corvette62

Thanks to Plasticman, Jeff and others guidance, I have located and just installed one of the New Process (MY6) tansmissions from mid eighties GMC & Chevy diesel pickups. Gear ratios - 1st 3:09 / 2nd - 1:67 / 3rd - 1:1 / 4th - .73
I like the transmission very much, however I now feel I need a higher differential ratio because 2nd gear is a little low in ratio. I currently have 3.2? differential ratio in the stock housing. I believe the next ratio would be 3.7?. I would want a posi-trac unit. Are used ones available? Approximate cost? This total restored car vette is only ran on the street. I do however, enjoy running through the gears frequently. Have 410 CI sb engine, with approximately 425 HP. What would be your suggestions. I want to keep the stock gear housing. As always cost is an issue.

Plasticman

Congrats!

I am running a 3.55 rear gear with smaller dia. rear tires than "stock" (26.1" vs. stock 27.1" or so), so my effective rear gearing is about 3.7. Have a 350 engine with all the goodies, but it has a very broad torque band (can cruise in OD (4th) as low as 1800 rpm, but it will wind (very quickly) to 6K at a whim.

wmf62 has a LT1 350 that has FI, and he is running tall rear tires (probably pretty close to stock dia.) with a 3.36 rear gear. The LT1 cam does very nicely in his combination.

Rear gear assemblies are available, since both wmf62 and I have purchased used ones (I think his came from Ebay, while I got mine at a swap meet). Both are posi units and ran in the area of $400.

Note that mine was a bolt in, since a Posi was already in there. On wmf62 Vette (note! With a 56 rear end, which is different than the later rear ends!), we had to do some removal of the driver's side axle length at the splines (cut off somewhere around 3/16" to a 1/4"). We are now told that "something" is wrong there, in that we should not of needed to remove as much as we did (even though we measured twice and cut once, and it has been working great since we did it a couple of years ago).

See this recent thread for additional info:
http://forums.corvetteforum.com/show...highlight=axle

In other words, yes you can change the rear gear via a used center section assembly, and you could even just change the ring & pinion, but that would not get you Posi. There are also Posi center sections available, and you can modify the existing case to accept the Posi center section and a new ring & pinion.

Another option is to change (lower) the torque band in the engine with a different cam. You certainly have enough cubes!

Plasticman

DZAUTO

The 55-64 pass cars and 56-62 Vettes used the same style removeable rearend center section--------------they are all a 100% bolt-in swap.
The gear ratios that were available from the factory during those years were: 3.08, 3.36, 3.55, 3.70, 4.11, 4.56. SUPPOSEDLY, there was a 3.27 ratio in 1956 ONLY in the Corvettes, NOT in the pass cars. From my perspective, the 3.27 would not be a choice simply because of its very limited use.
Positraction was available during 57-64 models, they are becoming somewhat scarce, expensive, PLUS they are 40+yrs old and the strength of these old posi units may be suspect after all these years of use, abuse and stress.
My personal choice is to convert a non-posi rear out of some little ole lady's 4dr family sedan that hasn't been abused, with one of Eaton's posi units that is now available for these early rears. I've done several of these conversions with the Eaton unit and have been quite pleased.

GCD1962

By the time you fool around with getting a stock unit with posi, the gears you want, etc. you will have almost as much in it as a 9" Ford rear. If you are putting out over 400 hp you really can't bang too many times on the gears before either the axles or rear disintigrates. I went to a 9" Ford rear the end of last year (mine also a '62 with approx 425hp) and I can finally "get on it" without any worrys. The entire set-up does not look much different than the stock. I used all the original mounting point, shock plates, brakes, backing plates and even the same brake lines.

DZAUTO

Sooooooooooooooo, whadda say we just throw a kink in everyone's thinking about these old style rears.
Waaaaaaaaaaaay too many people forget that these rears were FACTORY installed behind 63-64 425hp/409s.
And, here is Aubrey Bruneau's 62 409 drag car with a "stock" style rear. Yes, I admit, he has beefed it up somewhat, but when properly built and setup, these rears can take a lot more stress than a lot of people give them credit for (TURN UP the volume).
One of the quickest and easiest things that can be done to beef up one of these early rears is to add saddles to the carrier bearing caps.

http://www.youtube.com/watch?v=xt5vBtCK1Ao

Plasticman

Yep, the stock bearing caps were the death of our 57 Chevy bracket racer rear end (but we also twisted "several" sets of stock axles too, and even broke a set of Strange Engineering axles - all with a 332 SBC).

BUT, that is with a sticky set of slicks (in a 4 door body, with good ladder bars, and relocated battery for great traction), with very high rpm launches (but the bearing caps failed in the shift into 4th!).

Street tires in a Vette are great drive line savers. I break loose my 235 rear tires significantly when shifting from 1st (3.09 ratio) to 2nd (1.68 ratio) on a blast through the gears.

Plasticman

L78racer

tom, a guy in canada was selling saddles. are those his? who else has them? thx, paul

DZAUTO

Paul,
Yes, that is Aubrey in Canada. I do not know if he make the saddles, contracts someone for them, or if he uses a common source. But, I've not located another source, so I buy them from him. He also includes easy instructions with them.
Those pictures above are posi rears that I have done. The first picture is a stock DANA posi, the other 2 are Eaton converted.
I have also converted several NON-posi rears with the Eaton 55-64 posi units. The Eaton posi units have 17-spline axle gears for the stock axles. But, for an upgrade to a stock DANA unit, Aubrey has 28-spline axle gears and axles available. He also has spools for drag racers.
The important thing about doing these upgrades, is that it allows the retention of the "stock" type rear. Soooooooooooo, if this kind of strength and durability is the requirement, why in the world do all of this to a stock housing/rear??? 'Cause some rules/classes/car owners require a stock rear. I'm one of those people.

L78racer

so far, tom, i haven't needed them.(knock on wood) we run on street tires so i'm not really shocking the diff too hard and the car is light. a friend has Aubrey's saddles on his '64 409 and it's held up fine. it took forever to get them shipped. i think he does farm them out. pr

DZAUTO

That has been my exact experience-----------------------but I do finally get them. As a result, I've learned to keep a set in stock. Thus, when I install them for someone, I immediately order a set.

GCD1962

The 1st to 2nd shift is a big drop. With 3:70 gears lst gear would be wound up in a split second.

We used to sell the posi-rears with special steel bearing caps, but even these could only take limited abuse. We ran an olds rear in a '61 and a 302 ci motor. Even with only 7" slicks (max that could be used in M/P at the time) it would chew the R&P up in short order. I know the 409s used them, but they can't take extended abuse without the axles or r&p giving up. I still think a 9" Ford rear is a better investment and will last under repeated abuse.