downforce

I've read very few posts, magazine articles, etc. regarding the limited slip differential. I know that Nissan has designed a great LSD for the Z with 245/45 18 tires, in stock form with 274 lb/torque. But what happens when you start unleashing HP and add some wide tires?

From what I've read, Sport Compact Car Mag (who has written the most about this so far, although I know some don't agree with some of there testing or findings) said that the stock Viscous type LSD is great for the normal driver who may push it a little bit and it helps to keep traction, but when you really push it hard, (wider tires, possible more HP) the Viscous fluid breaks down and is ruined from excessive use.

I know this is a long post, so sorry, but hang in there!

What I eventually want to end up with is a very well balanced great handling NA sports car, by tuning it a little along the way i.e.; pwr steering/engine oil cooler, clutch/flywheel, lsd, cams, ecu, forged rims and so on. Not looking for a FI power monster or 1/4-mile times necessarily.

To the question; do you have, or know of someone who has, first hand experience/knowledge regarding this issue, or can point me in the right direction. Because I know that just adding a clutch type lsd is not just a simple solution. There are 2 way 1.5 way lsd, there are multiple plates to stack different ways to yield results for width of tires, understeer, cornering vs. straight-line acceleration. I don’t want to be uneducated when it comes to making a decision like this.

Any help would be appreciated, especially from Performance Nissan and the like, who do this all the time!

Thanks!

Audito350Z

iTrader: (1)

do you plan to do HPDE's or track events (road course, not drag strip)??? If so, then you may benefit from a cluth-type LSD as you driving ability increases and you reach the limits of the car in it's OEM state of tune. For example, my NISMO LSD has increased my traction as I power out of turns at Texas World Speedway... the VLSD would allow too much tire spinning, even with r-compound tires.

If you do one or two HPDE's a year, the OEM VSLD is just fine, even with engine mods and bigger tires. The clutch-type lsd is good if you do 8 or 10 (or more) track events a year. The clutch-type lsd is agressive for the street. You have to put up with the cluncking, the jerky turns at low speeds, dragging tires at medium speeds and new noises that were not present with the VLSD.

What I would like is to have the NISMO diff for track events and an OEM VLSD that I can swap in for street driving.

Hope that helps,

PeteH

Z1 Performance

iTrader: (564)

I run the Cusco RS LSD - adjustable from 1.5 way to 2 way, and i left the clutch plates as they came from Cusco. MUCh better grip overall than the stock LSD, though I only notice it on very aggressive drives.

downforce

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Resolute

iTrader: (2)

Downforce-
This is a pretty big question for a lot of people, so I'll try and drop some info to help you and others decide which course of action is best suited to your needs. The first thing you must do, as Audito 350Z has stated, is decide what you are going to use your car for the most and the priorities you place on that decision. For track use, is the 1320 your primary, or Solo II? For street use, for aggressive driving or adverse wheather driving? The modifications you make on your vehicle will impact the operation of the LSD as well. With the planned mods you listed above, I believe your street or track priorities are the biggest factor in deciding to keep the Viscous or not.
First, with the forged rims, I am assuming they will be wider than stock and wrapped in better rubber. If this is the case, I think a higher bias LSD will be needed to extract the most performance from your engine and for any decent track use. This is because the stock LSD does not have a high enough bias ratio to allow the inside wheel to accept the same amount of traction now possible on the outside wheel due to the stickier rubber. Consider that in an open differential the wheel with the least resistance will recieve, by default of physics, the most torque. If we introduce a device to limit this power transfer ( Limited Slip ) so that both wheels, up to a point, will recieve power- then we still have two wheels with different resistance acting against each other. The wheel on the outside of the turn has more traction with the road surface and therefore has greater resistance than the inside wheel. Imagine, if you will, an extreme situation where the inside wheel is unladen - off the road surface completely. The torque going through the differential has to be directed to the wheel with traction, but the unladen wheel is obviously much easier to turn. The amount of torque that this unladen wheel can accept compared to the outside tire is called the bias ratio. Obviously, at some point the ratio becomes too extreme and the LSD cannot compensate for the differences in resistance between the two, thereby releasing all the torque to the unladen wheel as though an open differential is in place. A bias ratio of 6:1 is about the peak of a LSD abilities and will allow the outside tire in our example to recieve six times the amount of torque that the inside wheel can accept before it starts to turn, or break traction depending as the case may be. So, with no other improvements but stickier and wider tires, you will not be as fast on a course as you could be because the extra amount of traction on the outside can now recieve more torque than the inside tire can accept without breaking free. Once the inside tire breaks free, then the bias ratio is beyond the LSD abilities to transfer torque, and you lose speed. With a higher bias in the diff, the outside tire can recieve more torque without the inside tire breaking free. Of course, too high a bias ratio and the inside tire will break free because the torque that can be accepted by the inside tire is just as much as being delivered to the outside tire, causing both to spin at the same speed in turn.
On a Clutch type LSD, this bias ratio can be adjusted by the plate configuration. This is the single greatest advantage to the clutch type. As the car accelerates, the differential begins to rotate. The differential houses a special case with a "ramp" for each pinion. As the pinion is forced into the ramp, like a wedge, this case is spread apart and pushes several clutches against other clutches attached to the side gears. Each side gear, there are two, is attached to an axle shaft. This is the most common 1-way LSD. The amount of friction in these clutches distinguishes the character of the LSD. The friction can be changed by the design of the clutches, the material, the number, and the configuration. These all contribute to determine how much bias the LSD can hold, the amount of overall torque that can be transmitted, and the smoothness of engagement. A clutch-type may also have the ramps in two directions, on either side of the pinion. This allows for engagement of the LSD effect while decelerating as well. A 1.5 way has slight ramping and the 2 way has full ramping in the same effect as when accelerating. The shape and depth of the ramps can also determine how much pressure is exerted on the clutches, and therefore how smooth the engagement is and the maximum torque recievable.
A viscous unit can do everything the clutch type can, but better. Just not any stock viscous LSD. The stock unit is manufactured by GKN, an Hitachi? subsidiary, and sucks. Most, if not all, stock viscous LSD suck for hard driving. Consider that the VLSD is quiet, maintenance free and works well enough to do the job for most stock vehicles. And is cheap. Helical units are great also but expensive and generally overkill for most cars. And clutch types need maintenance, are expensive, and can be too noisy for most people. Now, in performance applications installing a clutch type or helical as stock makes sense and is done for some models, but generally all we get is a crappy varient of a mass produced viscous LSD. The way these work is incredibly simple. The case and side gears all have alternating plates with holes in them that rotate in a silicon based goo. As the differential case spins and the axles spin, all is well in the world as far as the diff is concerned. Until, you make a turn or hit some ice and one wheel wants to spin faster than the other. The fluid resists this difference in motion as the plates on the unladen side gear try to rotate faster in between the plates attached to the casing. This fluid thickens up the more sheer is applied, resisting the amount of torque the unladen wheel can accept, and allowing the wheel with the greater traction to recieve more torque in the process. Eventually, the resistance can become so high, bias ratio again here, that the fluid pushes the plates into each other like a clutch pack and this is known as the "hump mode". During this period, rediculously high bias ratios can exist. The fluid does NOT function as a property of heat- don't believe anyone who says otherwise. The resistance to sheer is on a molecular level and designed into the fluid, it does not increaes its resistance with heat, but rather, increases it's resistance to shear as more force is applied. That's the cool part. The way this benefits you is a low moment of inertia ( aka low parasitic loss of torque ) and smooth, quiet, well-controlled engagement. The diff can be designed for any power and traction level and custom tailored for your car. Pre-load can even be incorporated to provide a level of slip protection even before it is registered by the plate rotation difference. But nobody does that for the Z, or most road cars. It is too expensive. Hence why the good VLSD are made for professional race teams in F1, Champ, and touring cars by Xtrac and FF developements, (Xtrac currently supplies VLSD to Penske's IRL cars) and we get generic models that are cheap to mass-produce and work across a broad spectrum of cars. The downside of the VLSD is the inability to make changes to the character of the limited slip yourself, and crappy stock units tend to overheat with decent power and traction running through them - which reduces the ability of the fluid to resist shear since as we just learned is NOT based on fluid temps.
In short, viscous units are wonderful if you have a good racing budget to have some engineers custom build one for the track you want to run on and for your car's performance level, and clutch type's are all that's really available for the Z right now. The NISMO is made by the same manufacturer as Cusco. Cusco does not make it, I have the manufacturer listed somewhere but I can't find it right now, but supplies both with the differential - NISMO as it is and in the case of Cusco just the housing, clutches and gears. Cusco units are then assembled in house by Cusco and modified and changed as they see fit for the different models. Both are good, well made diffs though. The Cusco comes in two flavors, but it's one a.m. and like the manufacturer's name I can't remember what they are called. One comes with pre-load springs that apply constant pressure on the side gears, and the other has a more aggressive clutch pack. The one with pre-load springs will probably be the most civil for you on the street. The Kaaz is really similiar to the NISMO, but they do in fact make their own and not from stock housings like some companies do. My personal choice is OS Giken, awesome reputation and they build a clutch that is supposed to be as smooth as the viscous but with greater bias and, of course, adjustibility.
I really hope this helps and will check back tomorrow or next to see if I can answer any questions or clear something up.

FritzMan

Is that to say then that it's not possible to simply swap the viscous "goo" in the OEM unit for something with higher sheer characteristics?

curtiside

Thanks for the great information about viscous and clutch LSDs.

I have heard good things about torsen gear-driven LSDs. As yet, I don't know enough about them to determine whether it's worth waiting for Quaife to produce one for the 350Z. How do they work and how do they compare to clutch and viscous LSDs?

Thanks!!

PS - I had an LSD (don't know what type) on my 86 Corvette and I had a clutch-type LSD on my 95 Tahoe (I know it's not a sports car). I have to say that in my opinion an LSD is one of the most important mods that can be made for spirited driving; it takes power that's already in the engine and ensures that it gets to the ground where it matters.

Audito350Z

iTrader: (1)

OS Giken makes a carbon disk clutch-type LSD for the 350Z.... completely noise free and none of the jerky-ness that may be present in a clutch-type diff. But, it's about $5000.

Resolute

iTrader: (2)

Unfortunately, that is correct. The VLSD is a sealed unit, so any changes that can be made will not be simple to perform. The type of fluid used, the number of plates, and the design of the plates all define the characteristics of the VLSD, however, the changes would have to be tested in order to see what works best. This is the inherent cost associated with them for performance use. Even if a higher shear-resistant fluid was exchanged, and the case re-sealed, then you wouldn't know what effect it was going to have until you drove it in all sorts of circumstances. Then, the fluid might not work well with the type of plates you have, or the number (remember, higher shear fluid increases it's resistance the greater the strain is put on it, so with a strong fluid and too few plates the strain won't be enough to really utilize the fluid's properties). If that happens, then you take it apart again and change something else, - it would be like trying to tune your suspension from scratch rather than buying a kit already set up to provide better handling.

Resolute

iTrader: (2)

Gear-driven LSD have one distinct advantage over the VLSD - they don't need slip to occur before limiting it. Now, this can be cured in a VLSD by ading pre-load to the side gears, but obviously the amount of slip controlled bewteen the two wheels is then dependent on the stiffness of the springs used and the quickness of the fluid to register and resist the shearing forces in addition to those springs. With a Helical unit, there is no guesswork and testing needed to determine the optimum engagement for the car's power level and driving habit. The slip is registered instantly and limited appropriately based on the torque inputs from each wheel. Remember the bias ratio? Well, let's say that the unladen wheel wants to spin faster than the laden wheel because it has the least resistance to recieving all that torque from the motor. The side gears in the differential then want to spin at different speeds, but they are surrounded by worm gears that also rotate in conjunction with the diff casing. Worm, or helical cut gears, only transmit torque in one direction, and therefore resist the ability of the unladen wheel to accept more torque than the laden wheel can recieve to accelerate. Thus, the inside wheel never actually slipped or broke free at all under power, because the gears allowed torque to be evenly distributde before the side gear on the inside wheel could ever break free. Thus, helical units like the Quaife are known as torque-sensing diffs. This is the root of the name used for the first such dif, the TORSEN. The TORSEN is still being made by Zexel for a large number of race teams, as is the Quaife unit out of England. The beauty lies in the abilities to govern bias ratio rather than just slip, so the outside tire is allowed to spin faster in a turn, but not because it recieves more torque. Most other diffs limit the slip only and use the bias ratio to determine how much slip. The helical gearing will allow any tire to rotate faster, but when torque is applied, it regulates the bias between how much each side recieves regardless of the speeed of rotation differential already in place. This makes it quiet and instantaneous engagement, and no maintenance. This also means that it cannot function under braking, and can force to high a bias ratio to exist, especially on low traction cars, and act like a spool at low speed turns. Both wheels recieving the same amount of torque will cause one or both to break traction in a turn. The unit is also heavy and has a very high moment of inertia, meaning all those gears take a lot of energy to turn them. Like the VLSD, a generic version for all would not perform well, so they are manufactured semi-custom for each vehicle based on expected power and use. Stock units in front-wheel drive cars like the new SRT-4 and the SE-R Spec V are well suited for their applications, but would likely buckle under the strain of a high power rear drive. Most Quaiffe units are civil on the street and perform well on the track as is the TORSEN, depending on who you talk to. Some feel the high bias in low speed comes out too much on the track and puts the car in understeer, depends on the application I suppose. The clutch type is lighter and allows engagement based on acceleration and decleration, negating some of the advantages of the torque-sensing diff.

downforce

Wow! This is all great information, I am still soaking it in and will reread it several times.

Keep it coming!

Any comparisons done with the Z and different type LSD setups? (VLSD, CLUTCH, TORSEN) Streetabilty vs. Performance or a little of both.

I do plan to drive mostly on the road but hope to get as much track time as I can (or what the wife and kids will allow) oh and my pocketbook

dklau33

iTrader: (17)

Currently there is no Torsen type diff out yet for the Z/G's. But several people on the G35 Driver board as well as here are trying to round up a stock 350Z VLSD diff and send it to Quaife to see what they can do. Hopefully Quaife will make a production diff for us sometime in the near future.

downforce

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downforce

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Hunter Z

Resolute -

Thanks for the wealth of info about LSDs. You can see my current set-up below. I have just about every imaginable N/A mod short of forged internals. I want to get the 3.9 final drive if EVO ever comes out with it. They are about 6 months behind schedule now. That installation would be the perfect time to add a better LSD.

I do HPDE and other track events about once a month. My car is barely streetable now due to the Tilton rally clutch and 8 lb. flywheel. So a noisy/jerky LSD won't bother me too much. The Z is not my daily driver (got an '04 Audi S4 V8 quattro sedan - great car), but I do drive it to work once a week, and a lot on the weekend.

I don't drag race because the clutch/flywheel set-up makes the car too hard to launch from a dead stop quickly. I drive road courses and do canyon runs with a local Z club.

Please recommend the best LSD for me. I can afford a good one, but not $5000 for an OS Giken.

Thanks!!!

Resolute

iTrader: (2)

Thanks for the compliment Hunter Z. I dig your avatar btw, the new Lotus is really tempting me to ditch the Z. Anyway, right now the choices are pretty limited and even, but also very good. The stock VLSD is not going to provide the best performance for you, and no aftermarket viscous unit is available. There is no helical unit available either, so this leaves us with only clutch-types. The good news is I have yet to see a poorly designed clutch-type for the Z. The OS Giken is my preference, but on hearsay alone from those who have seen it at the various auto salons, and the price tag will probably not make it worth while for me anyhow. The NISMO and Cusco units are very well built, and I have a personal preference for them over the Kaaz unit. Realisticaly, they are all similiar and the Kaaz is a good LSD, but I have personal experience with the Cusco and, if it is available for the Z, would recommend the RS version for you. The RS uses springs in the diff to exert moderate pre-load on the clutches. The other type built by Cusco is the MZ and uses conical clutches outboard of the clutch pack against the diff case. Conical clutches typically allow a smoother, more progressive "grab" while maintaining high friction for big power, while the pre-load of springs is negated by big power but is much smoother and consistent- especially on the street. Since you are not making exceptional torque, I think the RS will be more streetable and, with up to 20 clutches depending on the model, more than enough to keep the inside tire in line on the track. I don't know the Cusco availability for the Z though, but I believe this would be a good choice worth checking out. Hope this helps you out Hunter Z.

Hunter Z

Resolute -

Cusco does make an RS LSD for the 350Z. I called Jason at Performance Nissan in Duarte, Calif. to ask if he could get me one. He said he could get me one out of one of their 350Z race cars, because they are switching to the Nismo LSD. I asked why and he said "because they don't burn up like the Cuscos."

I think I will go with the Nismo LSD. Thanks again for all your help.

Resolute

iTrader: (2)

Wow. I wonder why the Cusco is not holding up. The RS is my old S14 held up very well and I was very pleased with its performance. The differential on the Z and the S14 even share the same part numbers. I wonder how the plate numbers vary between the diffs, but then, my S14 was not making as much power either. This is good to know about the Cusco in the Z , though.

Z1 Performance

iTrader: (564)

if it is bruning up, it is not set correctly.

I have the Cusco one in my Z for awhile now - no problems whatsoever.

Resolute

iTrader: (2)

I thought the same, but Jason really seems to know his stuff, so who knows? In any case, Tomei has a new diff out for the Z and S14 that uses a 24 clutch pack. Sounds like a high hp diff, certainly a lot of heat involved. And, Kaaz is supposedly coming out with a diff for the Z that includes the stubbies required for base model Z's to install.
EDIT: threw in that last bit about the Kaaz