Hey guys this is my first post so be kind. I was wondering your opinion, which is better, limited-slip or posi. Daily driver car 86 firebird 2.8L v6

 

Welcome!

Lot of good folks here. Lots of people will tell you to search.

Posi is short for posi-trac, which was a GM branded name for the limited slip differentials used in 60's (?) era cars.

They are the same thing.

 

A Friend of mine told me that on a posi rear both wheels always spin sometimes causing the rear to "hop" in a turn while a limited-slip slows the one wheel down to reduce this. Another told me that a limited-slip is a lil slows on the take off because only one tire spins at first then the other kicks in seconds later.

 

Forget the above and read the second Post.

 

Nah, Limited Slip and Posi (Positraction) are the same thing. What your friend describes as a Posi that "hops" would actually be a locked rearend with a spool or something. If it has Limited Slip (or Posi if you wanna call it Posi) then it would not "hop" around turns. As far as Limited Slip being slow to lock it depends on how its setup and the condition of the parts. If its installed wrong or worn out then it might work as described. But if its installed properly and setup properly both wheels should lock at the same time going forward off the line. If you are unsure about your car just do a burnout while your friend watches both rear tires (or record it with a video camera and watch it later), thats how I check if my Limited Slip is working the way I want.

 

Thanks Guys uve all been a big help.

 

Limited Slip differential (LSD) = Posi

If you dont belive me, look it up on google. Posi-traction was Chevrolet's trade mark name on the Limited slip things. I belive MOPAR had Sure grip, Ford had Tru-Trac, Pontiac had Safe-T-Trac and i think AMC had Twin-Grip. Posi just stuck for some reason.

Three basic types of diffs:

Open Differential (which i think the first poster mistakenly called limited slip).
LSD (Or Posi), and Locker type differentials.

Lockers are the simplest, they constantly transmit all power to BOTH wheels, ALL the time. Perfect for 4 wheel drive type things. Problem with these is, on pavement, when you do a turn, the inside wheel which has a shorter distance to travel than the outside, will spin slower than the other. This is what the HOP is.

Next up is an Open. Famous for "one legging". This is a bit more intuitive. It solves the problem of the HOP, but doesnt solve the problem of traction. The way it works is, it will transfer power to the wheel that has traction, BUT this power is equal to the power needed by the wheel that DOES NOT have traction.

If youve, ever tried to push a car, or move a big peice of furniture, you will know about Friction. Static, and Kinetic (still, and moving). When something is at rest (stopped) it requires MORE force to move it. BUT if the same object is in motion, it requires much less force to KEEP IT MOVING AT THE SAME SPEED.

Case in point, the wheel that is spinning and has no traction, requires MUCH less power to keep spinning, than to get the other wheel spinning from rest. So if you ever get caught in mud, or snow, and floor the hell out of your open rear, youre not goin anywhere. Cuz the power transmitted to the wheel that ISNT spinning, is the same as the wheel that IS spinning, which isnt enough to get the car moving. So its best to VERY slowly let on the power, so the power transmitted is maximized.


Third type is the LSD. Through a series of clutches and springs, this type of rear end, transmits ALL AVAILABLE POWER to the wheel that DOES have traction. There by "positively increasing traction". This is why both wheels usually spin on a posi, because once one wheel loses traction, the other one gets the power, and if they both lose traction, they will both spin.


Go to www.howstuffworks.com. They have some cool visualizations to help you understand better, is reading isnt ur thing.

 

This isn't exactly true. Lockers engage and disengage. In a straight line they are locked mechanically as opposed to a posi/LSD type unit which uses a clutch type setup. When going around corners they unlock allowing the wheels to spin at different speeds. There are also variations that are controlled by the driver for lock and unlock. That's why the name "locker" since they have the ability to "lock"

Your description would be of a spool type which doesn't unlock.

 

Yea, so the "detroit locker" which uses gears instead of clutches, is noisy when unlocked. Like a ratchet clicking, when you go around a corner sorta deal.

Spool, mini spool, "lincoln locker" (welded spider gears) are the PERMANENT lock style. You can't drive those on the street or else it's quite dangerous.