Rear LSD question

[James614]

Okay, so my JDM Forester has a rear LSD. I can tell because the wheels turn together when its in the air and you can feel the resistance if you try to block one and turn the other.

My question, do VLSDs do that? Everything I've read about viscous units says they require heat to thicken the silicon fluid to transfer torque but I've never seen any mention just how low this threshold is. Is the fluid's viscosity thick enough under normal temps to transfer enough torque to spin both wheels and create noticeable resistance when the car is lifted? I've never heard of being able to test viscous LSDs this way.

Looking at the R&P would tell me for sure but I'm not about to go through the trouble of dropping the diff if its not 100% necessary.

[Mister Anderson]

Does it have a 4.44? Some of those JDM 4.44's were a mechanical type LSD R160 (I think only STi models) . I had a 3.90 VLSD in my Legacy before I swapped to a mechanical LSD R180. I do believe the opposite wheel did move in the same direction as the other, but only slightly when both wheels were off the ground with the VLSD. With my mechanical LSD the wheels feel more "connected" to each other, as in I can't break one wheel loose from the other with just my hands. With the VLSD I could easily do that. The diff cover does have an ID tag, but that doesn't mean the cover was used on another rear-end that someone could have swapped in. I would pull the cover off and look at the differential carrier if you want to be 100% certain.

edit- You can just pull one of the plugs (drain/fill) to look at the carrier.

[James614]

Your description, and the fact that the viscous LSD is standard on Foresters, pretty much confirms mine is viscous. Honestly I was 99.99% sure it was viscous since a clutch-type LSD doesn't seem like the kind of mod the PO would have thrown on there. I was just confused as I don't have experience working with LSD's on my own vehicles, and from reading/hearing other people talk about it I don't really have a good grasp of things like how much strength it should take to break loose different types by hand. To compound my confusion, the description of VLSD's acting "open" when cold makes me think "wheel spins backwards", which is what I was expecting from the VLSD when I had the car raised.

But now that I think about it more, being able to break loose a diff by hand means it will act more or less open when on the ground anyways, since the strength of my one arm isn't going to be powering my Forester out of a snow bank, lol. Hopefully that I feel decent resistance means the fluid hasn't been badly sheared and will still work decently this winter.

FWIW I'm pretty sure all the JDM Foresters are 4.44.

[Mister Anderson]

I think those VLSD's are kinda worthless in terms of functionality. If you could find a mechanical R160 that would be cool. They are around, and most of them are 4.44 ratio. The R160 MLSD's are a lot easier to find verses the R180 4.44 MLSD's. When I was researching the R180's out of earlier STi's (V.3-V4) I came across a decent amount of the R160 versions.

[James614]

For any performance application, I agree. They are very slow to react, and even in good working condition are unable to control wheelspin very much past their designed power capacity. I plan on trying to find a clutch type for my Legacy. I also plan on getting a 5MT DCCD trans for the Legacy to do away with the awful center VLSD.

My Forester is just straight DD though, I won't be full-throttle in corners or anything crazy. So slow reaction is acceptable as long as it makes both wheels spin.