RallyJon

Please pardon my noob questions...

Of the three differentials, I understand that the center has a viscous coupling (like an older Subaru) type lsd. The front diff is open. But is the rear diff an lsd, too?

Does the C4 have a rear lsd as standard, or is it just like the C2, in that you can add a conventional or torque biasing lsd?

Finally, does anyone know the specs for the stock viscous lsd? ??kgf-m/100rpm.

tjhcom

Good question -- my mechanic told me that my C4 does not have the same LSD as the option on the C2 -- and that the viscous coupling provides 'limited slip' functionality! I'm speculative about this, however, and am interested in the (REAL) answer.

Thanks!

RallyJon

A center lsd will only transfer torque from one end of the car to the other. That's OK for good traction conditions, but when there's little available traction, you have only two wheel drive (one front, one rear). In fact, if an AWD car has an open front and rear diff with a center lsd, you can literally park diagonally on a highly crowned road and spin one front and one rear wheel. That arrangement was common on the first generation of AWD cars in the '80s: Toyota Celica Alltrac (viscous center, open front and rear), Audi Quattro (Torsen center, open front and rear), DSM Eclipse/Talon (viscous center, open front and rear).

Anyway, I'm sure there's some device limiting slip between the rear wheels...

mborkow

i am told the the lsds on these cars wore out quickly anyway and that by now (even if it had it originally) the lsd would not be functioning

NotTwinTurboYet

Does someone know the answer to the initial question? (is the rear diff a LSD on a C4?).

Thank you!

RallyJon

From further research I've read that the 993 C4 was the first use of the ABS controlled faux-differential traction control called "ABD".

According to Peter Morgan: "The new [1995 993] Carrera 4 came with standard ABD applied to all four wheels. When a rear wheel started to spin, a viscous clutch transferred the extra torque automatically to the front wheels. An asymmetric rear axle differential lock applied a 25 per cent locking effect to a spinning rear wheel on acceleration and 40 per cent when on the over-run."

Note that Mr. Morgan seems to have confused cause and effect, which is right in line with Porsche's marketing materials. Irritating.

In any case, if all C4s came with ABD, that strongly suggests that the REAL rear diff is open.

Pretty surprising that no one here knows the real story. Someone must have rebuilt a C4 transmission, right?

kdurg

http://vista.pca.org/stl/c4.htm

RallyJon

I think that was just reprinted from a Porsche brochure. It claims "The center differential measures engine temp, rpm, and power output before deciding how much slip there will be between the front and rear axles." which isn't how a passive VC works at all, but probably incorporates some Porsche sales-speak about how the whole ABD system works.

It does say, "A locking differential in the rear administers the torque split between the rear wheels."

This site says the same, adding the word "conventional" which I assume means a plate-type: "Torque split between the rear wheels is regulated by a conventional rear locking differential"
www.autoworld.com/Carrera4Cabrio.htm

TheOtherEric

All C4 models have rear LSD. And AFAIK, the rear LSD in a C4 is the same as the LSD in a C2 with the LSD option. That's the conventional wisdom here on Rennlist.

InTheAir

New-to-me link, Keith. Thanks!

RallyJon

Here's a surprising additional piece of information: "To make the viscous-coupling always engaged the front wheels, the rear tyres were made marginally smaller in diameter, enhance established a small speed difference between the drive shafts to front and rear. With the speed difference, the viscous liquid normally transferred 5-15% torque to the front axle..."

One could affect the torque split just by increasing the difference between front and rear tire diameters.

Edward

Yep, but only up to a point. Given too wide a difference then the ABS computer will protest. There's a bit of wiggle room there, but not much.

Edward

RallyJon

Well, hopefully Adrian Streather's new 993 book will have the 993 equivalent of the amazing writeup he did for 964 C4s:
http://www.adrianstreather.com/docs/...overviewR2.pdf

Another quote: "the link between gearbox and front wheels was revised with drive now carried by a viscous coupling. For it to carry any drive to the front wheels though, meant they had to rotate slower than the rears, meaning they required a slightly larger rolling radius. The net result of all this was between five and 15 per cent of drive passing through the front wheels under normal circumstances, but that figure could climb to nearly 100 per cent if the rear wheels were experiencing no grip at all. That seemed an unlikely eventuality when you consider the amount of attention rear-end traction was given. Not only did the 993 have a 25/40 differential (under load/overrun), it was also equipped with Automatically Braked Differential (ABD). The latter’s inclusion meant that should the diff not be able to cope with the amount of slippage, it would step in and, via the ABS system, apply the relevant brake to improve grip. It also helped to control lift-off oversteer into the bargain."
from here: http://porsche964.co.uk/rev_rep/gt/GT_fourplayeb.pdf

RallyJon

Here's a thought: Using 225/45-17 front tires instead of 205/50-17 would cause a dead zone in the center diff takeup since they are slightly smaller than the rears rather than slightly larger. The VC limits slip whenever there is a differential in axle speeds F to R. With 255/40-17 rears slightly smaller than the fronts, rear wheelspin will increase the torque transfer. But, with the rears slightly larger than the fronts, wheelspin will take the VC through a brief period where it's transferring no torque at all before it begins to slip in the other direction and starts transferring torque again.

I wonder if that phenomenon is perceptible to the driver?

chris walrod

Percentage output from a VC device is generally heat controlled. More heat equals more VC output.

When the rears lose grip, this spins the VC internally and generates heat (rpm difference between inner and outer housing). As a result, more 'coupling' towards the front. And, that front drive, in really slippery conditions, is fundamentally one wheel drive. A typical street car LSD at the front would make the car wicked tricky to drive. Understeer, especially on-power.

Its too bad there isnt budget for road cars to enjoy the benefits of negative preload, ramp type differentials.