posi & LSD

[Jaymatic]

I know that there is a difference between Posi Traction and a Limited Slip Differential but I don't know what the differences are. I have searched all over the net, but all the sites that have any info about them seem to be describing each as though they were the same thing(which a lot of people think although they are not the same). Is there someone who could give a little explanation of what the differences are or maybe direct me to a website that really knows what each is and has an explanation of them.

Thank you in advance for widening my knowledge of this.

[Lord_Verminaard]

Good question, and since so many people use different terminology different ways (and different also depending on where you live!) it is hard to know for sure. Here's the best way I can describe it.
Posi-traction, basically is a differential that applies a certian amount of pre-load on each axle which is done by springs and clutches in the diff. (or cone-type clutches if you have an Auburn unit) So, if you have a Posi with a 180 ft/lbs split rating (just pulling a number out of my arse here for an example) and one wheel gets locked up in a rut, the other wheel will only spin with 180 ft/lbs of that split and not send anymore torque to either wheel, even the one that is locked. Eaton sells posi diffs like this, and many trucks in the past came with Posi-traction rear ends. It was also a popular upgrade for muscle cars in the 60's and 70's.
Limited slip diffs differ in that the difference in slippage between the axles is Limited by a gear type mechanism. Otherwise acting like an open diff until one wheel slips, causing the gears to engage and allowing the other wheel to turn a limited amount. These diffs arent usually used in many performance applications due to their historically weak strength, but on the other hand, for normal driving conditions, they were good and often used in passenger car applications. Our Torsen differentials are essentially an upgraded version of the limited slip differential, and while the term "LSD" is sometimes used to describe the Torsen, it's not really the best way, but it's a lot easier to say LSD than "Torsen". To prove my point go to your local old-school hotrod shop and ask them to describe a Torsen diff. [img]smile.gif[/img] Torsen stands for Torque-Sensing, and the gear mechanism can "sense" which wheel is getting the best use of torque and can send more torque to it. If both tires can use it, it sends equal torque to both wheels, like in a straight line burnout. Torsens, while not nearly bullet-proof like Posi's, are more durable and last longer than old-school Limited slips, and provide the best amount of traction under nearly any driving conditions. If you have a front wheel drive car, Torsens are the only way to go for a differential.

I guess that about covers it. So, on our message board, if someone mentions they put an LSD in their car, usually they are talking about the Torsen, although there are more than a handfull of folks on here running Auburn and Eaton units. If I'm wrong, please somebody correct me. [img]smile.gif[/img]

Hope that helped.

Brendan
2000 Camaro L36 M49

[SOONverySOON]

I was told by my teacher in my powertrains class(was going to school to be a mechanic) that they are the same thing just chevy called them one thing and ford called theirs something else and from what I have seen they are the same both are just locking differentials and that can be accomplished in many ways.

[Jaymatic]

Hey Brendan, Thanks for that post. It was really helpful in my knowledge of the differentials.

SOONverySOON, I was always sure that there was a difference in the two. I have always heard that though, that one company called it something different than another, but my take on it was that they pretty much functioned the same way, but differed in their methods a little bit, and that is why many people say they are the same. It was a very confusing thing to me and I was unsure of it and that is why I posted this topic. On the Modifications Encyclopedia under the differentials section of this site it does say that they are two different things but doesn't say why. That is why I posted this topic, I was wondering what the difference was and was having a hard time finding the info on the net. Brendan had a good explanation.

Thanks Brendan

[Lord_Verminaard]

Glad I could help. [img]graemlins/burnout.gif[/img]

Brendan
2000 Camaro L36 M49
84 VW Scirocco 8v

[V6Bob]

"I was told by my teacher in my powertrains class(was going to school to be a mechanic) that they are the same thing just chevy called them one thing and ford called theirs something else and from what I have seen they are the same both are just locking differentials and that can be accomplished in many ways."

Pretty much true. The buzzwords are generally used like this:

limited slip differential - the general term, includes every differential that can adjust where the power is sent based on what the car needs, like Torsen, positraction, clutch pack, locking differential, etc.

positraction - Not a technical word, just GM's trademark for some of their LSDs. I've only seen it applied to clutch type LSDs.

locking differential - aka "Detroit locker" another type of LSD, not much used for street cars

There are still more LSDs (like the viscous LSD) but they're not often seen on our cars

[hou-tex-six]

They are the same chit, dont overcomplicate things, be it by clutch packs or gears. Positrac is the GM trademarked name for their LSD back in the day as mentioned above.

Cp

[97rs]

no one has read this page? i think it was posted on here a while back. http://www.angelfire.com/my/fastcar/diffey.html

[Jaymatic]

That is an amazing article