996/997 GT3 Limited Slip Differentials

 

A few observations on limited slip differentials   

 The Porsche 996/997 GT3 street car is a marvelous product for street driving enthusiasts who also enjoy weekend track events and or occasional club races. A great chassis coupled with plenty of power makes for a great driving experience.   However, all of that horsepower doesn't matter if you can't get the power down. The power from these cars when accelerating off corners causes the inside, or unloaded tire to loose traction; this slows the rate of acceleration, causes unwanted corner exit oversteer, and kills the rear tires. The problem only got worse when the newer 997 GT3 was made available. An even stiffer chassis platform and more power put more demand on the already limited-limited slip.

  In the past, we used torsen or torque-sensing differentials, made by various manufacturers, Gleason and Quaiffe being most familiar to correct the loss of traction. These differentials seemed to be the end-all. After all what could possibly be better than a unit that actually sensed the torque, and applied it opposite to the wheel that slipped? The key with torque-biasing differentials is that the wheels must remain in contact with the pavement; even a minuscule amount of traction is needed to engage the diff. Early 911's flexed enough and kept the rear wheels on the ground maintaining the miniscule amount of traction needed (just look at  the inside front tires off the ground in older 911s when racing). When the 944 turbo came around, with its stiffer chassis, cars would lift a rear tire in tight or bumpy corners and the torque sensing diffs were no good. We learned this first hand when we built the first Mini Cooper S's for Grand Am. Same problem with front wheel drive, when one drive wheel looses contact with the track, say over a curb or bump, all the torque went to that wheel.

 

Moving onto the GT3's and their limited slip differentials. A limited slip is a clutch type differential that, with stiff chassis 996/997 based cars, over curbs and in corners, is designed to apply the same force to both rear wheels. Porsche uses a four clutch disc pack in all GT3's. If you jack up one side of a GT3, put a torque wrench on one rear wheel axle nut, you may find it takes less than 20 pounds feet of torque to turn one rear wheel. That's called the breakaway torque. And, that's on a brand new 997. We have checked the differential breakaway torque on two dozen 996/997 GT3 street cars and after one or more track weekends the preload torque is at or near zero. Keep in mind, there are ramps inside the differential that, when drive, or accelerating force is applied, it forces the differential clutches to bind up and act to lock up the rear. However, little or no lock up occurs on deceleration. So, on hard braking, as the weight goes to the front wheels the rears get light, and the inside rear wheel will temporarily lose traction. This causes a slower entry into the corner and generally you will feel the anti-lock brakes engage when this occurs. But what if we could get the differential, to partially engage on deceleration and engage more aggressively the harder we accelerate off a corner, this would be having your cake and eating it too, wouldn't it? You bet it would!

  The expensive solution was discovered by all of the long distance enduro GT3 Cup cars years ago. Install a billet, fully adjustable Guard limited slip differential. This nearly indestructible unit is also available for the street cars. Keep in mind, this solution requires removal and disassembly of the entire transaxle. However, the cost of the unit and labor can exceed $7500. This is a bit of over kill for street, track days and even club racing. The expense of the billet housing is way beyond most club race GT3 budgets.

  However, using the same technology and top quality parts we can improve your differential beyond its original ability, and make it last for many track days. We can remove your differential (without removing the entire transaxle) and build the same adjustable ramp diff as the pro's have. The adjustable ramps in this unit allow for 40% lock on acceleration and 60% on deceleration. For more aggressive situations the same diff can be set to 50% acceleration and 80% deceleration. The friction plates in the new diff are far stronger than the Porsche factory parts and won't show near the fade. The break away torque is set to 80 pounds feet new, and after a few track miles they settle in at around 60-80 pounds.

996/997 GT3 Limited Slip Differentials

A little more technical analysis of GT3 diffs


 If you've come this far, maybe you're interested enough to go a little deeper into the differential operation and benefits of our GT3 upgrade. There are small technical differences between the 996 and 997 based GT3 street car differentials. Porsche apparently noticed the poor performance of their factory limited slip differential with the 996. From what we have heard, the warranty claim rate on the 996 differentials was fairly high. This to replace the friction discs inside, presumably to restore the designed amount of limit to the slip. Every instance ended with the same results; almost zero break away torque static, very limited ramp angle force applied with power on, and just about zero lock on deceleration. The 997 has a slight improvement in acceleration ramp angle (more on that later), but suffers the same fate when multiple track days are run, a differential with almost the same feel as an "open" diff. I think the Porsche engineers reckon that with traction control the diff just isn't as important. Why else would they, for the first time in the history of the 911, offer the 2005 997 without a limited slip differential option?

Most professional race teams, and even well funded club racers have several differentials on hand for various tracks. This highlights the importance of balanced acceleration and balanced deceleration from both rear wheels. The differential is very important to race car set up and handling. If you have ever chased corner off oversteer by continually tightening up the front end, only to find corner entry or mid corner push, then you probably can reduce your lap times with a correctly built diff. Since our customers run various tracks and do street driving, we keep the differential fairly tame. As opposed to drivers in the 80's who learned to steer 911's with their right foot as well as use the steering wheel. Because usually they had an 80% locked diff or what is known as a "spool". This allowed for zero slip and both rear tires rotated equally, talk about low speed corner push.

  Pro teams running 12 and 24 hour races with 450 hp Cup cars need a "bullet proof" diff that can be tailored or adjusted to suit their needs with a housing made of the finest billet steel available. In fact, if you ask most Cup Pro teams they will tell you that the factory diff is a weak link in their drive train. Plunk down serious big money (money being relative, Farnbacher/Loles lost the Rolex in 2009 because of a failed factory diff, what did that cost?) and you can have one. Our estimate for one of these would be $7500, all in. And because, it requires re-set up of the ring/pinion backlash and bearing preload, it is best done with the transmission removed. Most GT3 owners don't need a differential of that nature. However, they do need a significant upgrade over the stock Porsche unit. Our friends at Guard Transmission have developed engineering and materials that far surpass the units provided by Porsche.

  The differentials we build have four variations of "ramp" or lock up: 40/60, 50/80, 60/40, and 80/50. These ramps are part of the "guts" of our improved differential. We have found for the type of driving that most of our customers do both street and track days that the 40/60 works best. What does 40/60 mean? The first number, or percentage, is the capability of the diff, under full power to provide lock up on acceleration. The second is its theoretical lock up under deceleration. As opposed to an open diff which would be 0/0 between the rear tires and the aforementioned "spool" at 100/100.

  We have developed a technique and procedure to modify GT3 differentials without transaxle removal. The differential and ring gear are removed. Everything is very carefully measured and logged. We install Guard friction plates, intermediate plates and spring washers, along with the multiple choice ramp section into your differential housing. This eliminates the need for altering the factory ring/pinion relationship and need to change or remove the differential side bearings. Other benefits include easy service, the differential can be removed at any time in the future to try different settings or replace the friction discs when necessary. We have some high mile cars out there and we have not had to replace any worn discs yet. The car is in our shop for three days, or you can send us your gearbox or differential. The cost complete is in the $3,000 area. Of course if you do the removal and install yourself it will be less. We recommend and fill our gear boxes with Torco MTF fully synthetic gear lube and we add a friction modifier to keep low speed (parking) chatter to a minimum for street drivers. This gives the same driveability and handling improvements as the pro diff, at a substantially reduced cost.

 You may have to readjust the balance of your sway bars, or shocks to suit the new diff, track or your style, but the benefit will always be faster lap speeds, and that's what it's all about

 

-Mike

 

Call or e-mail (mike@bodymotion.com) me for more specifics or to schedule your GT3 for an improvement way beyond the $3,000 cost of the job complete.