LSD Question

[zuperair]

I've searched this and came up dry, so I hope someone has an answer for me. Lots of talk about the LSD's on this site, and I read this very informative post through and through as well...

 

http://forums.hybridz.org/showthread.php?t=114798&highlight=differentials but it doesn't answer my questions...

 

....with my limited knowledge I've understood there to be "one wheel wonder", "limited slip", "Posi Traction", and "spools" in rear drive cars. A one wheel wonder car will only spin one tire period (we've all presumably had one or two of these)...a limited slip car will mostly spin two...a posi will mostly always spin two, and a spool of course will always spin two.

 

So, since there wasn't a limited slip diff it's said... (http://datsunzgarage.com/engine/ ) until 1987....why have all the z cars I've owned (and there've been plenty) spun two most of the time? If this isn't limited slip...then what is it, and why is limited slip better? What advantages are there to an LSD, if I've already got a diff that will spin up two wheels?

 

2nd Question... I bought an r200 3:90 Diff supposedly out of a Japanese Skyline... I'd like to know how to Identify it as such, and to Identify it as a true LSD diff but haven't been able to search out any answers. Any help (or schooling) would be greatly appreciated.

[JMortensen]

1. The independent rear suspension in the Z loads the rear tires much more evenly than a solid rear axle, so it is very common for a Z to leave two black stripes. If you want to check for an LSD by driving the car, do some donuts. Turning will unload the inside tire and that will lead to spinning one tire if there is no LSD.

 

2. Pull the cover, compare the carrier to the pictures in the thread you linked to.

 

Not LSD:

 

LSD:

[johnc]

Back to the basics:

 

http://auto.howstuffworks.com/differential.htm

 

And an open diff will spin both rear wheels if traction is the same on both sides - something an independent rear does. Also, Posi-Traction is a GM brand name for a clutch pack limited slip differential. Its no different then any other CLSD.

[Roostmonkey]

Jack up the rear end so both wheels are off the ground and give a tire a spin. If the opposite tire spins in the same direction, its an lsd.

[johnc]

If the opposite tire spins in the same direction, its an lsd.

 

Or its a welded diff, a spool of some kind, or full of sawdust and grease.

[zuperair]

I appreciate the responses and the "how stuff works" link. The only question I have left really is.... If an Open Diff is most common, and the Z spins two as a result of the "equal torque" being applied to both wheels....why would say my old Buick always spin one, or my old Mustang, or the list goes on. The Z's have all laid down two, although the "Donut" test is a great example since they would lay down two as long as you were going relatively straight. The "one wheel wonder" that many of us grew up with never would spin two....and I mean never. Were they just super cheap open diffs that power would always make one slip? Just wondering, since the Z question was answered perfectly...thanks!!!

 

Oh, and looks like I'm good...thanks for the pic example jmortenson, here's a shot of the 3:90 LSD that I've got.

 

 

Thanks again guys!!!

[FlatBlack]

Hey zuper have you run the 3.9 yet? I am about to get one but im thinking interstate travel is going to blow - I run at near 3500 RPM around 75 mph and its painful now with my loud exhaust.

[johnc]

Were they just super cheap open diffs that power would always make one slip?

 

No. As alluded to above, the independent rear suspension on the Z more equally loads the tires under straight line acceleration. A live axle car loads the tires unevenly (left side more then the right under acceleration) so the right rear tire breaks traction before the left and thus gets all the torque from the open diff.

 

An open diff is an open diff regardless of which car its installed in. The open diff will always send all available torque to the tire with the least amount of traction. If both tires have equal traction, they will both get the same amount of torque until there is a traction difference.

[zuperair]

Thanks John, makes sense. I definitely understood what Jmortenson was saying there given the independent rear. It just seemed a bit odd that I've never got any of those cars to spin two. But those solid axles do torque twist when your hard on it...so that would explain my "always" scenario.

 

 

Hey zuper have you run the 3.9 yet? I am about to get one but im thinking interstate travel is going to blow - I run at near 3500 RPM around 75 mph and its painful now with my loud exhaust.

 

No, I haven't put it in yet. Timeline now puts the install probably around 4-6 weeks, I'll definitely let you know what I find if you haven't done one by then. Hopefully the snow will hold off so I can check it out. I've done a few calculations for RPM given certain speeds and it will be a lot different than the 3:36's and the 3:55's that I'm used to. I'm not going to do much road tripping with the car, or I might be a little concerned since I'm going to be running a loud exhaust as well. But then again I loved the sound of my 260, fully open with webers....so not sure if it's gonna bother me or not. If it's a problem I'll just swap it back, but I think the added zip will be a riot.