craig71springsport

Looking for some info on the 9 inch. What exactly is the difference between an open and posi center unit? I was always told an "open" rear meant you didn't have posi traction at all, and only one wheel propelled the car forward at any given time, but I was recently told I was wrong on this.

Also, which is better to have, open or posi? I'm not even sure I'm using correct terminology, so much differing info coming from different places that I get all kinds of confused.

Thanks for the help guys!

Couper

Kinda depends on what you mean by better, if you have a straight stock six, no need for anything other than stock. Ford performance cars come/came with posi or trac loc, maybe equa lock if its old.

Posi is a GM term,...some guys just LUV to point this out. Either way, posi is the generally understood term to imply that power is distributed equally to both wheels, Ford, GM, whatever. Equally distributed,....unless you go around a corner then the inside wheel will slip so that it doesn't "chirp" as what would happed if you had a "true" posi.


Following is an excellent NINE INCH FORD link:

http://www.kevinstang.com/Ninecase.htm

2+2GT

I agree, a lotta mis-information is floating around out there.

Your original view on the function is about correct. Stop listening to the person who told you that you were wrong. In an open rear, both wheels are driven, but the power goes to the wheel that has the least traction. Usually, this doesn't matter, but when you do a burnout, or drive over sand at the edge of the road, or drive in snow or rain, this becomes apparent.

As stated above, "posi" is slang for Positraction™, and General Motors trademark. As in the Beach Boys singing about their "Four speed dual-quad Positraction 409".

The engineering term for this is "limited-slip". Early Ford limited slip, as seen on the 64-67 Mustang, was Equalock™. This was an internal clutch pack, kept in friction by a Belleville spring, that transferred a set amount of torque from the spinning, low-traction wheel to the other wheel. Even the 64-66 six-cylinder cars could be ordered with this.

Ford then introduced Traction-Lok™, which had a center spring block transferring a set amount of torque at all times, but mainly used axle gear thrust to exert pressure on the clutch pack at times of increased torque. This improved handling, since you didn't have high torque on the clutch pack when driving around curves in the rain, for example, but had even more when accelerating. Positraction™ always worked this way, something where GM was ahead of Ford. I have Traction-Lok in my 66, and when it was a driver, that T-Lok got me home through 8-12" snow one night when I was caught out in a storm. Made a real believer out of me. The Mustang had a reputation years ago (as did the Camaro, etc) of being really lousy in snow, or even rain. It was true, too. An open-rear Mustang could get stuck in the middle of a level road, I swear. With limited slip, though, even a six cylinder Mustang suddenly became an all-weather car. Limited slip is good for a whole lot more than drag racing.

Ford occasionally offered the Detroit Locker™, a differential that mechanically locks solid when wheelspin occurs. Neither Traction-Lok nor Positraction does this, despite urban legends to the contrary. If you don't know what it is, the clunk that results from a simple intersection turn can be unnerving, which was why Shelby dropped the locker as standard after 1965.

craig71springsport

So open rear = both spinning all the time, and Track-Loc? = limited slip?

bud41pen

I HAVE A 68 J-CODE GT FASTBACK W/A 302 ENGINE, 9 INCH REAR END, 3.25 POSI, COBRA HI-RISE INTAKE AND BORED TO 30 K. WHAT ARE THE BEST HEADERS TO USE? SHORTIES OR TRY Y'S AND WHY? OH YES C-4 AUTO TRANS TOO.

JULIAN BOMBACI
FORUM MEMBER

bud41pen

craig71springsport

Make your own thread ding dong. Thread jacking is a good way to get people to dislike you.

Couper

go away, very far away

craig71springsport

ANYWAY, which is better, open or Tracton-Lok? Am I correct that open just means they both spin at the same rate, and Trac-Lok just chooses the wheel with the most grip?

Also, if I find a rear that's a few inches smaller than what I'd need, could I still use it? Originally, I would need a 61.25 inch, but I found a 57 inch for nice and cheap. I'm assuming it'd kind of be like getting a bigger one and just shortening it a little? Possibly making it a plus since it could theoretically fit a wider tire?

Thanks guys!

andrewmp6

Open means only one wheel will ever spin at a time.For both to spin at the same time takes a lsd aka limited slip differential and there is a few different types it depends if the car is going to be street driven the most or a drag only car.