this is the beginning of an lsd FAQ.please do not respond to this post.i am stealing all kinds of info from everywhere i can find it..
What is a Limited-Slip Differential, and what are the different types?
A differential of any type allows two output shafts to spin at different speeds (See http://auto.howstuff...ifferential.htm
for more information). This is important when going around corners, as the inside and outside drive wheels are spinning at different speeds. Most differentials are of the "open" type, meaning they have no limitation on the difference in speeds between the two output shafts. Without a limited slip differential, when one drive wheel gets stuck in a ditch, it could spin 100mph while the other drive wheel stands still. By their very nature, open differentials send power to the wheel with the least grip. An LSD is a differential that prevents one rear wheel from spinning while the other just sits there, but still allows for a variation in speed between right and left wheels as the car goes through a turn. In other words, the LSD unit limits the speed difference between the two wheels, allowing torque to be applied to a wheel when the other is spinning without traction. Why would anyone want an LSD differential? They allow power to be applied through two tires instead of one, and often means you can apply the power sooner coming out of turns. It is important to remember that although we steer the front wheels of a car, the car actually rotates around the rear axle. Limiting the differential's slip limits this rotation somewhat. Drivers preferring to drive the rear end of the car (oversteer) prefer a locked or severely biased limited slip, whereas drivers that prefer to drive the front end (understeer) of the car prefer a looser differential setup. Both differential types can be fast but consider that the looser differential is gentler on tires and may be easier to drive. What are the downsides? An LSD installed into an already balanced chassis can cause a dramatic increase in understeer on dry pavement, and may even cause oversteer on wet pavement, requiring changes in swaybars, springs, and shock settings to return the car to neutral. There are several variations of LSD differentials, which are very well discussed on Gordon Glasgow's LSD tech page (http://www.gordon-gl...rg/lsdtech.html
) or at http://auto.howstuff...fferential6.htm
, but I will briefly list them below: Welded Differential:
Actually, this is not an LSD, but the modification is a popular with road racers as a low-cost way to make a locked rear end. By welding together the side gears and the spider gears within the differential in several places, the rear wheels are both forced to spin at the same speed. By definition, this is no longer a differential, as the rear wheels cannot spin at different speeds. This is fine at higher speeds on the race track, but is really unsuitable for street driving, as the rear end of the car skips/hops across the pavement as you go through low-speed turns since one wheel cannot spin at a different speed from the other. It also can cause severe understeer in a 510, not to mention breaking at the worst possible moment when those welds let go. Detroit Locker:
A popular option on the Detroit Muscle cars, this is mechanical differential that acts like an open differential until power is applied, at which point it locks up and gives power to both rear wheels simultaneously. Unfortunately, this differential either gives you complete lock-up or no lock-up at all. These are available for the Datsun/Nissan pick ups, but will not be discussed here. Limited Slip Differentials:
These come in several varieties: Clutch-Pack
(or Salisbury), like the Subaru LSD, Kaaz, Cusco, ATS, and Power Brute units, Viscous Fluid units
(as used in the early Miata, 240sx, 300zx, and '91-92 Subaru Legacy turbo 4WD sedans, as well as all LSDs found in '00 and up Subaru Imprezas, WRXs, Legacies, and Forester GTs), and the Mechanical Torque-Sensing
units like the Quaife and the Gleason-Torsen (the latter came in the '94 and newer Mazda Miatas and 3rd generation RX-7s, Lexus IS300s, HumVees, etc.). All of these combine the streetability of an open differential with the advantages of a locked differential, but there are differences between the three types of LSDs. A brief history of LSD technology and applications can be found here
.The Clutch-Pack ( Salisbury) LSD
is what we're interested in. This is the kind of R-160 unit you can buy new from Nissan Motorsports or Subaru for $800, or used from a salvaged Subaru for under $300. These LSDs have an assortment of friction disks and shims inside, arranged so that the limited slip typically has a factory breakaway setting of 45 ft-pounds (allowing the rear wheels to turn at different speeds if they have to). See exploded diagrams of this differential from the Subaru Manual here
and a close-up diagram here
. The major downside is that as these clutch disk units wear, their breakaway torque setting gradually lessens, so they become more like an open differential until you rebuild them again to get the breakaway back up to 45 ft-lbs. Gordon Glasgow's web page
(link URL is at the beginning of this FAQ) tells how to rebuild these units, but it is tricky, as the only way to adjust the breakaway is to use different thickness shims, reassemble the entire LSD, and then see what you've got. When Bluebirds list member Gary Savage had his LSD rebuilt, adding only one extra shim sent the breakaway up to 180 ft-lbs, which is a bit too much for everyday street use. For reference, the 2.5 Trans-Am race series BRE 510s had breakaway settings adjusted to 150 ft-lbs. Too high a breakaway pressure will cause clutch-pack LSDs to function as locked differentials, as well as creating severe understeer at corner entry as well as power-on corner exit. Anything under 100 ft-lbs should be fine for a dual-purpose street car. Pro:
Cheap (used Subaru R-160), easily bolts in. Con:
Hard to find, limited ratio (3.7), unless you swap R&P, clutch discs wear out, hard to adjust breakaway setting. For serious on-track duty, these setups generate HEAT, so a differential cooler may be in your future. See the section on Other Datsun 510 LSD Options
for more information on the various aftermarket LSD units.
(stands for TORque-SENsing) units are often regarded as the best LSD. Their locking action is via a complicated worm gear setup (read the technical paper here
or go to the Torsen
site), so they have no clutch plates to wear out and do not have the delay in locking up that some report with the viscous fluid LSD units. In a no-slip condition, the differential splits the torque 50:50 between the two drive wheels. When wheel slip occurs, the unit sends more torque to the axle with more grip (via the torque-multiplying characteristic of the worm gear mechanism), which in an autocross or race track situation is the outside drive wheel. By design, these LSDs separate the speed differentiation and torque distribution functions of a differential, resulting in a proactive LSD that actually prevents excessive wheelspin under acceleration. Different applications come with different torque transfer ratios (torque bias ratio, or TBR), capable of transferring torque to the non-slipping wheel at a ratio of up to 9:1. The locking action of these units occurs only under acceleration, and is instantaneous and progressive in nature. Under braking, the differential behaves like an open unit. However, there are no OEM applications that easily fit under a 510, and they are very expensive (Miata Torsen units are $1200 used, $3000 new, and I have no idea what a Lexus IS300 Torsen would list for). There are rumors of both Miata and 3rd generation RX-7 rear suspensions with LSDs being installed under 510s, but I have not seen any of these in person. Probably the easiest solution (if you can afford it) comes from Quaife. Quaife
now has an LSD unit that fits in a Nissan R-180 or Subaru R-160 case that sells for $1,500 new, though they can be found at times for $900. Ted Hedman has one of these R-180 units in his 200hp Autech-powered SR20DE 510. One thing to know about the Torsen or Quaife units: they require BOTH wheels to have some traction in order to work as an LSD, as the differential "senses" the difference in torque between the two. If a drive wheel comes off the ground, it will spin just like an open differential, as zero available torque (a spinning wheel) when multiplied, is still zero. Pro
: Durable unit, smooth, quick action Con
: Pricey, no traction if wheel comes of the ground, cannot "tune" locking action.Viscous LSD
units are popular OEM LSD solutions, as they are relatively simple and cheap to produce. They come in many performance Subarus, Nissans, Mazdas, Toyotas, etc. The LSD unit consists of stacks of thin plates with holes or slots, all suspended in a special silicone fluid. They have no clutches to wear out, and locking characteristics can theoretically be changed by varying fluid viscosity. Generally, however, these units are non-serviceable, and require no special maintenance. As the differential spins, the plates shear the fluid up to a point, after which the fluid provides some resistance to shear, allowing 15-25% torque transfer to the other wheel. The downside is that these units don't act like a limited slip until one wheel actually starts slipping (i.e., they don't prevent slippage), which means the VLSD action often kicks in after you've already exited the corner. Compared to the proactive nature of the Torsen LSDs, the VLSDs are reactive units. They do not prevent slippage, they merely sense differences in rotation, not torque. They also don't allow for very much torque transfer, compared to mechanical or clutch-pack LSDs. They do still work well for starting from a dead stop in slippery conditions. It is important to note that VLSDs locking characteristics occur both during acceleration AND braking, as it can't tell the difference between the two, but merely reacts to the rotational speed differences between the two drive wheels. Another problem with VLSDs is the limited availability of applications that easily fit 510s. Subaru USA lists the '91-'92 Legacy 4WD Turbo 4 dr sport sedans as having an R-160 viscous 3.9 LSD option. All OEM Subaru LSDs since 1991 are viscous R-160 units (Note: No U.S. Subarus between 1995 and 1999 came with LSDs). These LSDs have their half-shaft axle stubs held in with internal C-ring retainer clips, not bolts as described below. See Adobe Acrobat pdf pages from the Subaru Repair Manual with diagrams of these LSDs here
. The viscous LSD can be fitted under a 510 with major half-shaft modifications, as the 510 rear track is about 50" and the Subaru rear track is about 56" wide. The modifications would include either a way to use the 510 bolt-in axles stubs with the circlip LSD diff (how?), or shorten the stock Subaru half-shafts and somehow make an adapter so it bolts to the 510 wheel hubs, or choose to make custom half-shafts from scratch -- you choose. I have never seen one of these conversions in the flesh, but when I do, I'll report on it here. Pro:
Readily available from newer Subarus (R-160), Nissans (R-200), smooth action, no special maintenance needed. Where Does Subaru Fit in?
Subaru is also partially owned by Fuji Heavy Industries, and through miraculous good fortune for us 510 owners, decided to use the R-160 differentials as the rear differential in many of their all-wheel-drive cars starting in about 1985. Cars which use the R-160 include the BRAT, Loyale, GL, RX, XT, Impreza, WRX, Legacy, and Outback. Most of these R-160 differentials are NOT limited-slip, but as they come in 3.70, 3.90, 4.11, and 4.44 ratios, they are an attractive replacement unit for a tired 510 differential, and can often be purchased for less than $50. Note that LSDs were NOT available in any 1995-1999 U.S. Subarus. Installation of an Subaru open differential would be the same as the LSD instructions that follow below. How do I find a Subaru LSD?
The hard part about finding these LSD units is that almost any Subaru could be ordered with one, yet very few actually were. I'd guess that less than 5% of the cars came with LSD units, judging by what I've seen in yards. Perhaps those of you in mountainous/snowy climes might see more LSDs than those of us in flat/hot areas. What this means that there is no "one" Subaru that for sure has an LSD unit of a given ratio. Most likely clutch-pack LSD candidates are the '85-89 EA82 platform 4WD turbo cars, often with the 4AT (4 spd Auto) tranny. Rumors have it that all Turbo 4WD RX coupes and Turbo 4WD GL-10s came std. with LSDs. High-buck XT-6s, XT Turbos, and possibly even Brats may also have them. Anyway, the LSDs you'll find will be 3.70 ratio. This is fine for a street 510, and will actually make freeway driving less buzzy, as your engine revs will be lower at any given speed (compared to the stock 510 3.90 ratio), but it may hurt your 0-60 acceleration times. For an auto-x or road-racing car, you'd probably be happier with a 3.90 or 4.11. I've heard rumors of 3.90 and 4.11 clutch-pack Subaru LSDs, but never actually found one myself, nor seen one. As an aside, most manual transmission Legacys have 4.11 R-160s that are non-LSD (Auto tranny cars have 4.44 ratios), giving you a 4.11 ring & pinion you can drop the 3.70 LSD clutch unit into (using the special LSD bolt set described below). I did just this by purchasing a used Subaru 3.70 LSD unit and a used legacy 4.11 open R-160 differential and creating a 4.11 LSD unit from the parts. I paid a rear-end shop $120 to drill the six 10mm holes out to 11mm so the LSD unit's bolts could be used, and to set up the newly assembled unit with the correct tolerances. Gary Savage did put a Subaru LSD carrier from a 3.70 ratio differential into his 510 using the 510 differential case, the NISMO LSD bolt set and the Nissan 4.11 ring & pinion to get the 4.11 LSD he wanted. '00 and newer Subaru Foresters and Legacys are available with 4.44 R-160s (open or viscous LSD), with rumors pointing to the finned rear cover being a clue to the identity of the LSD differentials. 2002-2003 WRXs are available with 3.54 Rear Viscous LSD.
Used Subaru R-160 LSDs go for between $100-$300 at the yards (when they have them), though I've heard of smart shoppers getting them from U-Pull-It yards for as little as $30. The good news is that most of these rear differentials are barely broken in, so they shouldn't need rebuilding. A major problem is that most yard folks don't know much about them, and don't know how to tell an LSD from an non-LSD unit. Furthermore, I've heard from several yards that there are different universal listing code numbers for an open and a locked Subaru R-160 differential, but that there is just a single code for all 3.90 ratio Subaru differentials, making it impossible for them to search via teletype for 3.90 LSDs. Many people I know have been sold LSDs that actually weren't, so make sure it's an actual LSD before you pay for it or at least know what the return policy is before you leave the yard. For these reasons, I prefer to buy from a local salvage yard and let them deal with getting the LSD from a far-away locale.
However, Subaru made it easier for us to tell what kind of differential is installed in their cars by just looking under them. Almost all of the older Subaru differentials (both LSD and Non-LSD) have a gold or silver foil sticker on the outside of the rear case cover stating the Subaru differential part number, the ratio of the differential (i.e. 3.70, 3.90, 4.11) and whether or not it is an LSD (if it is, it will have "LSD" in 1/2 inch-tall block letters on the left side of the foil sticker, as you can see in the picture below). The above ratios are the ones I've seen on Subarus in yards around the country. Sometimes the gold foil gets really grimy, but you can gently scrape it with a screwdriver to pull off a clear covering from it (like a helmet visor tear-off) to get a better view. The foil sticker makes it really nice and easy to see the differential ratios from under the car without counting ring and pinion teeth or driveshaft/rear wheel revolutions (see picture below). Later subaru differentials, by the way, do not have the gold foil stickers on them.
Foil Sticker off Rear Case of Subaru R-160 LSD Differential (Left),
Image of same sticker from Subaru Manual (Right)
Sure Ways to Tell if it's a Clutch-Pack LSD:
- Check the sticker! It should have the letters LSD on it (see pictures above).
- Rotate one of the half-shafts (both half-shafts rotate the same way if it is LSD. If one rotates backwards and one forwards it isn't)!
- Drain the fluid from the differential and peek inside through the fill or drain hole with a flashlight. The clutch-pack LSD unit looks different from the open unit. You could also stick a finger inside the fill or drain hole and feel the difference between the two. The clutch pack is pretty obvious. The internals of some Subaru viscous LSD units will look the same as an open unit, as the viscous plates are hidden behind the ring gear.
What if you find a 3.70 Subaru LSD, but want a 3.90 or 4.11 ratio? I have heard that the 3.70 Subaru units will fit inside the 510 R-160 cases, and that the 510 ring & pinions will fit inside the Subaru R-160 cases. I have not seen nor done this myself, so I cannot offer specifics, nor guarantee that this is the case. I am sure that Subaru LSD units will interchange inside all of the Subaru R-160 cases, using various Subaru R-160 ring & pinion ratios. I purchased a used Subaru 3.70 LSD unit and a used legacy 4.11 open R-160 differential and created a 4.11 LSD unit from the parts. I paid a rear-end shop $120 to drill the six 10mm holes out to 11mm so the Subaru LSD unit's bolts could be used, and to set up the newly assembled unit with the correct tolerances. You MUST
use either the Nissan Motorsports LSD 10mm x 1.25 ring gear bolt set (Part # 99996-D3100, bolts cost $8 each and you'll need 8 of them), or just BE SURE
to use the bolts that came with the LSD unit, not the ones from the open differential whenever you do this or your LSD innards will chew themselves to pieces