Help me determine the right rear end gear for my build

[coachtek]

I have an Ly6 matched to a 4l80 trans in my 68 cutlass

im looking for quick acceleration or should I say stop light / backstreet road action but also maintain good rpm on highway cruising.

in the future I intend to do a cam swap or maybe supercharge or turbo so I want to have room to grow

My rear tire size is 285/30ZR20 from my calculations I think that equates to a 26 inch tall tire?

[bryant]

a 3.73 will be a good all around gear ratio for your described use. the only thing i see that i would change it for is if you go turbo. for the turbo you will be better with something around a 3.25 ratio. you didnt say what rear end you have so exact ratios may very depending on what type of diff you have, but the closest ratio to what is state above will deliver what your looking for.

[coachtek]

My rear end is a 12 bolt but after doing some research I found some olds 12 bolt weren't true 12 bolts depending on if it was a Canadian built car or US built.

[bryant]

ive seen a few olds diffs that have 12 bolt covers but 10 bolt ring and pinion sets in them. dont remember if the 8.5" gear set works in them. you need to pull the cover and count the ring gear bolts.

[Nothingface5384]

My rear end is a 12 bolt but after doing some research I found some olds 12 bolt weren't true 12 bolts depending on if it was a Canadian built car or US built.
That would be an olds type O rear
they take their own gears..one could mod a ford 8.8 diff unit to fit in the olds 12bolt type O center housing too

Cutlass didnt get the rare hybrid 8.5 10bolt till 71 upto 72
68 through 70 shall be type O 12bolt
And i imagine 64-67 would be a poncho 8.2 rear
Canadian built shall be chevy 12bolts?

[coachtek]

That would be an olds type O rear
they take their own gears..one could mod a ford 8.8 diff unit to fit in the olds 12bolt type O center housing too

Cutlass didnt get the rare hybrid 8.5 10bolt till 71 upto 72
68 through 70 shall be type O 12bolt
And i imagine 64-67 would be a poncho 8.2 rear
Canadian built shall be chevy 12bolts?

So with a type O rear end can I still fit the 3.73 gears? Oh and forgot to add that I want to add a posi in it too

[69cutlassrkt]

The olds 12 bolt type O is is a 12 bolt cover but the carrier and ring is only 10 bolts. I think that's right. And I think the Canadian ones used a different rearend.

[Nothingface5384]

Contact monzaz
Hes the rear end guru very respected guy over at v8buick forum
http://www.jdrace.com/diff_id/12bABodyOlds.htm
If you do have an olds type O, you may buy a 3.42, 3.90 or 4.10s ring and pinion
Or like i said but a modified ford 8.8 posi..think monzaz does a modified eaton and a tru trac too

[69cutlassrkt]

Parts for the olds rear end are so expensive why not just get a ford 9in rear? Idk if you have a posi, but they are around $550-600 last time I looked and the r&ps were close to $300.
What stall of a torque converter do you plan on going with?

[coachtek]

Parts for the olds rear end are so expensive why not just get a ford 9in rear? Idk if you have a posi, but they are around $550-600 last time I looked and the r&ps were close to $300.
What stall of a torque converter do you plan on going with?

No I don't have a posi I plan on adding one

How much is a ford 9in? And does it come loaded with posi and the gears I desire?

[69cutlassrkt]

Just looking real quick looks like about $3200 with the trutrac diff and with wilwood discs brakes complete bolt in rearend. Looking at drive train specialists for the olds 12 bolt the carrier is $554, gears $318 and install kit $90.http://www.drivetrainspecialists.com/gm/gm-85-o-oldsmobile/
In the mean time you could do a torque converter with a moderate stall speed and a powertrax no slip unit. The powertrax isn't too hard to install. I just realized the engine and transmission is already installed I was thinking it wasn't for some reason, so the torque converter would be a lot of work.

[andrewb70]

I vote is for 3.42s. This will keep your drive shaft speed down. As I recall, the 4L80e doesn't have a very deep overdrive, so the 3.42s will also make for comfortable highway cruising.

Also, double check your u-joint operating angles.

Andrew

[Nothingface5384]

Why not just go s60 and end all lol
But For a chevy 12bolt or 9in ford
Check out
Www.Scandc.com

[coachtek]

Well this turned out more complicated than I expected lol

Buying a new rearend is not in my budget.. Just want to install posi and some new effective gears for my type O axle, that will improve accelerating from a stop and still be good enough for highway cruising

[69cutlassrkt]

We'll spend your money for you lol.
I agree with andrew 3.42 gears.
Found this for sale http://realoldspower.prophpbb.com/topic4188.html. It's got a spool though and 3.90 gears. That guy who is selling it sent me a retainer plate or whatever it's called before. Pretty knowledgeable guy with the olds rearend.

[coachtek]

Contact monzaz
Hes the rear end guru very respected guy over at v8buick forum
http://www.jdrace.com/diff_id/12bABodyOlds.htm
If you do have an olds type O, you may buy a 3.42, 3.90 or 4.10s ring and pinion
Or like i said but a modified ford 8.8 posi..think monzaz does a modified eaton and a tru trac too

Spoke to jim from jdracing he was very knowledgable with the O type rear ends.. for 900 i can get all the parts to rebuild it to exactly where i want it. Much better than paying 3k for a new ford 9inch.

Whats the average cost on labor to rebuild a rear end with new gears & posi?

[gmorris]

I just swapped my 3.73's over to 3.08's. With the 4l60 first was useless with the 3.73's and I was at 3700ish rpm at 80mph. This should get me down close to 2000rpm and help hook first a bit better (this is with a built 418 LS3 making roughly 630HP) and drop my DS speed as Andrew mentioned. I considered something around 3.42 but decided the change wasn't big enough to make being covered in gear oil for 2 days worthwhile lol.

[andrewb70]

Spoke to jim from jdracing he was very knowledgable with the O type rear ends.. for 900 i can get all the parts to rebuild it to exactly where i want it. Much better than paying 3k for a new ford 9inch.

Whats the average cost on labor to rebuild a rear end with new gears & posi?

A competent mechanic should not charge more than 4 hours labor to do a R & P swap.

Andrew

[coachtek]

I just swapped my 3.73's over to 3.08's. With the 4l60 first was useless with the 3.73's and I was at 3700ish rpm at 80mph. This should get me down close to 2000rpm and help hook first a bit better (this is with a built 418 LS3 making roughly 630HP) and drop my DS speed as Andrew mentioned. I considered something around 3.42 but decided the change wasn't big enough to make being covered in gear oil for 2 days worthwhile lol.

Why was 1st useless? And is the a difference between the 1st on a 4L60 and that of a 4L80?

[69cutlassrkt]

4l60e has a steep 3.06 first gear then a good drop to a 1.63 2nd. 4l80e has 2.48 first and 2nd is 1.48
Though the 4l60e has a 0.70 overdrive and 4l80e has 0.75 od. Wish I had a 4l80e.

[Nothingface5384]

My guy did a complete rebuild on my 72 8.5 10bolt with gear/posi swap , new axles, retainer plate studs bearings seals etc and disc conversion for 400 bucks

Well its apart have the guy weld the axle tubes..thats the single best thing to strengthen the rearend

[CampbellshotrodsAZ]

A 26" tall tire seems to be pretty short and low profile for a 20" wheel on a car like this, is that definitely the tire size? Those A-bodies can take a 28" tall tire pretty easily, I'd personally do a taller tire with around a 3.73. Otherwise with a tire around 26" tall, I'd go with something like a 3.42 then, as mentioned previously.

[coachtek]

Ok so I went with the 3:90 gears, and after doing a cam swap and stall converter I'm not happy with the performance.

It's slow out the gate and feels like it's pulling the 1st gear for extremely too long..then runs through the 2nd gear pretty fast into the 3rd. My highway Rpm's are 3k at 75mph.

I'm looking for a gear that will wake up the car off the line and still be good enough for highway cruising ..

[nokones]

In my opinion, the first gear ratio at 2.48 is a little on the tall side. I'm surprised that you went through second year real fast with 1.48 gear ratio and that may be how the valve body is set up. Sounds like you need a custom torque converter. Before you have torque converter designed for your setup, you need to determine the cam design, the rear diff gear ratio, and the rear tire size.

I would rethink the gear set in your tranny is you want to leap off the line. If you stay with the existing tranny and use a shorter rear diff final like 3.90 or 4.11's or shorter, you probably have a good launch but, you will be spinning the motor a lot cruising speed. I would consider a shorter 1st and 2nd gear set like the 700R4/4L60 trannies, with a low to mid range torque curve on the cam with a custom designed torque converter and 3.40 something rear final. It worked great for me in my 90 1LE.

[Sean Cooke]

4l80e & 3.73 is what I'm currently running, cruises about 2800 at 80mph with the converter locked up. Im also running a 3k stall converter from Yank performance. My buddy has a 69 and is planning on running 3.55s with his 4l80

[gmorris]

I mentioned before that I went to a 3.08 which worked Ok with the 4l60's tall first but after switching to the 4l80 it was a complete dog off the line and even shifting to 2nd dropped it out of the power most of the time. I swapped back to the 3.73's and it is much happier. My cam does not work with the taller overall gearing at all and the car just felt wrong everywhere. I'll live with the extra 600RPM cruising for a much better driving car overall. Amazing what a gear swap can do!

[coachtek]

after doing all my research and the responses here ...seems like my missing link is the torque converter, will look into that

[Sean Cooke]

I ran the stock truck converter for awhile, the 3k stall is way more fun & yank makes a pretty tight converter for street cruises but when you hammer on it. It's a good time

[69cutlassrkt]

yeah torque converter is the way to go. And if you upgrade the cam later you will need that or the car will be pushing through the brakes with a stock stall torque converter.

[coachtek]

yeah torque converter is the way to go. And if you upgrade the cam later you will need that or the car will be pushing through the brakes with a stock stall torque converter.

i already have a cam in it, i called the manufacturer today and learned that it is a lil too big for my application and is killing my low end torque, prolly get it regrinded