Swapping 2.56's for Lower Gears
 

I have an open rear in my Lemans. I have a complete posi rear with 2.56 gears. Can I swap the 2.56 gears in the posi for something in the 3.08 or 3.23 range? I've read many posts on this subject and was under the impression my 2.56 carrier could go up to 2.73 gears. I've also read that 2.56 is the high(numerically) end of my carrier. Can anyone clear this up for me once and for all? What's the deal with spacers? Also, if my open rear has 2.56 gears, will putting a posi with 2.56 in the car do much for me? Is it worth the cost? The car has the stock 350 with a 4 bbl setup, dual exhuast. Just a driver, but I'd like to get a little better acceleration from a stop while keeping it highway friendly. Thanks

What year of LeMans do you have? Is the rear a 10 bolt? 8.2 or 8.5? I know it can be confusing.

gear swap
 

My Lemans is a 1968 with a 10 bolt 8.2 rear. It's the original rear dated October 1967. The 2.56 posi is also a 10 bolt 8.2 with a date of December 1967, supposedly out of a Buick GS. However, I have a feeling the GS part is not correct since I doubt they would have put those gears in a GS. Just a thought.

My 1970 GTO had been converted to a peg leg 2.56. I put in an Auburn gear posi with 3.55 Richmond gears.
There are two different carriers. The info I found was that your carrier with the 2.56 gears will go up to 3.31. The next carrier goes from 3.36 to 4.56 gear sets.
A posi with 2.56 gears won't be of much value. The basic concept is to have two tires get traction rather than one. With a stock 350 and the 2.56 gears my guess is that you have one of two situations when you nail it from a standstill:
1) - the right rear tire grabs and the engine lugs until it gets some RPM.
2) - the right rear breaks loose and you have zero traction until the car gets rolling.
At least that's what would happen with my car when it had the 2.56 with a TH400 trans.
You can put in a posi carrier with 3.08's or 3.23's. The 3.08 set will be more highway friendly, but the 3.23 set isn't bad either. With the 3.55's I was turning about 3,000RPM at 70MPH. After putting in a 700R4 trans it now only turns about 2,100RPM at 70MPH with the 3.55's.
Spacers? What kind of spacers? To space the wheel out from the axle?

2.56 swap
 

I understand that with the 2.56 open rear I'll get one tire spinning. I was wondering whether replacing the open 2.56 rear with a posi 2.56 rear would give better acceleration from a stop. The other question was whether I could go to a numerically higher gear set with the 2.56 posi carrier? I've read where spacers are used to jump up to even higher gears, but I didn't know if you could go from 2 series gears to 3 series, as in going from 2.56 gears to 3.08 or 3.23. Thanks

The spacers are used on the carrier when you want to keep your 2 series carrier and use a 3 series gear on it. Here's an article that may answer some questions: http://www.chevyhiperformance.com/techarticles/48518/

going from a open 2.56 to a 2.56 posi is a waste of money.

when you buy a new ring and pinion it will say you have to have a 3 series carrier....

if you want to have a nice launch possibly look into an auburn unit......
I assume you have a BOP rearend ???

how tall are your tires ???

do a 3.55 ratio.......it's a huge difference from what you have now.
also...... do you have a stock torque converter???

2.56 swap
 

Yes, I have a BOP rear end. The open rear is the original in my '68 Lemans. The posi was reportedly from a 1967 Buick, but the date code is December 1967, so it's a '68 rear.

My tires are BFG Radial TA's 235-60-15's, not sure how tall. The trans is the ST-300 2 speed, all orignal. I was considering eventually putting in a TH 400 or a TH 350, but that's down the road some.

I don't know if I'd want to go up to 3.55 gears, but the lower 3's might suit me fine. Thanks for your reply

Buick's tend to like the 2.56 gears because they are torque motors. My 71 skylark has 2.56 gears and even though it is all stock and not posi, it does quite well for stock. It is also great for the highway. I was going to get a set of 3.08's, but was told that all highway power would probably disappear since stock buick motors dont really have upper RPM power. The Pontiac motor would do great on the highway with 2.73 or 2.93 gears as long as it is stock.

Acceleration with the 2.56 ratio won't be any different with your current open end or a posi. Having the posi is great as they aren't cheap. If you put on a set of 3.08 or 3.23 gears you will definitely notice the difference in take off acceleration and you won't have to buy a new posi carrier.
I also converted my 1976 Firebird Formula 400 to the 3.08 ratio and I like it a lot. The 3.55's would be better out of the hole, but the 3.08's cruise nice and it isn't a racer ... it's a cruiser. If you want speed buy a new carrier and 3.55 gears, but you probably won't like your car on the highway any longer.

I'm sorry if I hijacked but I also have a 2.56 non-posi in my 67 Firebird. Am I correct in assuming I can switch to 3.23 gears with only a new ring and pinion, not a new carrier? This would be great news as my car is only a fair weather cruiser. I am not concerned with fuel mileage/drivablility issues. Only looking to leave a longer black mark!!:D

There are 3 different carriers for the Pontiac 8.2" 10-bolt.

2-series: 2.56 & 2.78

3-series: 2.93, 3.08 & 3.23

4-series: 3.36, 3.55, 3.90 & 4.33

These are all factory ratios, the only ones available new aftermarket are 3.55 and higher numerically.

A stock 350 might benefit from the 2.56 posi if it currently seems to be spinning the passenger side rear tire on take off with the stock open differential (one-legger). However once you're moving the benefit is gone, no acceleration gain.:(

My brother had a stock 255HP 2-barrel 350 in his '69 Firebird (a 2-1/4" dual exhaust was the only change from stock), had a 2.78 open rear. We took it to the drags with both a 2.78 and a 3.55 rear. Surprisingly enough there was no gain in ET with the 3.55s, the car ran in the 16.70 to 16.80 range with either rear gear.

However once we swapped in a better cam (factory 068) and 4-barrel intake (stock '69 intake with a '72 750 Q-jet carb) it woke-up and ran between 14.80-14.90.:cool:

A swap to better gear ratio might help, might not with a bone-stock engine. If you do decide to swap gears go for 3.08 or 3.23 at the most.

You'll probably hate anything but the 2.56 gears on the highway now that you're used to them.:D

I think B-man hit it. My first car was a 69 Cutlass with 350 and 2.56's. Nice on the freeway, not too bad off the light. If you have a ST-300, you should have a switch pitch converter. If so, maybe Hughes may be able to rework it to give you a higher loose stall.

Yukon make new ring & pinion sets that fit the 8.2" Chev 10 bolt. These will fit the BOP 10 bolt, but you have to use the Chev diff yoke, seal & U joint. I just bought a 3.08 set & it looks well made. To fit the 3.08 & numerically higher gears, you need a 3.08 or higher case.

rear swap
 

Thanks to all for your replies and interest in this thread. Looks like I have alot to consider. Do gears from an open rear fit on a posi carrier? Say, if I could find 2.73 gears from a '68 to '72 A-body open rear carrier? Anyone know what A-bodies might have come with 2.73 gears stock from the factory?

The gears are the same for posi and open rear ends. You're looking for 2.78s, not 2.73s that came in later 8.5" 10-bolt 'corporate' rears.

Should be able to find 2.78s in '64 -'69 rear ends in both A and F-body cars, in '70 I know at least the ring gear changed using left-hand fasteners of a different size.

Lots of cars came with 2.78s but it's hard to nail down any specifics since the wide variety of engines and options available determined gearing back then. I'm sure plenty of those 2.78 gears/rears have gone to the crusher since almost nobody wants them now for performance builds.:(

Have Hijacked this old thread, i have a 68 lemans 350 2b (now 4b) with 2.56 gears oas original, now for the ignorant questions, BOP? body of pontiac? dont know what BOP means. Know what do i need to purchase to put in 3.08 or 3.23 gears. dont care if i have to use chev and different unijoint and yoke just need to know exactly which parts i need ie do i need a new axle housing etc. Thanks

Buick, Olds, Pontiac. It is different than Chevrolet.

Mike

Mash. I ended up buying a used 3 series GM posi carrier with 3.23 gears from someone on this forum and had it installed in my stock housing. Had new axle seals installed at the same time. I'm happy with the change, better acceleration but still tolerable on the highway. If I remember right I'm running somewhere between 2700 and 3000 rpms at highway speeds, but I don't do much driving on the highway anyway.