LSD vs. TBD

Folks, I kind of want to discuss the differences between LSD and TBD, but I don't want to hijack Tess' thread where he's conducting his group buy. So, I've started a new thread. I've read a few posts where folks suggest that TBD is the better choice for street cars and that LSD would really be best for cars that are tracked extensively. My last Porsche , a 1986 944 Turbo had factory LSD . I am almost sure it was a clutch type LSD and I never really experienced anything that made me regret having it. That car was overboosted to 1 bar (14.5psi) and it made 285 ft.lbs. of torque. So it would have really spun the inside wheel if it didn't have LSD . With the LSD , that thing really bit into the ground. It was quite difficult to get the tail out, bit it would come out if you really tried. Otherwise, the LSD in that car seemed perfectly set up. The only negative experience I had with it was when I once got caught in a freak snow storm. It was nearly impossible to keep the tail from coming out. The way I understand the benefits and drawbacks of each are: TBD Benefits: Only applies lock under acceleration so it remains nearly invisible Adds no understeer Does not wear out or require any maintenance TBD Drawbacks: Will open up if a wheel is lifted Does not provide lock under braking LSD Benefits: Applys lock even if a wheel lifts Applys lock during braking Applys more consistant lock regardless of acceleration LSD Drawbacks: Clutches do wear, and do add wear material to gear oil requiring more maintenance More expensive than most TBDs Can cause understeer in high traction conditions Can cause oversteer in slipery conditions Will cause more tire wear than TBD Am I understanding this correctly? I know a few of you have clutch type LSDs in your cars. Can you comment? Would you do it again?

Hi Mike As this is my first post, I'd like to introduce myself to the group .... Paul Guard from Guard Transmission. I'm using the name "Geary", which is the same name I use in one of the other popular forums. The primary focus of Guard Transmission over the past decade has been the supply of special ratio gears and the innovation of new products for 911 -based race car transmissions .... this of course covers later " 911 " models, right through the latest GT3RSR and Cup. As of late, the fastest-growing segment of our business has been the development and sale of our high quality Limited-Slip differential line. Almost every one of our products carry the easily recognizable "GT" logo, two staggered letters within a circle. Regarding the thread that Mike has opened, I can't possibly give all available input to the subject that I have in one or two posts, but I will just start with a couple basic points : The "proper" definition of an LSD is a plate-style locking differential. It will typically have multiples of two friction discs, pressed into action by a pair of pressure rings actuated (spread) by the cross shafts, during acceleration or deceleration. The greatest lock-up occurs during high rates of acceleration or deceleration .... but a fair amount of lock-up exists even during cruising speeds. Salisbury and ZF are probably the oldest names in LSD manufacture, but dozens of LSD manufacturers exist today. A TBD or ATB differential is a gear-type locking differential. The most basic designs of these will have two sets of planetary gears that provide lock-up via the outward thrust of these gears within their individual gear pockets. The outward and end-thrust of these planetary gears provides the friction required to send available engine torque to the wheels during acceleration. Gleason is the company that developed and patented the TBD in the '40s, and this company exists as the Zexel-Torsen company today. (It's worthwhile to note that the only TBD ever factory-installed by Porsche was in the 968.) Many companies produce TBDs today, although the considerable promotion by Quaife and the wide range TBD models that Quaife carries has given them a considerable edge in the TBD market. While the term " LSD " has become bastardized by the general automotive community to now mean "anything other than open diff", I would hope that within the Porsche community , for the sake of clarity, a Torque-biasing differential would be called a TBD or ATB diff, and the term "LSD" would be reserved for true limited-slip differentials. I do my best to police (via education) the misuse of the term wherever I can, as it drives me nuts to see a TBD called an LSD. (To their credit, you will not find the term "LSD" on the Zexel-Torsen nor Quaife websites, although you will often see the term used by their dealers and users.) Enough for now, as I've got to get back to work .... Oh ... this may be the only time I fully list my name and contact info... I usually just go by the name "Geary". Paul Guard Guard Transmission ph/fax [phone removed] [email removed]

Paul, Welcome to the site, looking forward to your coming onboard as a sponsor and your continued educational series on TBD, LSD and all points in between.

Hi Paul When will you have a LSD available for the Cayman ? Regards Herman

Hi Herman We should be able to begin sending these out in 3-4 weeks.

Paul, I have a TBD installed right now. The four questions that I have are ... At what mileage frequency do you usually see street driven cars (with limited DE days) require maintenance. I understand that this will be an imprecise and possibly fuzzy answer. Your best approximation will due. Does the diff need to be removed from the transmission for the maintenance? Can a shop familiar with your diffs perform the maintenance, or does it need to be sent back to the factory? Does your LSD make noise when turning? I remember seeing this posted, but I'd like to hear your answer. I've heard great things about your diffs from the shop I usually deal with, Pat Williams Racing. In fact, he recommended your LSD over any other due to his experiences. I believe he said he had them in 30 or more cars, and all but one of his installs have been Guard units.

One thing that I don't believe you have covered is the acceleration ratio. I believe in a TBD, the lockup is dynamic at a 1:5 ratio. In a LSD, I believe you choose it before install. Some were adjustable such as the 9FF and others were sold with a ratio that you chose on the order. Someone please chime in if this is incorrect.

Smounic, I was under the impression that TBD didn't really require any additional maintenance. And as I understood it, with an LSD you might need to change the gear oil more frequently (maybe once every 50K miles) and might need to change out the clutches, but I have no idea how often. I'll let Paul provide more expertise.

Paul, Welcome to the club. It will be great to have you here. You guys can help plug the hole, the largest vulnerability that the creators gave the Cayman , the lack of an LSD . Another vulnerability they gave it is a higher 1st and 2nd gear than the other cars, which I bet you can change too, but I am guessing that might be a little pricey for a street car (maybe I'm wrong). My understanding of LSD vs. TBD is very vague so my questions are a good starting point. I am sure there is a lot to add to what I've posted. I will have my car some time within the next month and I don't plan on putting it under the wrench right away. I'd like to get a feel for it as is first and determine what if anything I want to do to it. I am very conservative when it comes to modifications, and as many here can tell you, I am very skeptical about claimed gains, results, improvements. With that in mind, anyone who's ever driven a car with and without an LSD knows that there is a night and day difference. I still can't believe Porsche sent the Cayman out there with an open differential. :wall: I guess what I am trying to say is that if I were limited to only one modification, an LSD would likely be it. I really look forward to learning more about the topic from someone with so much experience. As you will see, one of our long standing members, Tess Tickle, has pioneered a Quaife TBD for the Cayman , brought it to market and gotten it to our members. The testimonials from those that have driven one are very compelling. I don't mean to question those that have embraced the TBD, but before I crack my transmission open I want to make sure that I clearly know and understand both options and which is better for me. This is not something I want to do and then later say, "Damn, I should have gotten the ______ instead......" I'm looking forward to becoming a differential expert......

Sorry for not getting back sooner, guys. I barely know where to begin ... First, I am quite familiar with all TBDs and LSDs sold for Porsches today, and have some pretty strong opinions on those that I see as being lower than acceptable quality. I've explained to both Ken and Mike that I don't intend on holding back the facts , when the right opportunity presents itself. So if this sort of thing offends you, I'm giving warning now... On the other hand, I'll gladly discuss the higher quality units also, as GT units aren't alone on this list. Other manufacturers have had slightly different approaches, which we note with interest. More on this another day... TBDs don't generally require any maintenance. A high quality TBD will drop off slightly in efficiency as the planetary gears seat themselves. A low quality TBD will continue to drop in efficiency as these gears continue to wear into their individual gear pockets. But even in these, the only "maintenance" is usually removal and replacement with an LSD.... unless the TBD has been hard tracked.... in which case repair or replacement will become necessary. More on this another day, as well ... An LSD will require friction disc replacement eventually. "How often" is quite difficult to say... We tell customers every 2-5 years, depending on how they're using the vehicle. I know this sounds vague, so let me present more input: Porsche used to recommend replacement of the friction discs in a 930 Turbo @ 30,000 street miles. Porsche ALMS teams probably replace their friction discs after every race. Those are the two extremes... but, I would say that most track cars raced on a regular basis probably "should" have their discs replaced twice a race season. Of the many hundreds of LSDs that we've sold over the years, an amazingly small percentage of these customers have ordered new friction discs.... difficult to say why.... I'm sure that many more are due for a refresh, but the customers seem to be satisfied with somewhat diminished performance. I'm sure that distant customers (specifically our German customers) merely replace their friction discs with factory OE pieces. We presently rebuild OE LSDs for 911 customers, and replace a LOT of the slick "carbon" friction discs with our plasma-sprayed variety. These customers are ecstatic with the difference in performance. I think these low quality "carbon" friction discs wear to a point that they're even slicker than a plain metal plate would be. (Keep in mind that the original 904 LSDs had just this ... plain metal plates throughout.) Very few of these rebuild customers come back for disc replacement, either. I'd have to guess that the typical customer who drives some DE events... but mostly street... will go as long as 5 years before needing to replace their discs. As more time passes, we'll have a better answer for this question. Tomorrow morning I'll perhaps offer an opinion on "Which diff type, LSD or TBD", as it pertains to the Cayman. BTW, Guard Transmission will be offering a TBD for the Cayman. This unit is a bit further down on our manufacture list, but it is in the que. The Cayman LSD has been one of our most requested units, so we put an extra rush on this a couple months ago.

Geary, Do you have a preferred lubricant? I think we put in a synthetic when we did my diff swap. Thanks again for the info. It would be great if you could get a Guard LSD unit into a car by Croctoberfest time and we could drive them back to back with the TBD in another Cayman and note the differences first hand. BTW, I'm curious about your statement that you think a TBD might not last if tracked hard. How hard and how short of a lifespan are you thinking?

Let's cover the lubricants another time, as it's a long story ... Regarding TBD vs LSD on the track: Feedback to Guard Transmission over the past decade has been primarily on rear-engined Porsches.... the 911 series. So once again, let me present two extremes ... ALMS race cars and street cars that see perhaps a few DE events per year. An ALMS race car will shred a TBD in short order .... perhaps a single test session. Quaife found this out in the late '80s here in the U.S., with a unit given over for testing to Porsche Motorsports handed back in a stinky bag. Even the Guard TBD given to Alex Job for testing in 1999 came back looking like a 10-year-old unit, after a single day of testing. A TBD works fine for street & autocross, typically in a car that has softer suspension that doesn't go fast enough to lift and spin tires . But once a drive wheel gets too light (as in a road racing car), the TBD will no longer transfer torque to the opposite, more heavily loaded tire, and the inside tire will spin. Planetary gears in a TBD will not tolerate this activity for very long .... certainly not for hours on end. A TBD will do a much better all-around job in a mid-engined car than in a 911 . Without engine weight hanging off the back, the tendancy for the rear end to swing around or for the tires to break loose isn't nearly as pronounced. It's been suggested to Guard Transmission that a TBD works pretty well as a track diff in mid-engined cars up to around 200-250 hp. This is from Alex Job crew chief, Greg Fordahl. I've had similar feedback from 914 owners, who have opted for the purchase of one of our TBDs over an LSD for economic reasons. So while there's no question that an LSD is a more durable type of differential allowing many possible settings, there's more leeway for using a TBD in Caymans that see primarily street use with moderate track time.

Very interesting. I know some of our UK members are having great results with the Quaife on the track. It will be interesing to see how they hold up over time.

I've also punished my TBD at the track several times and my suspension is certainly stiffer than stock, but that said I'm not running a dedicated race car either.

Any sort of locking diff is a big step up from an open diff. For high speed track use, a properly set-up LSD will always outperform a TBD. Guard Transmission carries high quality TBDs and LSDs for every model we cover, so we really aren't prejudiced one way or the other.... but we make certain that the customer knows which diff is best for their particular application.

Nice. I like your style Geary.

Please Geary, send out a version for the Tip as well, not just Manual. Thanks. :banana:

Paul (Geary), Does PSM or ABD or other gadgets that apply breaking to the spinning wheel help protect a TBD from damage?

titanic ... Perhaps a ways down the road we might consider a tip version. We would have to hear from a number of tip owners first. (We rarely get calls from 911 tip guys...) Greg .... One would think so, but I don't know.

Paul (Geary), it's good to see you posting on this forum...you and I had a lengthy phone conversation a few years back when I'd ordered my 2001 986 S and found that I couldn't order a limited slip of any kind from the factory...you were very patient and informative, and sent me some very good literature that I've kept and still refer to when I need information, even though I never did put any form of limited slip in my Boxster ...now that I have the Croc, though, it's a different matter, but it sounds like I'd better concentrate on correcting my suspension so that I can keep the inside rear wheel(s) on the ground before I do the LSD thing... Thank you!

Crocster .... While a TBD will spin freely like an open diff if one tire lifts, a true LSD doesn't suffer from this problem.

smounic asked: "One thing that I don't believe you have covered is the acceleration ratio. I believe in a TBD, the lockup is dynamic at a 1:5 ratio. In a LSD, I believe you choose it before install. Some were adjustable such as the 9FF and others were sold with a ratio that you chose on the order. Someone please chime in if this is incorrect." I'm not certain what you mean by "acceleration ratio", but I'll do my best to cover locking percentages etc. below: First, a quote from the Zexel-Torsen Company, the originators of the TBD: "What does torque bias ratio mean? Torque bias ratio (TBR) represents the "locking effect" of the differential. More specifically, it indicates how much more torque is sent to the high traction wheel (or axle) then (sp) is sent to the spinning wheel. For example, when a differential has a TBR of 3:1, 3 times the torque the spinning wheel can maintain (75% of the total torque) will go to the higher traction wheel. If a wheel is spinning and can support 100 lb-ft, the other wheel will get 300 lb-ft." Porsche uses a straight-forward approach to describing locking pecentages of LSDs installed in their cars, which makes a heck of a lot more sense to laymen than actual ramp angles. You've seen terms like "40%", "80%", "50/80" .... Porsche refers to these figures as "Percentage of anti-slip effectiveness".... which we normally just call "locking percentages". (TBDs and LSDs are different differential types, and there is no direct comparison between a TBD's "bias ratio" and an LSD's "locking percentage".) When you see a split figure (40/60) used to describe an LSDs locking percentages, this means 40% on acceleration, 60% on deceleration. Porsche saw the advantage of differing percentages of lock-up in the rear-engined 911 & later series, and introduced these split percentages with the 993 (perhaps a bit earlier). About 4 years ago, Guard Transmission introduced the split percentages to all earlier models of GT LSDs, as well. So now we have even the earliest 904 LSDs with modern split locking percentages. This has been an extremely popular design improvement. At the same time, a second set of ramp angles were cut into the GT pressure rings, so that our customers could now choose between two different sets of ramps, built into any Guard LSD that they purchased. The most popular choices by far are 40/60 and 50/80. In a mid-engine vechicle, the desire is to have slightly more lock-up on acceleration than deceleration, so a 40/60 ramp used in a 911 would be set as a 60/40 in a Cayman. A year or so back, a German company began producing a high quality "zero preload" LSD, which has the interest of a number of U.S. teams today. "Zero preload" isn't a new concept. As a matter of fact, any LSD can be set up to be a zero preload LSD.... but using the correct ramp angles and number of friction discs to achieve the desired result has to be part of the package. Guard Transmission has recently modified their list of LSD options to include any possible desired set-up in any GT LSD. Possibilities now include 2, 4, 6, or 8 friction discs, combined with 2.2, 2.0, 1.5, 1.0mm or "zero" Belleville washers. With all these new options, the old Porsche terminology of "40%" or "40/60" is out the door. Complete description of set-up makes more sense, at least when working greater-than-4 friction discs into the design. Don't let all this confuse you .... I would estimate that 97% of our Cayman customers are going to want 4 friction discs, with a 60/40 setting. This is the likely standard for Cayman 987 LSDs.

Geary, Any update on your progress? Are you guys getting closer to delivery?

Paul, I may be in the market for a LSD for my Cayman 2.7; I'm not sure yet since I am l due to pick it up this week, and it may not have enough miles on it to make it to my last DE event this fall. However, I have a question about how good the electronics are in overcoming wheelspin in a stock Cayman . The owner's manual states that even with PSM turned off, the car will still recognize wheel spin on one side of the car (relative to the other side) and apply brakes to the spinning wheel. This, of cource, transfers the power to the outside rear wheel with acceleration but would give you none of the problems inherent with the LSD (understeer at turn-in). Instead of replacing friction discs, you replace brake pads. So why is this a bad system? I know members on this site who have rushed to put a LSD in their car and then admit they are not sure they can tell the difference. I know your business is sales, but could you explain to us all what the signs are of needing an LSD and in what ways we will notice a difference over and above what Porsche provides as a solution to one-sided wheel spin. Thanks for your time. It is always nice to have technically savvy people on this site.

doubleclutch, I might be able to answer in part. Without my TBD unit when I would take sharp lefts at the track the driver's side rear wheel would spin and the car would cut power to that wheel. The problem is that it is an open diff and power is NOT applied to the other wheel to compensate so basically the inside wheel stops spinning, but the car loses power/momentum until it settles in and regrips with the driver's side rear drive wheel.

PSM cuts your throttle when it intervenes.

With PSM OFF? However, if the PSM is off, why would power be cut? The owners manual states (p. 74 in 2007 manual): "One-sided spinning of the wheels is reduced, even with PSM switched off, to enable optimum traction on all drive wheels." Do you really notice that with PSM off, that power is cut? If so, that is unfortunate. Ken, on those sharp left turns you talk about, was PSM off? I have always thought that just keeping the ADB (automatic differential braking) part of the equation would be the perfect solution: an electronic LSD, as it were. Without retarding the engine power, if you brake the spinning wheel, the open diff must by definition transfer the power to the other wheel. Is the owener's manual deceiving us?

Awesome thread! This thread is why I love the CaymanClub... I just started AutoX this year, and hope to make the jump to some DE events next year. The amount of info on this sight, like this thread, is astounding. I'm not jumping into the TBD vs. LSD debate, just watching and learning. For what it's worth, this thread finally motivated me to make my CaymanClub donation; money well spent.

doubleclutch, PSM was not switched off, but the car was in sport mode so PSM is retarded back almost to the same point as if you press the PSM off button.

Thanks, Ken. I will experiment with my new car with and without PSM on. If Porsche really can keep the ADB on while turning PSM off to act like an electronic LSD , then I will report back. I imagine a few skidpad sessions when I teach at the October school should give me some clues. Thanks for your report. My car won't have the sports chrono package , so I will have to deal with on and off only--unless I pay for some aftermarket software upgrade. --Eric

I find that sport mode is very different than turning PSM off. Sport mode doesn't seem to cut the throttle, but it does manage wheelspin. I think turning PSM off raises both thresholds significantly. Someone mentioned in a previous forum post that they run on the track with PSM on. They have said that if they need PSM, they are most likely doing something incorrectly. Food for thought ...

To All Tip Owners from Around the World: Please tell Geary that you want him to make one for you.:thanks:

Hi Paul, Thanks for posting over here. I've noticed your website is under construction for a few years. Can you post a list of which late model Porsche models you cover and what your coverage is? I have yet to find a reliable source of what is/can be changed on 986s/987s: TBD, LSD, R&P, which gears (if any). I've seen the GT3 guys changing out 4, 5, 6 gears for example. With the 3.4L, the 1 and 2 are tall by Porsche on purpose.

arenared We would be more than happy to mail an information packet to you. We carry over 90 different gear ratios for the GT3 . Give us a call on Monday ...