Coming from an Audi background, I am curious. There are aftermarket parts for most Audi's that change the 50/50 F/R torque split to something more aggressive and generally aid in helping to remove understeer. I am curious, has anyone seen anything like this for the 964 C4? I am assuming it's a purely mechanical unit (picks up Adrians book). I am more curious than anything at this point to see if there is any mods like this for the C4. Search is dry on this subject pretty much.

Here is an example Audi upgrade from the good guys at
StaSIS

 

I don't have a C4, but I assume it's all down to what sort of central diff it has and how it's controlled.
Just my 2p's worth

 

The basic front/rear torque split is done by an epicylic gearbox. During 964 C4 development Porsche tried various splits and settled on 31 F-69 R .
There is also an intergrated clutch pack which can lock the front to rear solid.

To change the ratio a redesign - numbers of teeth - would have to be done on the box.Possible but demand ?
The 993 went over to a viscous coupling where the split is determined by tire rolling radius . That means that tire wear changes the split !
The new 997 turbo has gone back to the 959 system where F R split is set by a slipping clutch in the F R drive shaft . The amount of slip is set by F R wheel speeds .Involves some complex control strategies !

I have looked at replacing the front diff with a Haldex system but there is a lot of vehicle dynamic stuff to do !

Hope that helps

Geoff

 

Hey Geoff,
I might be wrong here, but from everything I've looked into, the torque spit on the C4 is performed via a torsionally elastic driveshaft where the front driveshaft is thinner and thus a fixed percentage more flexible than the rear output shaft, so when there is equal traction front and rear, the torque is always greater at the rear from the less "elastic" driveshaft. The sun gears in the center differential make the diff act like an open differential (a 50/50 split) until the clutch packs are acted upon, changing the torque ratio when wheels do not have good traction.
So to change the split, you'd change the driveshaft thickness, no? The factory workshop describes as such, no?

 

kgorman, via suspension changes you can make the car understeer free... a much more predictable way than changing the handling characteristics influenced by the drivetrain...

Besides, the torque split is changing when the differentials are being locked and unlocked accordingly...

 

Garrett376,

I dont know where the drive shaft stuff comes from ! Which manual ?
The torque split is a straightfoward gear ratio sum from the epicyclic gearbox .
I have factory data if you are interested.
By using different gear ratios the factory played with the split to get the best end result .
It is a further adaptation of the 959 system but removed the need for the font tires to be slightly larger than the rears to achieve front drive .It also removed the need to calculate the clutch lock force .

Hope that helps

Geoff

 

Hoping not to sound smug or anything but the C4 does not use a center differential. Instead a multi-plate clutch connects the transmission output to the front drive via a torsionally elastic driveshaft and a simple non-locking front differential. The multi-plate clutch in its resting state will transfer enough torque to the front drive to produce the 31/69 split. The PDAS system can direct more torque to the front drive via a hydraulic actuator that applies a “squeeze” to the multi-plate clutch pack.

As the clutch pack wears, the amount of torque sent to the front drive is reduced and the only way to get things back into spec is to replace the clutch pack. At some point during production, Porsche changed the configuration of the clutch pack and added a tension adjuster to allow compensation for wear. Perhaps the tension adjuster can be used to alter the split and change the car’s behavior?

I have one of the older units sitting on my garage floor at the moment. It’s really quite a piece of work—in my opinion.

-doug

 

But my Owner's Manual says there is one :

.... The yellow indicator light on the traction switch shows that the differentials are operating....

...When slippage occurs, the normal 31% to 69% power distribution changes as the hydraulically controllled longitudinal and transverse differentials engage....



And Geoff, I see in the manual it says "Torsionally elastic drive shaft mounted in transaxle tube between rear and front drives. Constant torque distribution by planetary gear in rear axle drive:"

So Geoff, does that mean the input shaft portion of the center differential has a different number of teeth against the planetary gears? How would it work otherwise?

If you have more info, I'd love to read it! Thanks

 

Garrett376,

As a starter , from Porsche Service Training WKS 305 020

P3-06 Lock control for the transfer box ( distribution of drive torque to the front axle and rear axle ) as well as the transverse lock for the gaurantee of rear wheel drive to both wheels is known as the PORSCHE DYNAMIC FOUR WHEEL DRIVE CONTROL .

P 3-09

Torque distribution is according to the pitch circle radii of the sun gear and ring gear , namely
31% to the front axle ( sun gear radius ) and
69% to the rear axle ( ring gear radius ).
---------------------------------------------------------------------------------
This Page 3-09 is part of an explanation of the operation of the epicyclic gearbox responsible for front / rear torque split.

Page 3-06 is an outline of the lock up clutches used when loss of wheel grip sensed by the ABS sensors demands further action.
----------------------------------------------------------------------------------

Just looked in Paul Freres Porsche 911 Story , P149 " the front rear torque split in the cenrtal differential etc "
---------------------------------------------------------------------------
Manfred Bantle - 964 development Weissach -

In a Carrera 4 , on the other hand , drive torque from the gearbox is first directed to a centre transfer case designed as a planetry or epicyclic gear set. In this center transfer case the input torque - presumming the lock up control doesnt come on - will always be divided in the same relationship: 31% for the front axle, 69% for the rear axle.

---------------------------------------------------------------------------------
Just a look at the design of the torque split center differential will confirm that
these guys are right !!!

There are some good books on these systems which really help understanding of what is going on.The lock up control system really is very sophisticated and was , technically , a big step foward from the 959.
Interesting that the 997 Turbo with even more processing power , drive by wire throttle etc has re-introduced the 959 system.

As you can tell this is an area of the 964 C4 that really interests me , though there are plenty of others !!!!!!!!!!!!!!!!!!!!

Hope that helps

Geoff

 

DougB,
The 964 C4 has a center differential for front rear torque split . The clutch pack is used to redirect torque front or rear when required.

All the best

Geoff

 

Hey Geoff - sweet information! You don't happen to have an electronic (email-able) version do you? Thanks for the explanations - it also fascinates me - thus the reason I have owned 3 of them, and still own two C4s

 

Garrett376,

Unfortunately I have good old printed stuff but I will try to scan the relevant bits .
An excellent book is Carrera 4 - Porsche Allrad 1900 - 1990

ISBN 3-517-01124-X

foward is by Ferry Porsche and it covers everything Porsche 4WD up to and including the 964 . When you read what the development guys had to say about viscous coupling it makes you wonder how they enjoyed the 993 !!!!!!

I have looked at other systems for torque split that are independent of tire size

but the 964 setup really is good. It would be interesting the revise the torque

split ratio by resizing the epicyclic gears but the cost would be serious !

All the best

Geoff

 

Good info Red! Thanks.

Garrett, this is more an acedemic excerise for us, vs me actually trying to change the handliing characteristics of my car. Frankly with the changes I have made I am quite happy. I am just curious like you and trying to expand my knowlege.

This discussion is great. So it seems to me based on the above discussion one could possibly (re)build a center diff to change the torque split if they wanted to by changing the plantary gears. All theoretical at this point.

 

Thank you Geoff - fabulous reference; I will take a look as that sounds really interesting - thanks! I love the historical and technical info about these cars...

 

I couldn't find this book new, but I did find it used here

As it turns out the book that arrived was new!

Cheers,
Dave