[Ramairnacho]

12 bolt standard 331 cou rear end.

[unruhjonny]

Please do some searches here to be sure you get the correct one;
There are Camaro and Firebird rear ends - they have different depth spring perches;
I do not know if the spring perch is in play with the rear end.

I recently (as in a couple years back) got a 3.31:1 open rear from a Formula that was wrecked decades ago;
I have sold the 3.31:1 ring and pinion - and planned on replacing it (and the open carrier) with a 3.90:1 posi... or just maybe a 3.73:1 posi.

[Ramairnacho]

Well it's very confusing to me. My car didn’t come with posi and it is a ram.air auto. The 1970 chart only indicates 307 but the 71 contradicts it by saying only 331 both open. I guess the housing is the same
. Is yours a cou open? I do have a cos open

[unruhjonny]

first;
Don't look at 1971 stuff - that will just confuse you.

1970 Formula's, Trans Am's, and Esprit's with the optional 400(2bbl) all had 12 bolt rear ends;
All other models had the corporate ten bolt - as did all 1971 models;
With different rear ends, came different gear sets.

The Trans Am came standard with posi, but the Formula (and Esprit) had to have it as an option.

All 1970 F-body twelve bolt housings are exactly the same regardless of the original installed gear set (and "pumpkin") - with the sole exception of the spring mount;
I had recently been made aware that some Camaros had housings with mounts for four leaf springs (vs three leaf springs all Firebirds, and most Camaros got;
Again I don't know if the Camaro rear housings with the larger spring mounts had different codes to denote the different shackle mount - but I would suspect they do.

I guess the housing is the same
. Is yours a cou open? I do have a cos open

I got my car (summer 1995) with a COX (3.55:1 posi), which when I replaced the ring and pinion some four years later, appeared to be all original parts;
Athough there is a slim possibility that there was an assembly line error, it's more than likely it came from the same car that the YZ engine came from.
I also owned an original COW (3.55:1 open), and sold it some years ago when I needed funds;
I bought this rear to put a more aggressive gear set in;
You made me go out and look, I am pretty sure I am reading a C0U.
If I am reading the info I have handy correct, a C0S is a 3.07:1 open twelve bolt;
If that is correct, then yes, it's exactly the same housing as a C0U.

['ol Pinion head]

Originally Posted by unruhjonny

first;
Don't look at 1971 stuff - that will just confuse you.

1970 Formula's, Trans Am's, and Esprit's with the optional 400(2bbl) all had 12 bolt rear ends;
All other models had the corporate ten bolt - as did all 1971 models;
With different rear ends, came different gear sets....

1970 10 bolts used in Camaros & Firebirds were CHEVY 8.2 10 bolts not "corporate" (8.5) ten bolts. The lowest factory ratio for this year 8.2 Chevy 10 bolt F-body rear was the 3.36.

The c-clip axle Chevy 8.2 10 bolt continued to be used in '71 Novas & Ventura II's, as well as under '71 & '72 low performance Malibus, Elco's, & Monte's.

[unruhjonny]

Thank you for the correction;
I had attempted to convey that they were a Chevrolet design ten bolt rear end, as opposed to the Pontiac rear that was much more common under American built Pontiac's prior to 1970.
I am a bit unversed in 8.2 vs 8.5 ect.

[poncho-mike]

Why don't you just go for a 3.31 limited slip rear end. A ram air car needs a limited slip.

[Ramairnacho]

well, oviously I really don't know what Im doing. I feel like Im going in circles sometimes . I could have swore that as a kid I counted about 3 turns of drive shaft to 1 turn of tire but I was wrong. I bought a cos 307, when it should have ben a 331 cou. I plan on making it posi and have rear end rebuilt. I want all rebuilt drive train is my goal and am courious if there is a 331 out there. The 70 suppliment showed the open end to be a 307 or 331. I guess there could be a 307 forumla ra3 bur the 71 suppliment shows 70 as an open 331 and 307 open was sports option what ever that means on esprits and formula auto tran cars. It also says the 70 formula HO got 331 open and the formula with mountain package got 331 open. Im thinking the formula HO is a formula ram air 3 but not 100% sure either. Maybe Ill just run what I got but a 331 would be Ideal. Yes Im confused

[poncho-mike]

I had an original 1970 Formula 400 with a 400 YS motor (non ram air). It absolutely had a 3.31 open rear. I've had a number of 1970 Formulas and Trans Ams, every a/c ar had a 3.31. I think it would be very unlikely your car came with a 3.07. There are always exceptions to the rule. The very early 71 T/A had a 3.07 12-bolt limited slip, but I suspect Pontiac was using up leftover parts from 1970. There were a lot of odd things going on at that time. I suspect Pontiac forecast it was going to sell a lot more Firebirds in 1970 than they did, and with the Firebird coming late to market, they had excess inventory in the factory.

[70FormulaCA]

My 70 Formula 400 4spd with A/C that I owned had a 3.31 open rear and my current 70 Formula 400 auto non-A/C is a 3.07 open rear. Both non-RA cars.

[Ramairnacho]

Interesting I was thought the 4 speed had the 308. Did it have a sport option? How is the 307 gear? Can you tell me more or less rpm o freeway T 65 and tire size?

[unruhjonny]

@ 'Nacho;

This is my understanding of how gear sets were applied to the Formula model:

L78 + M20 = 3.55
L78 + M20 + C60 = 3.31
L78 + M20 + G90 = 3.73 (wo C60)

L78 + M40 = 3.07 (w or wo C60)
L78 + M40 + G90 = 3.31 (w or wo C60)

L74 + M20 = 3.55
L74 + M20 + C60 = 3.31
L74 + M20 + G90 = 3.73 (wo C60)

L74 + M40 = 3.55
L74 + M40 + C60 = 3.31
L78 + M40 + G90 = 3.73 (wo C60)

[Formulabruce]

Question and correction.
1. Why this thread Ramairnacho? You have 2 , 1970 12 bolt rear ends already, right ?
2 Correction to post #4. There were NEVER ANY F bodys with 3 leafs, Ever. No offense, but have to keep history correct.
The 1970 Camaro Z-28 shared the Whole complete housing and perches rear end with All Firebirds that came with a 12 bolt. These are 4 Leaf perches.
The 1970 Camaro SS used same housing but had 5 Leaf perches. These seem to pop up for sale more than 4 leaf perch rear ends. If you see one in a Firebird typically the lower shock plate is bent in a " U" and the center spring bolt isn't doing a thing to keep rear in place.
So, 4 leaf All Pontiac ,AND Camaro Z-28. And 5 leaf Camaro SS only ( small and Big Block)
The New Trans am shared the F-41 suspension with the Z -28 and shows in some 1970 documents.
All 1970 12 bolts seem to have Chevy gear ratios.
I have had both types here, side by side .

[Ramairnacho]

Oh what is l78 and l 74? I understandm40 but what is c90 or c60?

[Ramairnacho]

Formulabruce I thought I bought the right rear end and now was told I need a 331 ,mine is 307. I put my original I a 72 formula I built and lost the car. It got taken from me by police and sold. So I'm puzzled by the correct gear. Hoping maybe I could find a cou 331 open. I have a 307 open in an esprit I bought for the rear end and other minimal parts then put a 75 10 bolt in esprit and sell it.

[unruhjonny]

@ Bruce:
Thank you for the correction. Five vs four was what I meant.

@ Nacho:
L78 - RPO for 'base' Formula/GTO engine
L74 - RPO for RAIII
C60 - RPO for air conditioning
G90 - RPO for 'performance axle'

EDIT: RPO = Regular Production Order code

[Ramairnacho]

Thanks then I made mistake buying the 307 damit

[unruhjonny]

mistake?
you got a 12 bolt rear end for your car to replace the missing one;

My cousin's '71 TA has a 3.08 & M40;
it drives great.

[turbo69bird]

I’m sure this will get everyone all riled up but this came directly from the owner of the dealership back in 71 or 72, caused a real ruckus w Pontiac. Read about the rear differential in this RAV pro stock firebird. I have the drive train from this car in my 69 RAIV raced by the same dealer .

[Ramairnacho]

That's superstore race car must be fast.5.14 gear ratio? Wow.