gjvander

I was wondering what the difference is between the two AWD models. From what I have read the 993 has a viscous coupling in the diff. Not sure how that works. Normal limited slip diffs are either speed sensing or torque sensing. I think in the 964's case it is speed sensing via wheel speed sensors vs gears that load up under centripital acceleration in a mechanical speed sensing diff.

Does the 993 also have active diffs?

I read the road test report where they complain about the 964's understeer. How much of that goes away if you disable the AWD system (the active part, not the normal 31/69 split), or is it even possible to do that? I have experienced severe understeer with light trailing of the throttle into a sharp turn.

Maybe some guys who take their C4's on the track have some experience with this.

Any thoughts?

Regards

Geo

Adrian

Dear Geo,
You are talking chalk and cheese systems here. The 964 C4 has a permanent all mechanical AWD system with electronically controlled, hydro-mechanically operated traction control.
The 993 C4 has a viscous coupling which is basically a unit filled with silicon fluid. This fluid hardens when swirled up by a set of discs. One set of discs is driven by the rear drive shaft and the other by the front drive shaft. If one set speeds up the silicon hardens and graps the other set effectively locking them together. Due to the nature of the fluid under normal conditions the drive ratio is 5% front and 95% rear.
The 993 C4 is basically a RWD with variable front to rear and rear to front traction control. The lateral traction control on the rear wheels is provided by a 20-100% lock all mechanical limited slip diff.

The 964C4 traction control can easily be disconnected. Just remove the connector from the control unit.

964C4 understeer can easily be adjusted out by making some small modifications.

The 964C4 AWD system is heavier but a far superior traction system to the 993 system which is the Subaru system. The 993 suffers from less initial understeer but once the front and rear driveshafts are locked together the resulting understeer is the same. The disadvantage of the 993 is also that this system cannot be disconnected for track work.

Ciao,
Adrian
964C4

rutz964C4

Adrian by disconnecting this then will the car act like a RWD?

THANKS
ALEX

delhi

in regular driving, you will not notice the difference b/t the two. the VC works just as well but minus the weight and maintenance. I say this cus I owned a coupla subarus and the AWD system never give any trouble whatsoever.

Adrian

Dear Alex,
No this just disconnects the traction control. You cannot disconnect the AWD because it is mechanical.

Dear Delhi,
Have you driven a 964C4 and 993C4? They handle, feel and drive completely differently under all circumstances.

Ciao,
Adrian
964C4

joey bagadonuts

Hey Geo,

Give the "Wringing out the Carrera 4" review a read from John Miles's website: http://www.porsche964.co.uk It explains how the C4's AWD system works and how to drive these cars quickly at the track.

delhi

adrian,
no haven't the chance to drive a 993 c4 to compare. i'm refering to the types of awd system found b/t the two. both models differ so much that of course there will be differences when comes to handling and accelerations. but if you put a 993 awd system into a 964 or vice versa i will be surprised if there would be significant difference.

C4TT

Those are fantastic articles on John Miles' site. Well worth the reading.
Thanks.

Greg

gjvander

Joey,

I did read the articles, and that kind of peaked my interest in the AWD system. I am replacing my G64 transmission and have been taking an inside look at the diffs. (I can post some pics if people are interested in seeing the inner workings). I have some experience modifying the Zexel Torsen (Torque sensing diff. Does Porsche use it?), but the C4's is really ultra trick.

I am wondering what kind of control they gave the driver on the 959 - I think they had cockpit controls for the diffs. on some of the racing versions.

I plan to do some DE days next year at Mid-Ohio. I have some experience on the track on a Yamaha TZ250 with AMA Pro. Would like to do a few laps with and without the active system to see how it compares. Like I said before, I have scared myself a few times in the C4 with some violent understeer when the rear diff. is locked up.

Regards,

Geo

joey bagadonuts

Glad you guys found the articles of value. I have employed the techniques described at the track and autocross and once you understand how the system reacts to braking and throttle applications, the car really comes alive. Bigger tires (and 8" wheels) up front also make a huge difference in handling and eliminate much of the understeer. Tire pressures will also keep the setup neutral; I like HOT pressures pretty much the same at all four corners. Don't be misled--these cars can be a blast to drive as long as you know how to bring out the best in them.
Enjoy and be safe!

the 964 KID

I've never driven a 993, but i always heard they don't feel quite as nice on the road (although I understand there are some advantages to them - lower maintainance for example) - I know this is a 964 forum, but i'm just curious as to the impressions of those of you have. I do think the 964 was the last of the classic looking 911s, which was what led me to seek one in the first place. I don't wanna spark a war or anything... just curious.

Ken.

Speedraser

Adrian,

Why is the 964's AWD a "far superior traction system" to the 993's?

User 4621

I have a 993 C4S, but I have also driven a 964 C4 and have always had a soft spot for them because it was the first 911 I ever drove.

They feel very different to me. The 964 system is a "true" awd. The car feels like it has awd, more like, say, an Audi. I would guess that it is far superior to a 993 in the snow, although I have not tried it myself. The 993 system is more of a "standby" awd, in that it doesn't feel very different from the 2wd car under anything but the most extreme conditions. It is much simpler, lighter, and more transparent. It only gives about 5% to the front under normal conditions, and only because the tires are carefully chosen to have a slight difference in circumference.

The 993 C4 (and C2) understeers when stock, but it's pretty easy to dial that out with suspension tweaks and managing tire pressure. I have PSS-9's and my car is set up for mild oversteer. I do not know what is or can be done to improve the 964 C4 tendency to understeer, perhaps someone can chime in.

Hope this helps,

Adrian

Dear Todd,
Without getting technical the 993 viscous coupling is slower to respond than the constantly monitored 964 system. Mechanical reaction versus hydraulic power.
A mechanical limited slip diff is basically useless in the snow or on the ice whereas the differential locking system of the 964 ensures almost instant transfer of drive to the wheel which is not spinning. Again mechanical versus hydraulic.
The 964 system operates at much lower changes of load when compared to the 993 mechanical system (more sensitive).
The final point is that the 964 always has under normal conditions 31% of its drive up front. This provides superior front wheel traction as compared to 5%.
The disadvantages of course are weight and a 993 C4 will out accelerate a 964C4 in all circumstances except, heavily contaminated surfaces. The 993C4 is a much better track performer but also remember that the competitive 993s of the 1990s were all RWD versions. There was no 993 C4RS like there was a 964 C4RS. A 964 C4 even qualified for LeMans and raced in many endurance races. Run by the Quattro team.
Ciao,
Adrian
964C4

TB993tt

Adrian
Just how slow is the 993 system in transferring drive to the font ? On my favourite bend I like to power slide out of the bend in 2nd at about 50mph with my LSD holding the back end out - bringing the backend back in again always feels very easy and controlled and I put this down to my trick LSD (I think its 40/65 or similar) but also I presumed some of the drive was transferred to the front effectively pulling me out of the slide. Is this what is happening ?
Thanks