Hi,

I had a question, so I searched the archives and didn't find anything on this, so here goes :)

I have passenger car 12 bolt rear in my camaro. I wanted to know if there is a way to put a gearset lower than 3.08 in this rear. I'd like to install 2.56 gears. Since I have 3.73 gears right now, do I need to change the carrier housing to accept the 2.56 gearset?

Thanks in advance,

Mike

Yes.

Casting number on the carrier should read ED32088. It will support a 2.73 down gear set. The lowest you can go with your current carrier is a 3.08 gear which was the highest gear set availbale from the factory in the late seventies for a passenger car.

When you say you wish to install a lower gear, its normally understood that you want to install a higher numerical ratio, ie. 4:10, 4:56 , etc.

If you say you wish a installer a higher speed gear , we understand going the opposite direction, lower numerical numbers. Confusing?

To install a higher speed gear , ie. 2:56 , I think you need to use a truck carrier of the same 8.875.

Why would you want to go lower that 3:08? A good all round gear is 3:55 or 3:36. Excellant performance is 3:73 or 4:10.

Is this for your '69 (car in sig.) ? If so you really don't want gears that high (2.56) with that car ! 3.73's should be about right ! If the car is not original, I would suggest an overdrive trans. and leave the gears alone.

Guys,

Thanks for your input. I know this sounds crazy, but here's my idea...

Yes, it's for my '69 camaro. I have a SB with about 350HP, an M21 trans with a 3.73 12 bolt currently in the car. Great performance, but the car is a pooch on the highway.

A Richmond ROD is a big itch to fit in, plus they're expensive. A TKO swap will also easily be $3K+ when it's done... My idea is to find an old Nash 4+1 trans and use 2.56 gears in the rear - like overdrive in reverse :thumbsup:

The 3.27 first gear in the Nash would give me the good launch and the rear would give low RPMs on the highway and low driveshaft RPMs... Good idea or just crazy? :clonk:

So.... To do this, all I would need to buy is a new carrier for the ring gear to accept the 2.56 gearset???

Thanks again,
Mike

Mike,
I think that I might have a used 2-series posi with a 2.56, if you don't find a new one . I'll have to dig thru some boxes.
I also have an old mag case DNE with a 3.50 1st gear.
Call me if you are interested in looking them over.
937-748-4254
Eric

I have another solution in mind to OD; the MoparA833 4spd manual transmission. It is actually a new process Mopar transmission that was used in Mopar muscle cars from the 60's up and was used behind big block Mopar engines.

In the late 70's the gears were changed to give an overdrive gear and the case was made out of aluminum (It weighs about 80lbs). The tranny is the Mopar version of the Muncie. The transmission was used in Chevy trucks in the early 80's. It was almost exclusively used in the GMC and Chevy diesel trucks with 6.2l engine. It was used in some gas Chevy trucks though. It used a larger truck aluminum bell housing which allows the use of a 12 inch clutch (instead of the 11" for BBC or 10.5" disc for SBC).

The bell housing for this transmission is unique, the bearing retainer hole is larger than a typical GM bell housing as is the 4 bolt pattern that bolts up the tranny from a Muncie. Also the 6.2 L diesel flywheel will not work on a gas powered Chevy engine (wrong bolt pattern at the crank), so you may have to buy a flywheel if you don't find one off a gas powered car in the bone yard.

The gear ratios are 3.09 1.67 1.1 .73


Larger Dave

2.56’s might be too tall for a 350hp small block. With a 1:1 fifth gear and 27 inch tires your engine will only be turning 1966 RPM at 60 MPH which is a bit to slow for most performance small blocks.

A 3.07 rear end with the Nash 4+1 would give you a 10.03 (3.27 X 3.07) first gear which is the same as an M21 with a 4.56 rear end! In fifth gear (1:1) your engine will be turning 2358 RPM at 60 MPH which is perfect for cruising. You can also save some money by using your 3 series carrier with a 3.07 gear set.

For comparison your current M21 / 3.73 setup only yields a 8.206 (2.20 X 3.73) first gear while at 60 MPH in high gear the engine is turning 2865 RPM.

Guys,

But aren't the ratios on the Nash just right with a 2.56 or perhaps a 2.73 rear? I'd like to be at about 2200 RPM at 70 MPH. I can downshift to 4th if needed at 55MPH and I'm back to about 2100 RPM. But I'm thinking you might be right - a 2.73 might be better. I think it's 2190RPM at 70MPH with a 28" tire (I'm basing my calcs on 28" dia tires since I have 255/50R15 on my car.)

The TKO600 however is ridiculous - It's a 1000RPM drop from 4th to 5th when it goes from 1:1 to .64:1 at 70 MPH

Any ideas about getting that 2.73 into my 12 bolt rear?

PS - Eric, I'm interested and left you a message with my number. Give me a call when you have a chance.

OK, my '80 P/U has a P/U 12-bolt under it with 2.76 gears.

I figure that you will need a 2-series CAR 12-bolt carrier like already said to be able to use 2.56's or whatever.

And a 12-bolt uses 4.10's, 3.31's and 3.07's vs the 8.2" 10-bolt at 4.11's, 3.36's and 3.08's...

And the A833 OD 4-speed is almost too weak to use.. I think it is only rated at 275 t if not mistaken..

And Eric, please PM me if Mike backs out b/c I'm interested in your 2.56 posi too..

Heck, I figure my "tow-truck" 496 motor will pull it fine!!

pdq67

There are no new 2 series differentials for the car 12 bolt, so you will have to find a used one. The truck 12 bolt is different, so don't use one of them.

Thanks for all the replies! I need to find a 2 series carrier

I contacted Eric - I think he has what I'm looking for :-)

Mike

I new a guy who had a 68 elcamino with a 1980s Z/28 BW 4sp with the really low first gear and a 2:73 rear gear. He called it a poor-mans 5sp
it worked pretty well with his 300hp 350cu in engine

Probably a 3.50 ratio 1st gear Sag. 4-speed....

It will do fine until he HOOKS once or twice!!

pdq67

PS., I replaced my old P/U's 3.50, 3-speed with a 3.50, 4-speed....

A BW from an 80 Z28 would be a 3.43 ratio ST-10, I think.

I have one behind a 292 CID small block in my 38 Chevy Sedan.

pdq67,
Mike is probably going to buy the 2-series posi and a 2.73 gear.
Also I think the car posi and gearset wouldn't fit in a 12-bolt truck rear.

Eric

Mike, A 2.56 rear with a M-21 will be HORRIBLE in 1st gear! I had one with a 3.08 and it was barely workable (lots of slipping the clutch to get it moving). Do a little more thinking and math before you make your decision. Redoing the trans AND rear might be a bit more money than just putting the right trans in.

Kevin,

Yea, but I'm removing the Muncie and replacing it with a DNE 5 speed

Mike, comparing apples to apples, a new TKO 500 package is $3300, a new bare Richmond street 5 speed is $1900 + Approx $500 for new gears, carrier and R&R, so the Richmond is around $2400 a Diff of $900, but then you need a shifter, plus other misc.

2.73 with a Richmond 26.5 tires is RPM 2250, 3.73 with TKO 500 26.5 tires RPM is 1530. (at 65 mph)

Did you check out this page? http://www.5speedtransmissions.com/5sp_comparison.html

The A833 was used by Mopar exclusively behind the 440 engine and the 426 Street Hemi. It is the same box as used by Ronnie Sox to clean up in the Pro Stock wars before the NHRA changed the rules to let somebody besides Mopar win all the races (and this is from a Chevy fanatic).

It is more than strong enough, if you can still find a OD tranny from the eighties in a junk yard that was only put in light trucks (not cars) and mostly diesel powered trucks at that (not a hot seller from the salvage operators point of view, but it does prove it can handle high torque loads).

You could check e-Bay for a 2.56 or lower numbered 12 bolt passenger car gear but I do not think there is one left upon the face of the planet. There are a few open 12 bolt carriers left as people have pulled them out in ages past (like before '71 while still in production) and saved them (for reasons known only to pack rats like myself). That being said an OD transmission is the direction the automotive industry is moving in (I include gear Vendors as an option also). If you wish to swim against the tide I wish you luck and be sure to wear a Chevy orange inflatable device so those who wish to save you can find you again.


Larger Dave

LOL!! That was funny Dave! My life jacket is now inflated. :) But A833 is more work than the change I had in mind. It's also only a 4 speed.
Using an old DNE with a 10 spline is almost a no-brainer...

Kevin,

The gears I'd put in are not new since I think they're out of production (not sure), and the trans is used also. I posted in the "wanted" section for a DNE or Richmond 5 speed. Eric said he has a 2.73 gearset for me :thumbsup:

Also, 1530 RPM at 65 wouldn't work well with my engine. It needs to be slightly above 2000 to run right with the cam in it. With my setup it would be about 2300 RPM at 70 MPH

Peace,
Mike