996/997 GT3 Diff Rebuild: Anyone Done it? - Page 5 - 911/Carrera GT - PistonHeads UK

Hi Folks, It seems fairly clear now that the diff plates on these cars are made from chocolate; finest 70% coco variety I'm sure, but chocolate all the same. The method to test for a worn out diff appears to be inconclusive, but despite this, I am concerned that there's little or no assistance from my diff when coming out of light speed. I'm still getting lockup out of the corners, but the diff is far from aggressive in any way and the car is very nervous under hard braking (despite a recent geo set-up at JAZ). Conclusion: the diff's f ked. Now, where do I get it sorted (I'm London based)? Anyone got experience of this? The car's still under warranty (although my friendly OPC has since drawn a blank on any notion of an FoC rebuild), so on that basis, I'd rather have the standard LSD rebuilt with stronger internals than do anything too drastic. I've heard that the 'Cup' internals are the way to go? Thoughts?

Just had mine done. Take a short trip down the M23 to see Nine Excellence; get them to replace with Cup diff plates. Job's a goodun'.

If it's under Porsche warranty then its down to them to sort it out - but it'd be nice if you could just pay the cost different to change to the Motorsport part. I believe the part cost is similar but it requires a little more time & skill to set up correctly - but it is indeed much better quality.

Take it back to Steve at Jaz and asking him very kindly to put the cup plates in. Just don't drink the tea.

thegoose said: If it's under Porsche warranty then its down to them to sort it out. To replace a consumable part?

sleep envy said: thegoose said: If it's under Porsche warranty then its down to them to sort it out. To replace a consumable part? Indeed. It was worth asking, but alas, the diff packs are covered under the same umbrella as brakes and clutches; i.e. "Go forth and multiply. Sir". Edited by GentlePersuasion on Tuesday 12th February 12:23

Pretty much everyone with a non garage queen milaged GT3 should have had diff rebuilds. It's nothing to be concerned about. As you are aware, the street plates aren't very robust and track driving will finish them off in short order. Just use some of the Cup discs instead of the regular street ones when you rebuild. Fwiw the Cup also uses more discs in its diff stack so mostly you don't want it rebuilt actually to Cup spec or else you may wrestle with the diff on the street. There is an actual test you can do to check, Porsche give the exact values for brakeaway torque so its not anything mystical that's involved to confirm. . Instability under braking is the first sign of one being worn from the drivers seat however. Mike at Sports and Classic would get the usual nod but he may be too far up North for you.

JZM rebuilt the diff on my 996.2 GT3 with cup plates last summer and I was very happy with the results.

Cup plates the way to go - it's the stability of the back end under heavy braking that you will notice most (on track anyway).

Rebuild mine myself last year with Guard internals (Guard Transmission). It's not hard to do, but if you want it done 100% correct you have to take a measurement first to get plates with a perfect fit as the stack height is slightly different for each differential. I recommend you check out the LSD Buster thread on 6speedonline and/or contact Matt at Guards to get all the info you need for this.

gt3nor said: Rebuild mine myself last year with Guard internals (Guard Transmission). It's not hard to do, but if you want it done 100% correct you have to take a measurement first to get plates with a perfect fit as the stack height is slightly different for each differential. I recommend you check out the LSD Buster thread on 6speedonline and/or contact Matt at Guards to get all the info you need for this. Thanks, I've read about the Guard diff. Any idea on prices vs the Porsche Motorsport stuff?

I paid US$1123,- including overseas shipping for all parts needed from Guards. Don't know the exact price on the Cup parts, but I think they cost more. Note that you have to measure the ramps inside the diff before you order to get plates that are the correct size, as far I understand Guard is the only company that will send you internals that are correct for your ramp size - they are not all the same. You either have to leave the diff out while waiting on the new internals, or you have to do the job twice - but it will be correct doing it this way.

From your post in the other thread (for others reading) GentlePersuasion said: Not any more than they'd do the clutch (on the premise that Porsche considers the diff to be 'in spec' at >8 lbs torque) which to a normal human being, more often than not means it's f ked. Not that I'm bitter, you understand. That is the specification for the stock diff. With it removed and one end held in a vice the other side should provide a torque figure of between 5-15Nm (3.5 - 11lb ft). If it achieves this your diff is working as intended. Whether or not you feel it is enough is a whole other deal.

Roughly what does it cost to get a diff rebuilt with cup plates?

arcamalpha said: Roughly what does it cost to get a diff rebuilt with cup plates? circa £1200?

Cup plates are a great upgrade, first did mine in 2009. But as they are a race part and not road your gear oil needs much more regular changes. There is a coating on the plates that contaminates the oil. I change mine every 5k miles

Dave 321 said: Cup plates are a great upgrade, first did mine in 2009. But as they are a race part and not road your gear oil needs much more regular changes. There is a coating on the plates that contaminates the oil. I change mine every 5k miles I've not heard of this before, where did you get your information from?

Cunno said: I've not heard of this before, where did you get your information from? Experienced the oil contamination by just changing the oil & put it on another forum. Well respected porsche specialist with a lot of Porsche racecar experience commented saying about the coating on the plates contaminates the oil

common fallout with the use of Carbonetic diffs.

The Motorsport diff plates are steel with a plasma sprayed molybdenum coating. The street diff plates are brass.

jeremyc said: Just had mine done. Take a short trip down the M23 to see Nine Excellence; get them to replace with Cup diff plates. Job's a goodun'. +1 Ken did an absolutely superb job with mine earlier this year. Correctly shimmed motorsport plates along with some tweaks to the geo. The car now is the best handling GT3 I've ever driven...through all the phases - braking, turn in, rotation, and exit. You have the confidence to play with the degree of rotation with the throttle pedal (e.g. slight lift to tuck the front end in, etc) Ken can also source the Guards solution - though I opted for Porsche Motorsport plates in the end. Highly recommend a chat with Ken.

In my opinion the ramps in a 997 GT3 should also be changed from 28/40 to 40/60 (like the 996 GT3 has) when doing a diff rebuild.

gt3nor said: In my opinion the ramps in a 997 GT3 should also be changed from 28/40 to 40/60 (like the 996 GT3 has) when doing a diff rebuild. Why do you say that? I've not driven a 996 GT3 do haven't experienced the difference.

My post above was about a 996 GT3. However, be interested if anyone knows what type of diff a 4.0 has. My driving seat feeling is not as aggressive as the 996 GT3 even though the 4.0 has other evolutionary dynamics (turn in, rear bias, rear stability control, etc)

Indeed. This was discussed in Steve's thread. The ramps are reduced in the 997 because it now uses the rear brakes to replicate a mechanical diff effect. As the plates wear out, the brakes take on more of the work. Adding better plates and increasing the ramps will reduce the opportunities for the electronics to involve the brakes in this area. How much this matters on an individual level will depend on how you drive of course.

fioran0 said: Indeed. This was discussed in Steve's thread. The ramps are reduced in the 997 because it now uses the rear brakes to replicate a mechanical diff effect. As the plates wear out, the brakes take on more of the work. Adding better plates and increasing the ramps will reduce the opportunities for the electronics to involve the brakes in this area. How much this matters on an individual level will depend on how you drive of course. When we discussed this in Steves 997 thread, I believe you thought this was the case but not 100% sure. Is this still the case or do you have new info. Only asking as I'm looking at increasing the ramp angles in my car. one thing which is confusing is when you compare the 997 and 996 diffs the part numbers are the same, so how does the 997 have different ramp angles? Jon

gt3nor said: In my opinion the ramps in a 997 GT3 should also be changed from 28/40 to 40/60 (like the 996 GT3 has) when doing a diff rebuild. Have you done this? What difference did it make? What parts did you use?

Cunno said: When we discussed this in Steves 997 thread, I believe you thought this was the case but not 100% sure. Is this still the case or do you have new info. Only asking as I'm looking at increasing the ramp angles in my car. one thing which is confusing is when you compare the 997 and 996 diffs the part numbers are the same, so how does the 997 have different ramp angles? Jon Heres my post from Steve's thread: "The pad wear is related to the wheels being physically braked by the ABD to replicate a conventional diff. As the actual LSD in the gearbox wears out the amount of work that the ABD would have to do would increase adding to pad wear. The system monitors for a difference in rotational speeds of the wheels via the abs sensors and reacts when certain conditions are reached. I would imagine that a diff with more lock would require less intervention by the system when such conditions were reached and also reduce the number of times the ABD felt it had to to anything in the first place. I also agree that TC off means off with respect to the systems that operate via engine control, but am not 100% on whether this also includes the ABD portion. I believe that ABD remains on at all times but don't know unequivocally." My position is still the same. I believe that its on at all times but don't know unequivocally. All the literature I have on the systems (and its systems literature, not marketing stuff) say that ABD is not turned off when the off button is pressed. It remains active along with ABS and the electronic bias. Ive not seen anything personally to suggest that this is wrong but I have limited time with the street versions past the 996. How much pad wear you get as a result seems to depend on how you drive and the condition of your diff. Its logical that a stronger diff would prevent much of the ABD activation since it responds to ABS wheel sensor signals regarding slippage I am an engineer so you aren't going to get anything stronger than that from me with the info I have. I've not directly measured it nor have I seen isolated results from someone who has. Ive seen data traces from brake system pressure sensors that show pressure on the rear circuit with system turned off that suggest the literature is correct but it should be considered anecdotal.

I put 997 GT2 ramps (steeper than the 996) in my 996 with Cup plates in the end. It cured the lurch I used to get coming on the power out of roundabouts(!) which was sometimes so violent it used to feel like a wheel was loose. Oddly, I only ever felt it on the road, never on track...

fioran0 said: Cunno said: When we discussed this in Steves 997 thread, I believe you thought this was the case but not 100% sure. Is this still the case or do you have new info. Only asking as I'm looking at increasing the ramp angles in my car. one thing which is confusing is when you compare the 997 and 996 diffs the part numbers are the same, so how does the 997 have different ramp angles? Jon Heres my post from Steve's thread: "The pad wear is related to the wheels being physically braked by the ABD to replicate a conventional diff. As the actual LSD in the gearbox wears out the amount of work that the ABD would have to do would increase adding to pad wear. The system monitors for a difference in rotational speeds of the wheels via the abs sensors and reacts when certain conditions are reached. I would imagine that a diff with more lock would require less intervention by the system when such conditions were reached and also reduce the number of times the ABD felt it had to to anything in the first place. I also agree that TC off means off with respect to the systems that operate via engine control, but am not 100% on whether this also includes the ABD portion. I believe that ABD remains on at all times but don't know unequivocally." My position is still the same. I believe that its on at all times but don't know unequivocally. All the literature I have on the systems (and its systems literature, not marketing stuff) say that ABD is not turned off when the off button is pressed. It remains active along with ABS and the electronic bias. Ive not seen anything personally to suggest that this is wrong but I have limited time with the street versions past the 996. How much pad wear you get as a result seems to depend on how you drive and the condition of your diff. Its logical that a stronger diff would prevent much of the ABD activation since it responds to ABS wheel sensor signals regarding slippage I am an engineer so you aren't going to get anything stronger than that from me with the info I have. I've not directly measured it nor have I seen isolated results from someone who has. Ive seen data traces from brake system pressure sensors that show pressure on the rear circuit with system turned off that suggest the literature is correct but it should be considered anecdotal. Thanks, that helps, I think I'll give it ago.

Cunno said: gt3nor said: In my opinion the ramps in a 997 GT3 should also be changed from 28/40 to 40/60 (like the 996 GT3 has) when doing a diff rebuild. Have you done this? What difference did it make? What parts did you use? I did my own rebuild on my 996 GT3 with parts from Guards. My 997 GT3 came with a rebuild diff with 40/60 ramps and Guards internals already, so I don't know how it was on stock ramps. But it grips like h*ll now.

Cunno said: Have you done this? What difference did it make? What parts did you use? Have you ever had your diff plates replaced?

Pictures: https://www.facebook.com/media/set/?set=ms.c.eJw9y...

Cunno said: . one thing which is confusing is when you compare the 997 and 996 diffs the part numbers are the same, so how does the 997 have different ramp angles? Jon Good question. What are the ramp angles on each then?

jackwood said: Cunno said: Have you done this? What difference did it make? What parts did you use? Have you ever had your diff plates replaced? Yes twice, both times with cup plates

ttdan said: Cunno said: . one thing which is confusing is when you compare the 997 and 996 diffs the part numbers are the same, so how does the 997 have different ramp angles? Jon Good question. What are the ramp angles on each then? 997. 28/40 996. 40/60

gt3nor said: Cunno said: gt3nor said: In my opinion the ramps in a 997 GT3 should also be changed from 28/40 to 40/60 (like the 996 GT3 has) when doing a diff rebuild. Have you done this? What difference did it make? What parts did you use? I did my own rebuild on my 996 GT3 with parts from Guards. My 997 GT3 came with a rebuild diff with 40/60 ramps and Guards internals already, so I don't know how it was on stock ramps. But it grips like h*ll now. How often do you track the car and at what rate are the rear pads wearing out? At present mine eats it rears on standard pads every 5 TDs and R29s seem to last about 7 TDS, but the discs are taking more hammer.

Cunno said: ttdan said: Cunno said: . one thing which is confusing is when you compare the 997 and 996 diffs the part numbers are the same, so how does the 997 have different ramp angles? Jon Good question. What are the ramp angles on each then? 997. 28/40 996. 40/60 Thats the angles for the 996 cup car, street car the same them but with the same part number as the 997?

Get the Guards plates and washers, they are a way better than the Cup ones. Slightly more expensive but cheaper in the long run. Give Matt a call, he's so clued into what GT3 owners want, having supplied hundreds of diff and gearbox parts. The build quality of his parts are top quality.

ttdan said: Cunno said: ttdan said: Cunno said: . one thing which is confusing is when you compare the 997 and 996 diffs the part numbers are the same, so how does the 997 have different ramp angles? Jon Good question. What are the ramp angles on each then? 997. 28/40 996. 40/60 Thats the angles for the 996 cup car, street car the same them but with the same part number as the 997? So I'm told hence the confusion, will now have to have a look at the GT2 ramps

I am fitting a motorsport diff pack to my 997 and changing to a 40/60 asap. I have now collected my car. Very nice example but it was obvious straight away that the diff is not effective under both braking and acceleration - even at gentle road speed the car is relatively unstable under braking. It needs a lot more diff control for my needs. Lovely chassis though. Very tight. The PASM on normal is reasonably well judged on the road but the damping control isn't good - and it doesn't like a trail. but I knew that already. Pasm on sport is hopeless and the control is even worse. In normal, it's not a problem on the road unless your driving to avoidance and you need to load the nose. Going back to the diff - i'd definitely advise a stronger component set, and for a driver who wants more help out of his or her car at 10/10ths, i'd definitely go 40/60 as well. jury is out on the benefits of a final drive change.

The Guards ramps have 2 settings, 40/60 and 50/70 for more track focussed cars. I set mine at 40/60 and am delighted with the way the car handles. Race cars set the diff at the more aggressive 50/80 setting according to Matt.

ttdan said: Cunno said: . one thing which is confusing is when you compare the 997 and 996 diffs the part numbers are the same, so how does the 997 have different ramp angles? Jon Good question. What are the ramp angles on each then? The 996 and 997 part numbers are not the same and the ramps are very much different on both.

fioran0 said: ttdan said: Cunno said: . one thing which is confusing is when you compare the 997 and 996 diffs the part numbers are the same, so how does the 997 have different ramp angles? Jon Good question. What are the ramp angles on each then? The 996 and 997 part numbers are not the same and the ramps are very much different on both. Ok what am I missing? Ring set 950 332 983 05 Gear set 950 332 983 02 According to the Porsche web site and the 996 & 997 Kat these parts are the same numbers on both cars?

Cunno said: Ok what am I missing? Ring set 950 332 983 05 Gear set 950 332 983 02 According to the Porsche web site and the 996 & 997 Kat these parts are the same numbers on both cars? Thats not the diff you are looking at. Part numbers for the diff are in my post above.

nxi20 said: I put 997 GT2 ramps (steeper than the 996) in my 996 with Cup plates in the end. It cured the lurch I used to get coming on the power out of roundabouts(!) which was sometimes so violent it used to feel like a wheel was loose. Oddly, I only ever felt it on the road, never on track... IIRC the 997 GT2 uses the exact same diff as the 997 GT3 with the same 28/40 ramp angles. Edit: Did you use Guard product actually? They have three product lines for the GT3 that are labelled (GT1, GT2 and GT3). Perhaps you got a GT2 and misinterpreted it as being from an actual GT2? Edited by fioran0 on Sunday 4th January 20:48

fioran0 said: Cunno said: Ok what am I missing? Ring set 950 332 983 05 Gear set 950 332 983 02 According to the Porsche web site and the 996 & 997 Kat these parts are the same numbers on both cars? Thats not the diff you are looking at. Part numbers for the diff are in my post above. Thanks I now see were your getting the part numbers from. Is that not the number to buy a complete new diff? Can you not buy the internals as per the cup plates to upgrade? What part within the diff sets the ramp angle I thought it was the ring set?

fioran0 said: IIRC the 997 GT2 uses the exact same diff as the 997 GT3 with the same 28/40 ramp angles. Edit: Did you use Guard product actually? They have three product lines for the GT3 that are labelled (GT1, GT2 and GT3). Perhaps you got a GT2 and misinterpreted it as being from an actual GT2? I'm 99% sure it was from a 997 GT2; I know I have a Guard LSD in my 997.2 GT3. The ramp angles are much steeper on the 997 GT2 diff compared to the ones on the 996 GT3 diff (as you would expect with the greater torque of the GT2); istr that the locking is 40/60 on both. I will check with the man who knows tomorrow...

ttdan said: Thats the angles for the 996 cup car, street car the same them but with the same part number as the 997? Only the early 996 cup used a 40/60 diff. The later 996 Cup used a slightly more aggressive diff ramp angles of 45/65 that continued through 997 Cup and RSR. Worthy of note is the difference in preload between the street and race diffs. The Cup diff runs with a preload of 150Nm give or take. The spec for the 996 GT3 diff is 5-15Nm. Thats quite a difference. As per my earlier post. The 996 and 997 diff part numbers are also not the same. There is probably some confusion as the 996 part superseded to a 997 part number but there are two distinct numbers for each group of cars. 997 332 083 90 is the part for the 996, 997 332 083 91 is the part for the 997. The 997 part ending in 91 very much uses 28/40 diff ramps, the 996 part which is now on a 997 number definitely does not. You do not want to get this mixed up if you are buying a new diff for a 996 which needs the 40/60. Edit to tidy up the formatting of my phone post. Edited by fioran0 on Monday 5th January 01:47

Why are the cups with the ramp angles machined into them all listed as same part number for 996 and 997 GT3? I realise the complete differential part numbers are listed differently but the cups that determine ramp angles when viewed on pet system are same part numbers ?

nxi20 said: I'm 99% sure it was from a 997 GT2; I know I have a Guard LSD in my 997.2 GT3. The ramp angles are much steeper on the 997 GT2 diff compared to the ones on the 996 GT3 diff (as you would expect with the greater torque of the GT2); istr that the locking is 40/60 on both. I will check with the man who knows tomorrow... My PET list shows the same part on the 997 GT2 as for the 997 GT3 and GT3 RS (3.6 and 3.8) though of course PET is wrong plenty of times. I'm certain that since no one buys half the stuff in there any way Porsche don't care that its error strewn. The one thing all the 997 cars (GT3 and GT2) have in common is that they all have the electronic diff thing happening so the use of a common 28/40 diff as a consequence isn't so surprising. I wouldn't expect the GT2 with its electronics to have a larger locking percentage than a 996 GT3 though given it is already delivered with 40/60 lock up. The steepest ramps that Porsche use AFAIK are the 45/65 as fitted to the late 996 cup, 997 Cup and RSR. Heres a 996 GT3 40/60 diff ramp, a 997 GT3 LSD 28/40 diff ramp and a Guard 50/80 diff ramp for comparison. Photos were taken by Matt at GT. Edit: update with better pics Edited by fioran0 on Monday 5th January 09:16

Hi, I have done some research into the braking system on 997 GT3 for anyone that is interested see screen print below.. So it seems when TC button is pressed the ABD/ASR is deactivated. Mike

Mike, any chance of a larger scan? Cant read that even when zoomed in. Thx.

ttdan said: Mike, any chance of a larger scan? Cant read that even when zoomed in. Thx. Click on the picture, and then click on it again in thumbsnap to further open a high res version...

sportsandclassic said: Why are the cups with the ramp angles machined into them all listed as same part number for 996 and 997 GT3? I realise the complete differential part numbers are listed differently but the cups that determine ramp angles when viewed on pet system are same part numbers ? The short answer is that the numbers in PET are obviously wrong.

sportsandclassic said: Hi, I have done some research into the braking system on 997 GT3 for anyone that is interested see screen print below.. So it seems when TC button is pressed the ABD/ASR is deactivated. Mike Thats interesting and conflicts with my own info. The technical manual I have for the systems says ABD is not disabled with systems off but that it remains on along with ABS and EBD . The off switch disables EDTC and ASR. As I mentioned above, Ive seen data logging traces showing pressure manipulation on the rear with systems apparently turned off that suggest some ABD intervention was still happening but this can only be considered anecdotal. In the end however I don't see that its being on or off is that important. Its purely a distraction on the following grounds: If it is indeed completely off then the overall LSD effect has no chance of doing anything at all with the factory diff with 28/40 and brass plates. If it remains active then one would want to do all they can to minimise its influence. The outcome is the same in the end. If you track your car you need to look at doing something with the mechanical side of the diff. Whats interesting is that theres as yet been no mention of the relative performance of diffs. Its been taken that 40/60 is better than 28/40 but this is not as it seems. With factory plates in there, neither diff will do much after a track day or two and so the 28/40 is no worse mechanically than the 40/60 in the end. Fitting the plasma coated plates into the 28/40 will give you a diff that likely outperforms the 40/60 with brass plates even when new. After a day on track there will be no contest thats for sure. The 40/60 of course has the advantage in lock up where both have plasma coated plates. The summary is that the 28/40 diff doesn't necessarily need much done to it depending on the owner and use. A change of plates can be enough to give more than enough diff in many cases, though if you need more then you need new ramps as well.

Can someone explain how the car will act/react differently with the different ramp options? And what the benefits/drawbacks are for street use vs track use? Thanks

Hi, I have not had time to digest all of the comments regarding the diff's as been over busy with our new workshop that we moved into beginning of November last year. As I have mentioned the ABD system is disabled according to Porsche so that is ruled out of the equation for the 997 issue a member mentioned. Porsche diffs use a % value like 40/65% or 28/40% I believe these to be the ramp angle.. I hope, if so this is my understanding of things. The ramp angles that keep getting mentioned are measured in degrees and are as follows: 996 GT3 40/65 997 GT3 28/40 This means that the first figure is the angle for acceleration lock up and second figure is for coast (braking) lock up, the greater the angle like 90 degrees translates to a low locking action and the smaller the degree means a higher locking action. So if you compare the two units the 996 GT3 has a larger lock up angle for acceleration and coast compared to the 997 GT3, the 997 GT3 ramps are more aggressive than that of a 996 GT3 not the other way round. The 997 GT3 28 degree ramp angle will be able to exert more lateral force onto the clutch packs to increase the amount of torque required to allow one wheel to rotate independent of the other... same under braking the 40 degree angle would require more torque or tyre grip to overcome the clutch packs and allow one wheel to turn independant of the other wheel compared with 996 GT3 setup. So "upgrading" to 996 spec is a step backwards as far as ramp angles is concerned..? Mike

sportsandclassic said: Porsche diffs use a % value like 40/65% or 28/40% I believe these to be the ramp angle.. I hope, if so this is my understanding of things. The ramp angles that keep getting mentioned are measured in degrees and are as follows: 996 GT3 40/65 997 GT3 28/40 I think you'll find they're lockup percentages rather than ramp angles.

sportsandclassic said: As I have mentioned the ABD system is disabled according to Porsche so that is ruled out of the equation for the 997 issue a member mentioned. Porsche diffs use a % value like 40/65% or 28/40% I believe these to be the ramp angle.. I hope, if so this is my understanding of things. Mike My technical booklet for the ABS and PSM systems is Porsche issued and says that the ABD remains on. The Bosch stuff I have also says the same. I see your PIWIS screen says otherwise though so who knows. As per my previous post, its not particularly important either way as far as I see it. If you drive on the road then systems will be on. If you drive on the track regularly then you need to take the same action over the diff regardless of ABD. The values Porsche use on the diffs are not ramp angles. They are percentages of lock up or as Porsche calls it, "Percentage of anti slip effectiveness". The GT gear diff has a monster 50/80 % lock up. Its the biggest locking diff for the GT3 cars I have seen. If it were ramp angles it would have the lowest lock up of them all. The 997 diff is weaker than the 996 diff all things being equal. This is because it has ABD and is designed to allow it to operate. Just looking at the pictures are enough to tell you which diffs have more or less lock. The actual ramp angles on the 996 diff are approx 50 deg on the 40% and 37.5 deg on the 60%. The GT ramp angles are 45 deg on the 50% and 30 deg on the 80%. The 997 diff has 50 deg and way more than 50 deg.

here's mine installed in a 6 Cup box.

The following site gives a good overview of LSDs, ramp angles and crucially the effect of preload (which isn't discussed much in this thread): http://www.blueskymotorsports.com/index.php?option... My Caterham has carbon covered plates in the diff and although using fairly modest accel/decel ramps, it can be massively tuned just by the use of setting a higher/lower preload on the clutch packs. Simply fitting the "right part number" and re-assembling is only half the picture....

Ade, Pure diff pron!!! Fergus, I assume that the reason its not been mentioned much is because the mainstream thinking on the GT cars is that for a car driven on street as well as the track, the factory preload setting is about right. That the balance of the diff itself is fine (with the 40/60 diff) but that the ability of the street plates to both provide this lock up and do it for more than a few track days is the main issue. FWIW, the preload setting for the street diff is 5-15Nm. The preload on the diff in my Cup is 150Nm. So while lock up is more or less the same (40/60 v 45/65) and the plates fitted may be the same (assuming a street upgrade with motorsport plates) the behavior of the diff is quite different.

fioran0 said: ...balance of the diff itself is fine ... ability of the street plates to both provide this lock up and do it for more than a few track days is the main issue. FWIW, the preload setting for the street diff is 5-15Nm. The preload on the diff in my Cup is 150Nm. So while lock up is more or less the same (40/60 v 45/65) and the plates fitted may be the same (assuming a street upgrade with motorsport plates) the behavior of the diff is quite different. Neil Hi. Totally agree with all the points above, esp. re plate longevity. The diff behaviour not being solely a function of accel/decel ramp angles was the point I was alluding to in my post. As an aside, the preload for the "street" setting seems very low? Does it take a good stab of throttle to get the diff locked up? i.e. not much effect on low torque inputs/part throttle? I can imagine this is fairly frustrating when on part throttle on the exit from a corner, when you want the diff to be in the process of locking up and the transition may be all or nothing?

You need as much stability as you can get with a 911 in the braking zone and a well set up diff is the key. I like a lot of lock under braking but the down side is a car that would prefer not to rotate so there is a conflict and a balance needs to be struck. I'll take a car that is lovely and stable under braking that I have to work to the apex than something that is easy to rotate. All of the time in a 911 is made up under braking. The lock on accelleration is usefull but the layout is already on the drivers side anyway. For me at least it is the braking set up of the diff that is crucial.

Steve Rance said: You need as much stability as you can get with a 911 in the braking zone and a well set up diff is the key. I like a lot of lock under braking but the down side is a car that would prefer not to rotate so there is a conflict and a balance needs to be struck. I'll take a car that is lovely and stable under braking that I have to work to the apex than something that is easy to rotate. All of the time in a 911 is made up under braking. The lock on accelleration is usefull but the layout is already on the drivers side anyway. For me at least it is the braking set up of the diff that is crucial. same here!

fioran0 said: sportsandclassic said: As I have mentioned the ABD system is disabled according to Porsche so that is ruled out of the equation for the 997 issue a member mentioned. Porsche diffs use a % value like 40/65% or 28/40% I believe these to be the ramp angle.. I hope, if so this is my understanding of things. Mike My technical booklet for the ABS and PSM systems is Porsche issued and says that the ABD remains on. The Bosch stuff I have also says the same. I see your PIWIS screen says otherwise though so who knows. As per my previous post, its not particularly important either way as far as I see it. If you drive on the road then systems will be on. If you drive on the track regularly then you need to take the same action over the diff regardless of ABD. The values Porsche use on the diffs are not ramp angles. They are percentages of lock up or as Porsche calls it, "Percentage of anti slip effectiveness". The GT gear diff has a monster 50/80 % lock up. Its the biggest locking diff for the GT3 cars I have seen. If it were ramp angles it would have the lowest lock up of them all. The 997 diff is weaker than the 996 diff all things being equal. This is because it has ABD and is designed to allow it to operate. Just looking at the pictures are enough to tell you which diffs have more or less lock. The actual ramp angles on the 996 diff are approx 50 deg on the 40% and 37.5 deg on the 60%. The GT ramp angles are 45 deg on the 50% and 30 deg on the 80%. The 997 diff has 50 deg and way more than 50 deg. All through this posting you refer to the numbers as ramp angles.. ?

keep it lit said: same here! Ade. The diff set up on your car is the best that i've ever driven. Just perfect

I've enjoyed this thread and would like to add my 0.5p worth for the LSD in the 997GT2. The 997GT2 runs the 28/40 set up which Porsche state is designed (with the other systems and chassis configuration particularly at the front axle) to offer more neutral handling for increased driving dynamics (compared with the 996GT2 40/60 chassis set up) In effect this LSD does squat even when rebuilt with CUP plates, I am told it is because of the low pre load specified. The 7GT2 doesn't seem to really need an LSD as the electroncs, the TC, PSM and associated systems work well together but seem to interject before the LSD can come into play. Because my road car has a wide 900NM torque band the factory diff was showing signs of wanting to explode so our friend Matt specced me up his proprietary 40/60 billet unit as seen below which was teamed up with the billet side plate. The most noticeable thing about having this unit is (as has been said in previous posts) there is a definite tendency to understeer, the front end simply doesn't turn in as easily as before particularly off throttle. In terms of the LSD providing traction ? Well the electronics seem to cut in pretty fast and whilst the LSD is probably helping as part of the system the electronics dominate proceedings when trying to deploy some serious grunt through two wheels. I'm sure with the TC switched off the LSD would come into its own but unfortunately/fortunately I have a program glitch which has disabled the TC off switch and I haven't been in a hurry to get it fixed. I presume the discussion about LSD actions on 997GT3.2s are talking about running with all electronic nannies off ? Edited by TB993tt on Wednesday 7th January 16:34

TB993tt said: I presume the discussion about LSD actions on 997GT3.2s are talking about running with all electronic nannies off ? Which you have to do to prevent the car from utterly obliterating its brakes. With the standard diff and everything off it gets a bit frisky under braking, but under power it isn't _that_ bad IMHO. Edited by DiscoColin on Wednesday 7th January 19:03

This is an excellent thread. I think that it demonstrates the problems that Porsche engineers face and how they try to resolve them. From an ultimate performance perspective the 996 GT3 diff set up under braking is far better than the 997. It's a lot more stable and ultimately from my experience more predictable. But it requires a reasonably difficult driving technique to overcome the understeer that it creates and most drivers make that worse by turning in off of the brakes or worse still, on the throttle. It looks like to counter this on the next model (997) Porsche reduced the diff lock on braking and added some rear braking assistance that was programmed to help stabilise the car under braking but enable it to rotate. If I'm right, it would explain why when the 997 is driven in the classic 911 trail brake technique it does not reward or perform as the 996 does. In other words, the 996 is set up for ultimate performance but only when driven properly - the same as the cup car, but the 997 set up for the most commonly used driving technique. you could say that it's intentionally dummed down. That would explain why the PASM is so horrible on a trail - it's not designed to be driven on a trail. The primary assumption being that the majority of drivers wont drive it that way. Neil, I'd be interested to hear your thoughts on this?

It looks like JZM are launching a diff kit for the 997. Motorsport upgrade pack and 60:40 set up.

Info taken from this site : 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.

Sorry, missed the URL - http://www.bodymotion.com/GT3_Differentials.asp I have no idea if the information is 'correct' or not, just thought I would post it here for information

Sounds like plenty of sales guff in the reading,especialy on the early cars. You can get a lot of traction and brake stability with correct damping and springs,anti roll bar settings and then tune the diff to your needs.

Sounds like plenty of sales guff in the reading,especialy on the early cars. You can get a lot of traction and brake stability with correct damping and springs,anti roll bar settings and then tune the diff to your needs.

There is some truth in it but as you say, exit traction isn't a huge problem in 911's - old or new. Most LSD applications are mainly relevant under power on exit. But the 911 diff - certainly the water cooled cars - can be a key aid under braking and into the apex. I rely heavily on a diff set up to accommodate my driving style which I believe is quick and effective. Not sure about the multi diff set ups for pro's though. Most that I know that drive 911's are happy with a single set up. There is an interesting description of a worn diff on a 997 under braking though and it's pretty accurate. Either way, my view is that the 997 diff needs a plate upgrade and a 60:40 set up with a fair bit of pre load.

Steve Rance said: This is an excellent thread. I think that it demonstrates the problems that Porsche engineers face and how they try to resolve them. From an ultimate performance perspective the 996 GT3 diff set up under braking is far better than the 997. It's a lot more stable and ultimately from my experience more predictable. But it requires a reasonably difficult driving technique to overcome the understeer that it creates and most drivers make that worse by turning in off of the brakes or worse still, on the throttle. It looks like to counter this on the next model (997) Porsche reduced the diff lock on braking and added some rear braking assistance that was programmed to help stabilise the car under braking but enable it to rotate. If I'm right, it would explain why when the 997 is driven in the classic 911 trail brake technique it does not reward or perform as the 996 does. In other words, the 996 is set up for ultimate performance but only when driven properly - the same as the cup car, but the 997 set up for the most commonly used driving technique. you could say that it's intentionally dummed down. That would explain why the PASM is so horrible on a trail - it's not designed to be driven on a trail. The primary assumption being that the majority of drivers wont drive it that way. Neil, I'd be interested to hear your thoughts on this? Steve, Apologies for not getting on to this until now. It was a great post and I hope I didn't cause things to fall away too much by being tardy? Leading on from your first point a little, its also interesting in the context of what was happening with the cars as the appeal was widened. There has been a continuous and definite move with each iteration of GT3 to broaden its usability. This approach, coupled to the increase in ultimate performance that each iteration carries often does a good job of covering up some of what is actually lost by the process. On one level its hard to argue against it, clipping some of the peaks in exchange for access to more of whats left, more of the time is a positive in many respects. However you then get into something like a 997 cup and feel just what is left on the table with respect to the platform and its hard not to lament this approach. The 997.1 has it slightly tough in that regard as the performance jump from the previous iteration was actually quite small and as a result there is less camouflage there. With specific reference to the diff. Its an interesting one also. A diff with a high deceleration lock up % acts as a yaw damper on corner entry promoting understeer HOWEVER If you can get that diff lock up to really bite then you can use that same understeer inducing, yaw dampening diff to actually promote entry oversteer. The rear remains locked and the inside rear wheel breaks away. The 997 therefore responds less well to aggressive trail braking IMHO simply because there is less benefit to be obtained from it. The 40% deceleration side isn't really enough to provide that rotation that you experience with the 60% diff going in hot with the axle locked up. Trail braking is still something that benefits, especially in 2nd gear corners and some 3rd gear ones where it makes all the difference to rotation, but there isn't that extra level of reward available from hitting it really hard (as I am sure you do). The 996 by contrast equally benefits being trail braked in the same corners as with the 997 but can also offer that extra bit of rotation when worked aggressively as a result of its deceleration ramp offering 60% lock. This situation is magnified of course on the Cup cars because the preload is so high. Driving one of the guard 50/80 diffs combined with a large preload one needs to really lean on the diff lock rotation effect to get the car turned at all. The whole diff decision in the 997.1 was poorly judged by Porsche IMHO but thats for another post. I found your PASM comment interesting. I sometimes felt that it modulated in an odd way when worked heavily (and this would include heavy trail brake wrt to the front dampers) but I know many don't feel it. I am certainly not the most sensitive nor the fastest thats for sure so its certainly not that playing in. Its impossible to say whether its a limitation of system, design/testing, or a figment of imagination all together. Edited by fioran0 on Wednesday 14th January 00:18

Steve Rance said: There is some truth in it but as you say, exit traction isn't a huge problem in 911's - old or new. Most LSD applications are mainly relevant under power on exit. But the 911 diff - certainly the water cooled cars - can be a key aid under braking and into the apex. . This +1 The diff working to create rear end stability during braking is crucial on the 911. If you don't have this it gets incredibly twitchy back there as the rear tries to swap ends with the front. In fact the first indication that you diff has had enough is this sensation under braking. You almost never notice it under acceleration due to the weight advantage (and in my experience it will be throttle steering where you can tell rather than wheel spinning stuff). I always use the rear wing on the 997 cup at maximum angle for rear end braking stability too (amongst other reasons). With the wing flat I can feel the reduction in stability (even with the diff) when braking really heavily from high speed. My experiment in increasing Vmax was a short lived one Theres a second conversation in here somewhere about the 997.1 given its 40% deceleration lock up. Edited by fioran0 on Wednesday 14th January 01:07

Just to pop back in to this. I was rummaging around and found some hand over notes from one of the first 996 Cups I bought. In the notes for setup from the team that had previously owned it it listed: For general oversteer, soften rear bar. For general understeer, trail brake more. Hardly ground breaking stuff in terms of secret sharing but it was amusing seeing it after all these years in the context of this thread.

But it is SO effective and a whole lot of fun when you get it right. It's a pity that Porsche have tried to engineer this out of the 911. It was a technique that involved the driver as a fundamental in the extraction of performance. Surely what true driving enjoyment is all about??

Agreed and you would think so. Im not sure anyone at Porsche is much interested in that anachronistic view of driving any more sadly. Top Trumps style metrics are so much easier to pump out. Its a new day and a new target of owners.

(For the record, I agree with you)

Steve, Neil Thank you. I have learned so much from those past few posts! I have so much more to learn though!

I'm going to bump this up just to add that I read something by GT Gears earlier that was specifically discussing fitting a diff with more lock up % into the newer cars such as the 997. These are designed increasingly to have electronics do the job of the diff and are as a result given a very weak diff from the factory. He confirmed that doing this swap does indeed move the mechanical diff upstream of the electronics, reducing or removing the amount of intervention that takes place from the electronic side of things going forward. Much better situation from the driving seat and your brakes will thank you for doing it too.

Great news Old Chap. Thank you very much for posting

Steve Rance said: Great news Old Chap. Thank you very much for posting Steve have you convert your diff yet? If so which way are you or did you go guard or cup parts?

Not yet. I'm a little torn to be honest. Both options will be a huge improvement over the standard set up

Steve Rance said: Great news Old Chap. Thank you very much for posting No probs. Not worth a post on its own but felt it was worth filing in here.

Little bit of a thread resurrection… Cunno just told me about this thread, loads of great info I had cup plates fitted to my 7.1 GT3 around 2013 and then had that rebuilt a few years later as the diff never felt quite right, I’m not sure it was fitted correctly by the place that supplied and fitted the cup pack. The 2nd rebuild felt much better, but on my last trackday last year I noticed the car being a little wobbly at the rear again. Not badly, but not really together like it used to be So I’m having the diff rebuilt again soon, and the term Belleville washer has been thrown about. I’ve googled a bit but can’t seem to work out exactly what the Belleville washer is for. I’ve read about it helping (increase?) the preload, but I’m not sure exactly what is meant by preload either … I think I understand what the plates actually do in the diff, but increasing the preload will do what exactly, just tighten everything up so the diff works ‘sooner’? Ie Make it lock up earlier on acceleration/ braking? I thought that was dictated by the steepness of the ramps? In which case does the Belleville washer act as a bit of a cheat to help less steep ramps trigger like a diff with a steeper set up? Thanks in advance, apologies for the stupid questions …

Using a Belleville washer increases the static preload ie how hard it is for the wheels to turn independently with no power or braking torque being applied. The lower this preload, the more the differential acts like it is open when no torque (power or braking) is applied to it. Whereas higher preload reduces the severity with which the plates/diff locks when accelerating or decelerating (as they’re already preloaded)

Cheers slippy. So basically, using the Belleville washer means the diff will trigger with less torque being applied, so under less aggressive acceleration / braking? Do all diffs comes with a Belleville washer fitted as standard, or are they added such as in this instance to improve performance? I’m wondering if I already have a Belleville washer and it has worn out, or if I don’t have one and it’s being fitted for the first time (I could just ask the guys doing the work, but I like to research first so I don’t appear completely ignorant!)

(By “trigger” I mean lock ^)

The OE diff has a Bellville washer, but it's a pretty flat/weak/thin item, hence once the less than durable OE plates wear out, the diff quickly loses its preload. A quick, cheap er "fix" is to fit a Bellvillle washer which is thicker and more convex, thus providing more static preload, however, you're still relying on the less than brilliant OE plates, hence most go with the Cup plate upgrade, this comes with a thicker, more convex Belville washer and plates with a more durable (and aggressive) friction material. Increasing the static preload raises the threshold at which point the ramps are activated, and in the process softens their initial effect.

Slippydiff said: The OE diff has a Bellville washer, but it's a pretty flat/weak/thin item, hence once the less than durable OE plates wear out, the diff quickly loses its preload. A quick, cheap er "fix" is to fit a Bellvillle washer which is thicker and more convex, thus providing more static preload, however, you're still relying on the less than brilliant OE plates, hence most go with the Cup plate upgrade, this comes with a thicker, more convex Belville washer and plates with a more durable (and aggressive) friction material. Increasing the static preload raises the threshold at which point the ramps are activated, and in the process softens their initial effect. Another trick which Porsche uses is to flip the orientation of the bellville washers. Some diffs have the cones pointing towards the middle of the diff, some have them pointing outwards. Pointing outwards on average increased pre-load by about 10-12Nm. You can also play with the thickness of the dog plates which sit between the clutch plates as well with the actual clutchplates thickness. Setting it up involves a bit of simple maths and good quality micrometers/depth gauges. And a torque wrench. Needless to say, if you are tracking the car hard, transaxle oil changes need to happen a lot more frequently than Porsche's service schedule would suggest. The oil has a double hard task - it has to work with the gears and synchros on one hand, but also it has to lubricate the LSD. Motul 300V 75w90 LSD is probably the best...

Thanks guys. I thought this thread got deleted or something as I couldn’t find it, then realised it’s in the Porsche General chat not 911. Duh… Anyway, thanks for all the info guys, that’s been very helpful. My theory is that the cup plates weren’t set up 100% correct, so nothing a rebuild by someone that knows these cars well shouldn’t fix Thanks again

Interesting thread, thanks everyone. I have a 997 GT3 with the Cup friction plates and original 28/40 diff, the Cup friction plates are a massive improvement but would like a more active diff especially under braking so looking to install the 996 40/60 ramps. It's been suggested that the Cup plates will be too aggressive with the 996 ramps and that we should switch to the Guard plates instead. Does anyone have experience with this combination of 996 40/60 ramps and Cup plates?

VPD255M said: Interesting thread, thanks everyone. I have a 997 GT3 with the Cup friction plates and original 28/40 diff, the Cup friction plates are a massive improvement but would like a more active diff especially under braking so looking to install the 996 40/60 ramps. It's been suggested that the Cup plates will be too aggressive with the 996 ramps and that we should switch to the Guard plates instead. Does anyone have experience with this combination of 996 40/60 ramps and Cup plates? i have it in my 996.1 CS - it works a treat. No (s)PASM either... So perhaps it works well in a analogue car like the 996, but it's too aggressive for the nannied 997 platform?

Cheburator mk2 said: VPD255M said: Interesting thread, thanks everyone. I have a 997 GT3 with the Cup friction plates and original 28/40 diff, the Cup friction plates are a massive improvement but would like a more active diff especially under braking so looking to install the 996 40/60 ramps. It's been suggested that the Cup plates will be too aggressive with the 996 ramps and that we should switch to the Guard plates instead. Does anyone have experience with this combination of 996 40/60 ramps and Cup plates? i have it in my 996.1 CS - it works a treat. No (s)PASM either... So perhaps it works well in a analogue car like the 996, but it's too aggressive for the nannied 997 platform? Useful to know, thanks.

Reviving an interesting thread. Has anyone tried this 'friction plate upgrade kit' from design911? It's a Racing Diffs product and about £450. https://www.design911.co.uk/p/friction-plate-upgra... I seem to recall that the Motorsport/Cup plates are no longer available so are most people installing Guards diffs these days?

braddo said: Reviving an interesting thread. Has anyone tried this 'friction plate upgrade kit' from design911? It's a Racing Diffs product and about £450. https://www.design911.co.uk/p/friction-plate-upgra... I seem to recall that the Motorsport/Cup plates are no longer available so are most people installing Guards diffs these days? Guard sells upgraded plate packs for the OE GT car and Cup diffs IIRC.

I’d only fit Porsche motorsport or guards. If the two I’d have guards build me a diff to my ramp and preload specifications. From the 997 on, road car diffs are essentially made of chocolate, wear out early and then rely on abd as a secondary back up.

VPD255M said: Interesting thread, thanks everyone. I have a 997 GT3 with the Cup friction plates and original 28/40 diff, the Cup friction plates are a massive improvement but would like a more active diff especially under braking so looking to install the 996 40/60 ramps. It's been suggested that the Cup plates will be too aggressive with the 996 ramps and that we should switch to the Guard plates instead. Does anyone have experience with this combination of 996 40/60 ramps and Cup plates? To feedback for info - I did go with the set-up which I was advised - using 996 40/60 ramps with Guard plates. This was done back in Oct 2021 so have had plenty of track and road-trip time since, and I'm very happy with the results, now the mechanical diff seems to come in before the electronic diff with lower rear brake pad wear (traction control off while on circuit both before and after diff upgrade)

Excellent set up. Far more predictable than the electronic alternative