Installing Subaru Limited-Slip Differentials
When I heard that Subaru’s had LSD diffs that fit into 510’s, I thought “COOL!”. So I did some research, talked to some people (including Subaru and Subaru mechanics). Here is what I found, thought it may not all be correct, depending on the reliability of my sources. Subaru used both open and LSD R160 diffs in their cars beginning in ~1986. Almost all of the diffs (both LSD and Non-LSD) have a gold foil sticker on the outside of the case cover stating both the ratio (i.e. 3.70, 3.90, 4,11) and whether it is LSD (it says “LSD” on the foil). The above ratios are the ones I’ve seen on Subarus in yards around the country. Sometimes the gold foil gets really grimy, and you can pull off a clear covering from it (like a helmet visor tear-off) to get a better view. The gold foil makes it really nice to see ratios from under the car without counting teeth or revolutions.
The hard part about Subarus is that you could order them almost any way you wanted. Less that 10% of the cars came with LSD diffs, judging by what I’ve seen in yards. Perhaps those of you in mountainous/snowy climes might see more LSD diffs that those of us in flat/hot areas. This means that there is no “one” Subaru that for sure has an LSD of a given ratio. Most likely candidates are 4WD Turbo cars, often with the 4AT (4 spd Auto) tranny. High buck XTs also seem to have them, and possibly even Brats. Anyway, most of the LSDs you’ll find will be 3.7s. I’ve heard of 3.9s and 4.11s, but never actually found one myself, nor seen one. Subaru USA said that ’91 Legacy AWD Turbo 4 dr sedans were available with a viscous 3.9 LSD. Never seen one. Most Legacys have 4.11s that are non-LSD. Used Subaru LSDs go for between $100-$300 at the yards, when they have them. Most yard folks don’t know much about them. Many people I know have been sold LSDs that weren’t. Check the sticker! Check the rotation (both half shafts rotate the same way if it is LSD, one rotates backwards and one forwards if it isn’t)!!!
I have also heard that the 3.7 Subaru units will fit in SOME 510 cases, and that SOME 510 ring and pinions will fit in SOMES Subaru cases. I have not seen this done myself, so this is only hearsay. Perhaps some of you out there know more? I do know that you should definitely use the NISMO LSD short bolt set whenever you do this, or your innards will chew themselves to pieces (or so I’ve been told).
SO why do these things fit? Well, Nissan owns part of Subaru, which is actually a division of Fuji Heavy Industries, which uses (like Nissan) Hitachi R-160 diffs for some of their cars. How do they fit? The Subaru diffs (both LSD and non-LSD) bolt in just like the stock R-160 units we know and love except for the half-shaft connections. Get a friend with a deep Torx socket and an impact wrench (or a deep 6mm or 8mm socket can also be made to work–I forget which is right) to remove the stubs from the Subaru diff. Then remove the Nissan stubs from the stock diff, and bolt them on in place of the Subaru ones. Everything else just bolts right up. Cool. Just make sure your bolts are tight!
In summary, the 3.7 LSDs are around and available, though hard to find. Rumors of other Subaru LSD ratios may be just rumors. Almost any AWD Turbo Subaru can have an LSD (look at Brats, Legacys, RXs, XTs, Loyales, GL-10s, etc.), but it seems that the more loaded the car, the more likely you’ll find one.
Hope this helps. Feel free to correct me where I’m wrong, or add/elaborate where I’ve left something out. Don’t call Subaru and ask them about this stuff. I’ve done that many, many times, and more often than not, they have given me inaccurate info. Of course, you could just buy a complete Subaru LSD from them (for $800), or from NISMO (also $800). Junkyards are more fun, and they are much happier on my wallet.
For you Datsun 240Z folks, the R160 LSD from a Subaru will bolt right in, and is an awesome addition. Just make sure if you run on the track that you use a synthetic gear oil, otherwise you will burn it up in no time.