4 Wheel Disc

[tajoe]

This is just a "PSA"(Public Service Announcement) for anyone concerned with the 1980-81 Turbo Tran Am, and its 10 bolt rear-end. I had read, and been informed that "all" of these cars had a 3.08 "POSI" rear,  have just opened one up, and they were lying to us.
I bought a 1980 TTA with hi-mileage, abused and worn out when the car was a bit over 10 years old. I specifically bought it for the 4 wheel disc rear, to install into my 73 T/A. I didn't particularly care for the 3.08s, and have a 3.42 posi that I planned on putting the 4WD brake set-up onto. To my surprise, (disappointment) I finally opened up the rear yesterday, only to find an open carrier. 
I knew the owner that I purchased the car from, and he had gotten it from his brother, who bought the car new in 1980. It had "not" gone through any swaps or changes, but was the factory installed rear. Maybe I am remembering wrong, and people here might have heard differently, that the posis installed into this car was "optional", and not std.

[firebirdparts]

Definitely optional in 1981.  I would have believed standard in 1980, but I don't really know.

Turbo Trans Ams had rear disc standard, evidently, whether they had WS6 or not.  So at least in 1981, that opens the door to an oddball axle.  They only used one width for turbo wheels in 1981 also, so it got a lot harder to tell if a car was WS6 or not.

Obviously, after 40 years, who knows what's inside anything.  Could have been assembled wrong, and the dealer could have replaced it while Jimmy Carter was still president for all we know.

[5th T/A]

TAJOE,

What year TA was this differential out of?

From what I am seeing Posi was standard on all TA's 1970 - 1980 and became an option in 1981.

Here is a picture of the window sticker from my 1980, "Differential Limited Slip, NC".

IMG_8597 by Lawrence Alexander, on Flickr



This is from page 204 of Rocky Rotellas "Definitive Firebird & Trans Am guide: 1970 1/2 - 1981 saying limited slip was optional for 1981. I will do more research to see if the Turbo 301 had Limited Slip as a mandatory option.

IMG_8598 by Lawrence Alexander, on Flickr

[jonathonar89]

3.08 4 wheel disc is a nice setup for highway and acceleration in my opinion.  Interesting fact is the ‘79 W72 Trans Am with 3.23 was just as fast the ‘78 which came with a 3.42.  I’m actually wanting to install a 3.08 to be matched with my 4 speed T10….pretty much equates to 160mph at 6500rpm.  I have no business building a car to beyond that speed since most street z rated tires are 160mph 

[tajoe]

3.08 4 wheel disc is a nice setup for highway and acceleration in my opinion.  Interesting fact is the ‘79 W72 Trans Am with 3.23 was just as fast the ‘78 which came with a 3.42.  I’m actually wanting to install a 3.08 to be matched with my 4 speed T10….pretty much equates to 160mph at 6500rpm.  I have no business building a car to beyond that speed since most street z rated tires are 160mph 

A lower gear, given all else being equal, equates to more power to the rear wheels, and better acceleration. Fact, not opinion. As long as you can get the power to the ground. People with 3.08s claiming they're "quicker" than 3 1/2s+ have probably never had the ability to compare the differences. Naturally a 3.08 is a better choice for hi-way driving, and fuel mileage, if that's important to you. My Pontiacs have always had 3 1/2s, (3.55s, or 3.42s) and are outstanding for passing, or street strip use. and also around town driving. I've heard of people with quick drag strip times with 3.08s, and their tranny never gets out of 2nd gear. Or are going thru the traps at 3800 RPMs, in 3rd. Unless you spend a lot of time driving your T/A on the free-way to car shows, you'll be missing out, W/O  the 3.42s behind your stick shift. but hey, that's just me. As for hi-speed driving, on the freeway, good luck getting away with that.

TAJOE,

What year TA was this differential out of?
From what I am seeing Posi was standard on all TA's 1970 - 1980 and became an option in 1981.
Here is a picture of the window sticker from my 1980, "Differential Limited Slip, NC".

This is from page 204 of Rocky Rotellas "Definitive Firebird & Trans Am guide: 1970 1/2 - 1981 saying limited slip was optional for 1981. I will do more research to see if the Turbo 301 had Limited Slip as a mandatory option.

I have also learned the 81 car was "optional" for the posi, but this rear is an 80 car, and as mentioned above, I knew the PO of the car, and it came with the turbo motor, and this open diff. You were lucky to have a posi installed "NC" to your carbureted motor.

[tajoe]

One more piece of information, that might benefit some-one. I stripped the 80 4WD rear, for the brakes to put on my 3.42 posi that I rebuilt years ago. I wasn't sure how compatible the 4WD components were, to a drum brake rear. Now that I got it apart, I see all the pieces, (backing plates, rotors, calipers, and even the axles) are identical and interchangeable. If you're using used pieces, it's best to keep the axles, with the rear, if you're not changing axle bearings to new. We all know that for some moronic reason, GM decided to do away with the inner race of the axle bearing, and use the axle (journal) as the inner race. If you scrap a rear bearing, you also scrap the axle. Not a good thing.

[firebirdparts]

The disc axles are unique, and we've discussed that a lot here; they had a taller indexing ring on them because the rotor was thicker than the drum was.  The snowflakes especially are hub centered and it's not optional. Steel wheels are probably going to be okay lug centered but snowflakes cannot. 

on the question of the cheapness of a chevy 10 bolt, my dad could not believe that a car company made an axle like that. I laughed.  It's 50 years old.  But yeah, it's a cheap design.

[jonathonar89]

tajoe,

Definitely no arguing the math of torque multiplication.  On this subject, tire size is also something to bring up.  For the 70-81 platform, not many people run the 27.4" 225/70-15 tire anymore.  Many people switch to shorter tires such as 255/60-15 (27.1" diameter) or 275/40-17 (25.6" diameter).  If you do the math on a 4spd with a 3.42 rear, 3000rpm is 72mph with the stock height tire.  With the same rpm, that becomes 70mph with a 255/60-15 or, respectively, 67mph with a 275/40-17. 

As much as I love my 70's Trans Am, 80's and newer American cars really left us in the dust with overdrive transmissions. I can tell you first hand, it's not fun being in the right lane on the highway with a 70's sports car and you're getting passed by modern economy cars.  Heck, every time I drive a non-overdrive car, I avoid the highway for this reason.

In regard to me, my driving is a mix of local and highway so mpg is important for me.  As mentioned before, every time I get on the highway, everyone is doing 70-90mph on average so I feel having that type of cruising speed is important without winding out the engine too high.  Although my '79 Trans Am has the original 4spd, I have a 26" tire, lowered suspension, lighter LS1 aluminum engine and a bunch of other stuff going on that makes for different gearing requirements in my application. 

[tajoe]

I hear what you're saying Johnathon. And I agree we've been spoiled with the ODs of today. But I remember the 70s, and 80s, hi-way driving 65-70 MPH with 3 55s, and I got thru it OK. But...I didn't do it often. And of course the fuel mileage wasn't anything to write home about either.
And then there's the 83 C-30, 6.2 dually I drove for almost 30 yrs, (till recently), with 4.10s in the back, and "NO" OD. So I can relate to driving in the RHS lane, and watching even the trucks and busses flying by. So yeah, 3,08s are great for the free-way. (If you have no OD)

The disc axles are unique, and we've discussed that a lot here; they had a taller indexing ring on them because the rotor was thicker than the drum was.  The snowflakes especially are hub centered and it's not optional. Steel wheels are probably going to be okay lug centered but snowflakes cannot. 

Funny you bring this up, because the hub center was something I was concerned about, when I tore this rear apart today. Diametrically, they're the same, and even the depth was the same as my drum axle. So when you just brought this up, I went out, and re-measured the depth, and found, "1" of my axles, was deeper, than the other one! (what's up with that?)

I've noticed the rear cover gasket on this rear, had been sealed with blue RTV, (Which wasn't around in 1980...I don't believe?)
So it appears some-one has had the cover off, during it's life, for some purpose. Did some-one remove the posi carrier? Did they blow an axle bearing, and change to another axle. Who knows at this point. But I do know, when the discs are on the axle, there's a good 1/4" protruding, to pilot any wheel. (You can see it in the photo at the top of pg 1. And that axle has the short hub, compared to the other side, which is a bit longer.) This car had the original turbo wheels when I got it, and there were no problems with it. I have a stash of F-body second gen axles, and checked the pilot depth of the hubs and found significant differences. So it is a valid point to consider. Thanks for addressing it.

[scarebird]

...on the question of the cheapness of a chevy 10 bolt, my dad could not believe that a car company made an axle like that. I laughed.  It's 50 years old.  But yeah, it's a cheap design.

Of the Big 3, Chevy versions of GM are hands down the worst.

[tajoe]

on the question of the cheapness of a chevy 10 bolt, my dad could not believe that a car company made an axle like that. I laughed.  It's 50 years old.  But yeah, it's a cheap design.

Joe, are you referring to a comment I made in another thread, about comparing the F-body 69 chevy 10 bolt, to the 69 Pontiac F-body 10 bolt? Or the 2nd gen 10 bolts with no inner race on the axle brgs? It's not just the Chevy, but the Poncho too with that stupid design. If it were like the 1st gen rears, where the "complete" axle brg was held in place with a retainer on the axle, and you didn't have-ta open the center section to remove those crummy C-clips. Just remove 4 retainer bolts in the backing plate area, slide out the axle, and press off the bad brg. Much better, IMO.

[firebirdparts]

No, I was just responding to you comment above where you called it moronic.  It is pretty bad.

The old 1960's Pontiac axle was a masterpiece but certainly you could get lost in the parts book.  There must have been 10 or 12 different carriers.  You gotta respect anybody who would put in 4 pinions.

[tajoe]

The old 1960's Pontiac axle was a masterpiece but certainly you could get lost in the parts book.  There must have been 10 or 12 different carriers.  You gotta respect anybody who would put in 4 pinions.

Ahh, yes, the 4 pinion posi. An ingenious work of art. I gave one of those away to a good friend, a few years back. With a set of 3.55s. (Unfortunately it had a "cone" instead of disc clutches). But he had it rebuilt, and installed it into his 69 FB.

[MalteseFalcon]

The disc axles are unique, and we've discussed that a lot here; they had a taller indexing ring on them because the rotor was thicker than the drum was.  The snowflakes especially are hub centered and it's not optional. Steel wheels are probably going to be okay lug centered but snowflakes cannot. 

When I installed the 79-81 rear discs in my '78, I had to replace the factory axles with Moser axles made for the rear discs, I think for that reason.

[tajoe]

But a rally II wheel will still fit drum axles.

[b3nny]

I have an 80 TTA (ws4) with 3.08 posi and drums in the back. So there ya go. I also have the build sheet.

[tajoe]

I think the question, and concern is putting cast wheels, on a drum braked rear. Does your T/A have the turbo wheels? If so, wondering if the axles have a longer pilot because of it? Maybe the axles are different not because of the disc rotors, but the cast wheels compared to the steel rims. (?)

[firebirdparts]

The cast wheels are made in such a way that they have to be piloted, no matter what.  Rear drum axles can pilot any wheel, and all the old ordinary not-rally GM steel wheels were that way.  The snowflakes were made to fit on that regular ol' axle, the way GM did everything.

[Fbird]

I can add that the turbo wheels use a shanked lug nut which is very specific. I do NOT recall them being "hub-centric" but also have not messed with it in 5 yrs...(I am about to though.. )

[tajoe]

And bring on more info to the axle pilot. My buddy just bought some aftermarket wheels (Cragar S/S wanna-bees), and he mentioned the same thing. The wheel isn't piloted on the axle at all. But "shank lugged", as mentioned above.

[firebirdparts]

The aftermarket wheels kinda have to be.  They are making one casting and then drilling bolt holes for different cars. But then the shape of a cragar SS, like the shape of a Rally II, doesn't have any material back there to catch the hub.  They'd have to weld something on to catch the hub with.  So the cragars use a shank nut, but the Rally II's, being steel, used the cone.

For some reason, the Snowflakes and turbos were not made capable of being lug centered.  If you wanted to, you could bore the lug holes to a standard size for alloy lug nuts with a long shank.  That's what cragar did.  You could also countersink that hole so that the cone seat on the end of a GM-style lug nut would center it.  I think the aluminum is too soft for that, but in any case they didn't do either.  I never really looked all that close until we started running into that issue with aftermarket snowflakes and discussing it on here.