65silververt

I'm working on a 66 Coupe with a 327/300hp engine and an automatic.

Someone swapped the original rear end out for a 1967 dated 3.70 Posi-traction unit.
From what I have read in my books, the car should have had a non-positraction 3.36 ratio rear end.

So, my question is, why was positraction not an option on automatic cars in 1966?

Thanks

kenba

I believe you could get a posi. with any transmission.

SWCDuke

Please state what "books" you are reading to warn others of this misinformation.

Positraction (RPO G-81) was an unrestricted option with all engine/ transmission/standard axle ratio combinations, and all but one optional axle ratio were ONLY available with Positraction.

The one exception was 250/300 HP engines with either four-speed or Powerglide transmissions from '63-'65. The standard axle was 3.36, open, for all transmissions and Positraction was optional. You could also order a 3.08, either open (RPO G-91, about two bucks) or Positraction, but only with a four-speed or Powerglide, not the base three-speed manual.

Fewer than 4500 were sold over the those three model years, which is probably why it disappeared after '65, and I suspect some were ordered erroneously because it's real easy to mix up G81 and G91.

I think the reason this option even existed back in the day is that some "safety experts" thought Positraction was "dangerous" because on a slippery surface like snow or ice when both rear tires broke loose the rear could go sideways. True, but if you know what you're doing a Positraction axle will get you out of a slippery situation while the open diff axle will just spin one rear tire in vane and be hopelessly stuck.

Duke

AZDoug

I think the only axle ratio you COULDN'T get posi on was the 3.55 in the C1 cars.

Doug

DZAUTO

If you bring your car to my house (or just bring the rearend), you can have posi with ANYTHING!

emccomas

That doesn't sound right. I have a 1957 passenger car rear chunk with a 3.55 posi in it.

AZDoug

The carrier the 3.55 gear set came in (at least 1959 and later) won't accept a posi case unless you grind some of an internal support web away. I would suggest you have a carrier originally for a different ratio and someone put 3.55 gears in, if that is what you in fact, have. Or someone removed part of the web, like I did, so as to fit a posi case.

The 3.55 was only available with a PG, and non-posi in later C1s. i cannot say for sure about earlier ones.

Doug

65silververt

Thanks for clearing that up! I appreciate the replies!

62corvette

AZDoug,
You are usually right on. However, 62’s had 3.55 with posi. CC was the code with standard brakes, CL with metallic brakes. That wasn’t just on paper-I have an original CC here in the car.
Don’t know how to make that fit with what you said regarding the inner web—this is the original 3789812 case.

midyear

if, for originality's sake, you need a 3.36 non posi, I have one dated 66 or 67 in my assorted rear ends, free if you want to pick it up.

dennis

65vette396

My 67 has a 3:36 non-posi with base 300 hp and ac. Posi was an option but I guess most people ordering didn't think an auto needed posi as the performance wasn't there. Other than leaving 2 black marks on the pavement, a posi was only good if you got stuck and needed both wheels pulling to get you out of the mud.

AZDoug

Must have been a '62 only option, probably to account for the larger motor.
The web thing is covered here:

http://www.dallasclassicchevy.com/im...0TO%20POSI.pdf

Doug

emccomas

As best I can tell, 3.55 posi was available in passenger cars but not Corvettes until 1962.

Always learning something on this forum.

62corvette

Doug,
I read that all the way through. I knew about that web—but there was nothing about 3.55 posi at all. Only about changing from non-posi case to house a retrofit posi. Nothing germane to 3.55 ring needing that web cut down. What did I miss?

AZDoug

Up until 1962, all 3.55 were open diff, you had to cut the web if you wanted to use install a posi case inside the open diff carrier the 3.55 gear set came in (from the factory). It seem pretty clear from my other posts, except for the 1962 thing.

Doug

emccomas

Can you tell us what rear end code AK or BK for 1957 to 1961 represents?

65vette396

My books don't show the AK code being used until the 65 MY for 3:36 non-posi with manual or PG trans.. The AK continued until MY 69. I didn't see a BK referenced in the NCRS Specification Guide of numbers.

William Buckley

Chris,
The owner's manual in the glove compartment states that positraction is available with the automatic transmission.

emccomas

The AK and BK codes are listed in Chevrolet by the Numbers for passenger cars from 1957 to 1961 as 3.55 posi.

CorvetteMikeB

I am getting my 3.36 rear differential re-built. Is there any reason to make it a positive traction? Or just have it rebuilt w/o positive traction....