996/997 GT3 Diff Rebuild: Anyone Done it?

[nxi20]

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...

[fioran0]

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

[sportsandclassic]

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 ?

[sportsandclassic]

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

[ttdan]

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

[LaSource]

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...

[fioran0]

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.

[fioran0]

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.

[S1MMA]

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

[sportsandclassic]

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

[d41d8cd9]

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.

[fioran0]

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.

[keep it lit]

here's mine installed in a 6 Cup box.

[fergus]

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....

[fioran0]

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.

[fergus]

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?

[Steve Rance]

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.

[keep it lit]

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!

[sportsandclassic]

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.. ?

[Steve Rance]

keep it lit said:

same here!

Ade. The diff set up on your car is the best that i've ever driven. Just perfect