes, Torinos, Mavericks and anything else. All 8 inchers are 28 splines, and some 8 inchers came with a limited slip differential, but they are so rare that they aren't even worth the time it takes to look for them.

ultrastang · Jul 20, 2008

The 8-inch rears came out in 1962 and ceased production in 1980. All 8-inch 3rd members are interchangeable from one 8-inch housing to another. However, the 8-inch 3rd members from '62-'66 are the weakest design you can have.

In 1967, the 3rd member castings were revised to have more strengthening webs on the face of the 3rd member, both in a horizontal and vertical direction. This case is cast with the alpha-numeric "C7OW" on it.

Most 8- & 9-inch rear ends had open [single track] differentials. Some early models had Equalok/Limited-Slip differentials. After '68, the Limited-Slip was replaced by the much improved Traction-Lok differential. Whatever the case though, the 8-inch differentials only came with 2-pinion [spider] gears. --The 9-inch could have either 2-pinion or the much stonger 4-pinion differentials.

There are many who generically call any Ford differential that directs power to both rear wheels as a "Posi". Technically though, there are no Ford "Posi" differentials. "Posi" is an abbreviation of the word "Positraction" which is a General Motors trademarked name for the differential they produce. Mopar's trade name was "Sure-Grip".

spirockp · Jul 20, 2008

zookeeper said:
Stock '66's should have an 8 inch rear axle in nearly all cases. The quickest and easiest way to a limited slip rear is through one of these Powertrax 92-0680-2800 - Powertrax No-Slip Traction Systems - summitracing.com
If you're using slicks, you should step up to at least a 31 spline rear axle anyway.

Zookeeper do I have the option of upgrading the spline in my rear to 31 and get the Powertrax to match? I have searched the web and SummitRacing, but do not see any upgrades to 31 for an 8 inch rear. Wow, this is going to be a fun upgrade.

Just to follow my reasons for the upgrade....
I have a 66 Mustang with generally a stock 302. My Dad has a 65 Mustang with a slightly worked 302. We went to the drag for the first time last Friday and had a ball racing each other. Our biggest problem, more so my Dad's, was getting the tires to stick. So now you know why I'm inquiring about rears.

Peter S.

Decurion · Jul 20, 2008

Theres a 71ish Comet Ive seen at the local track that runs around 10.70s on an 8"(!) and an automatic. Then again, a buddy of mine blew up his 8" running mid to low 13s. His problem was wheel hop and a 5 speed. Somebody posted a link to an article that showed a Mustang running a 331, 5 speed, and 8" going something like 11.70s. My point is that an 8" can work just fine, but they do command a little respect or they can grenade. An 8.8 from an Explorer is stronger and stupid cheap, but pretty heavy. Either one will work fine and usually can be found on the cheap, so just look around, search the forums for info, pay attention, and chances are you cant go wrong with either.

bnickel · Jul 20, 2008

the 8" should hold up fine with an auto and street tires and less than 400hp. anything other than these and you'll want to upgrade to a 9" or an 8.8" from a late model.

the 8.8 is a virtual twin of the GM 12 bolt and has all the same pros and cons as well.

i like the Eaton Detroit True-Trac diff personally since it acts just like an open in normal driving conditions but when you nail it the limited slip function kicks in plus there are no cutches to wear and no slip gears to make noise.

zookeeper · Jul 20, 2008

spirockp said:
Zookeeper do I have the option of upgrading the spline in my rear to 31 and get the Powertrax to match? I have searched the web and SummitRacing, but do not see any upgrades to 31 for an 8 inch rear. Wow, this is going to be a fun upgrade.

Just to follow my reasons for the upgrade....
I have a 66 Mustang with generally a stock 302. My Dad has a 65 Mustang with a slightly worked 302. We went to the drag for the first time last Friday and had a ball racing each other. Our biggest problem, more so my Dad's, was getting the tires to stick. So now you know why I'm inquiring about rears.

Peter S.

First you need to know who much you want to spend. If $500 is your budget, go for the Powertrax, it'll make HUGE difference in traction. If you want to spend about double that, you can find a 9 inch rear, have it narrowed to fit your '66 then buy a limited-slip unit and have it set up. Then you get to have a custom driveshaft made, since your '66 uses a smaller U-joint. By the time you do the brakes, bearings and seals, you'll have easily spent twice as much for a 9 inch than you would for the 8 inch. There's no denying that the 9 inch rear is the King of stout, but if the 8 inch is good enough to run deep into the 10's, it'll be fine for the street. Let your wallet be your guide.

ultrastang · Jul 20, 2008

No denying that it's getting harder and harder to find a 9-inch in the wrecking yards, but if you're persistent, you may just find one.

I got this '80 Monarch 9-inch drum brake rear end assembly for $300.00 [same width as '65/'66 mustang rear]:
http://img384.imageshack.us/img384/7872/sn95discsglenhakanmonarou9.jpg

I picked up this 31-spline, 4-pinion Traction-Lok (Daytona) N-case 3rd member from a '75 Ford pickup for $75.00:
http://img170.imageshack.us/img170/398/ncases003xg6.jpg

After cleaning up and painting the rear end housing and 3rd member, fabricating some adapter brackets to adapt some Cobra rear discs [Cobra brake parts ~$400.00], it turned out like this [~$800.00 total invested]:
http://img170.imageshack.us/img170/1456/brakelines002fo4.jpg

spirockp · Jul 22, 2008

Not really trying to beat the system, but does anyone have a subscription to "FordMuscle" to view this install?

FORDMUSCLE webmagazine: Powertrax Performance Locker

PM me thanks.

Peter S.

Need some schooling on 8 inch rears.

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zookeeper

spirockp

BarnStang

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spirockp

Decurion

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