CalvinC4S

I agree that is not correct info on tires.

Put a call into the service manager (long time friend) at the local Porsche dealer to run back to there library to pull the 993 4 system intro book out.

orangecurry

Thanks Calvin and Chris.

Chris - does that mean the Streather comments I quoted are NOT talking about the 993 system?

Looking at those quotes again (from the 996 book apparently) does it mean that if BOTH rear wheels are spinning in say snow on a 993, is any power transmitted to the front wheels?

I repeat the quotes here as we are now on page 2......

According to Adrian Streather's "Porsche 996: The Essential Companion" (pg. 257)

"One major disadvantage of any viscous coupling system is that the vehicle actually has to be moving or both rear wheels spinning before drive can be transferred forward. The advantage that the 964 Carrera 4 still has over its later rivals is that there is always a minimum of 31% front wheel drive available. This was excellent if stuck in heavy snow or in sand at the beach. A viscous coupling cannot provide this.".

According to Streather (Pg. 259)

"In the standard (read original 993) viscous coupling the fluid hardens at 160C. When both sets of plates are spinning at the same speed and the friction level insufficient to bring the the fluid to hardening temperature, it will remain in a liquid state and 100% of the drive is directed to the rear wheels and nothing to the front wheels....

The ZF design (read new for 996) ensured that the friction generated within the coupling under normal driving conditions is sufficient to exceed 160C, causing the silicon fluid to harden around both sets of plates to provide a small percentage (5%) of permanent front drive. In reality this takes approximately 45 seconds to develop after driving off. "

CalvinC4S

"does it mean that if BOTH rear wheels are spinning in say snow on a 993, is any power transmitted to the front wheels?"


I know you asked Chris but 100% is transfered to the front.

ETA: Also to expand on that to get away from the snow/rain/dirt C4 stigma, if you are going into a over steer the front will pull the rear back into trail.

ReinerFink

I can spin the rears of my C4 on muddy grass (in the outside of turn 6 in Portland) and not have the fronts move at all. I did the test for my Center diff and it passed.

IMHO, the AWD system is there to help folks who lift in turns in 911's. If you learn not to do this, then you don't need it.

-reiner

CalvinC4S

In a oversteer the 100% rule comes into effect. you are accelerating beyond the tires capability while holding a corner. The front tires still have somthing to offer as they are holding the corner better then the rear.

The 4 balances the load and now brings the front to the full 100% capacity of the tires to match the rear.

Enter under steer...

Stealth 993

I thought the AWD system did 3-39% to the fronts?

As the fluid heats up (from slip) torque is transfered to the fronts. It takes miliseconds to do this. Supposedly you can wear out the diff & make it a 2wd car. I know driving in the snow & ice last year, the fronts will grip when the backs slips.

orangecurry

Thanks for the replies.

This is what I mean - everyone has a slightly different view. Does anyone have something concrete on exactly how it works, when the fronts cut-in, when they DON'T, what the percentages are...... etc etc.

Cheers.

Stealth 993

Unlike the previous AWD, the new system diverted power to the front only when signaled by sensors monitoring rpm, temperature, and wheel speed (part of the ABD and ABS electronics), regulated by the rear differential. Thus, if both rear wheels began spinning, as on ice, up to 39 percent of available power was sent forward. In most other conditions the front/rear torque split could vary between 5/95 and 25/75 percent.
The real beauty of the 993's AWD was that it weighed just 111 pounds, which trimmed C4 curb weight by a substantial 165 pounds. The new design also cut frictional losses by a sizable 50 percent. Even so, the new C4 remained a bit thirstier than the rear-drive Carrera and landed in the "guzzler" category, though only by one mpg.
Reviewers had nothing but raves for all the rejuvenated Porsche 911s. Let's start with the rear-driver, piloted for Car and Driver by Briton Peter Robinson. He noted that "no 911 has ever handled as well nor been as easy to drive. But do not think for a moment that this more friendly temperament has compromised driver pleasure...The [steering] wheel still transmits useful messages with typical 911 clarity as it writhes gently in your fingers, so that you feel there's almost no hydraulic help. Yet any artificial messages are eliminated, and so is kickback.
"The result combines razor-sharp turn-in response and sensitivity with staggering high-speed stability and a newfound sense of security. The contrived understeer engineered into the old Carrera 2 and 4 models has been replaced by a new agility...the greater roadholding and reduced understeer requiring far less steering effort...At its limit, the 911's not quite as serene as an Acura NSX, but it is far more predictable than a Ferrari 348" -- high praise considering those cars' pure mid-engine layout.
As for the Carrera 4, Road & Track's Joe Rusz tested it back-to-back with the rear-driver at Southern California's demanding Willow Springs Raceway, and his comments are interesting: "Without a doubt, the Carrera 4 is quicker, getting off the line with a fair amount of wheelspin but without the severe axle judder that seems to dog the Carrera...In fast sweepers the Carrera 4 feels more stable, its excellent awd enabling it to toe the line perfectly. The Carrera 2, on the other hand, gets very nervous and tends to oversteer at the slightest hint of throttle lift-off."
But Rusz found that "in slow, 90-degree corners, the C4...understeers, so much so that power-on exits often found the steering wheel reaching full lock. What's more, throttle liftoff only slightly reduces understeer -- in contrast to the C2 in which a reduction of power brings the tail out slightly and allows the front tires to bite. Why does the Carrera 4 understeer? Because when the front wheels are driven, less of their tires' grip is available for lateral traction. And with the rear tires getting such excellent traction, they propel the car forward, causing the lightly loaded fronts to lose their grip."


Whatever their flaws on the track, the new Porsche 911s were brilliant on the road. "[I]t seems Porsche's engineers have finally purged [the] flaws," Robinson declared, "and what's left is the sharpest 911 yet. And the most friendly." Said Rusz: "If I were a Porsche purist who knows how to drive...and who fully understands the meaning of rear-engine weight bias, the Carrera would get my vote. It's lively, responsive, challenging...But if I were the same fella and realized that there's a lot to be said for all-wheel-drive stability and all-weather traction, the new Carrera 4 would get my nod."
So as ever with Porsche 911s, "you pays your money and takes your choice." Only now you didn't pay so much. At just $100 shy of $60,000, the '95 U.S. Carrera coupe cost $5,000 less than its '94 counterpart, and the $65,900 C4 was a whopping $12,000 more affordable. Corresponding Cabriolet prices were $65,900 and $74,200. (Tiptronic S added $3,150 to either rear-drive model.) In convincing fashion, Porsche had made good on its promise of a better 911 for less money.
We're not forgetting acceleration, but gains here were relatively less impressive. R&T's rear-drive coupe matched the factory's 5.2-second 0-60 claim, which was a tick quicker than the departed RS America. The magazine's Carrera 4 clocked 5.7 seconds 0-60, 0.3-second off the official time, though about what you'd expect given a 111-pound weight penalty. Car and Driver got 4.9 seconds with a C4 coupe in the same sprint, which looked a shade optimistic.
As for skidpad cornering, R&T's Carrera 4 turned 0.85g versus 0.90 for C/D's car -- a big gap likely explained by different test sites. Porsche itself claimed "in excess of 1.0g on high-traction surfaces" for all 993s, which seemed a bit boastful in view of the above.
Overall, though, the 993-series was a remarkable advance on the 964 models, especially for being developed during one of the darkest periods in Porsche history.

http://auto.howstuffworks.com/porsche-911-history28.htm

CalvinC4S

"Whatever their flaws on the track, the new Porsche 911s were brilliant on the road."

top gear, march 1996

Dry course: C2's lap time is 53.43 sec.,C4 did it in 53.01 sec.
Wet course: C2 in 25.27 sec.,C4 did it in 25.05 sec.

The magazine quoted: "The porsche carrera 2 proved to be a very close match for its 4wd brother in every department, including the specially wetted surface of the steering pad, though the 4 wheel drive model maintained a clear edge in handling and wet grip."

It concluded: "The porsche C4 is a clear favorite. There is little to choose between the 2 911s but in handling and grip as well as our timed laps the C4 has the edge. The message is clear. Four wheel drive costs more- but for a keen driver, it's a price worth paying for the added reassurance it can bring."

orangecurry

Thanks Stealth - that article certainly seems to be bringing it all together...

I am a bit worried by the statement on the previous 'page' though...
While manual-shift 993s came with the usual limited-slip rear differential, ABD was optional for Carreras and standard on the Carrera 4.


LSD wasn't standard on the Carrera.

http://auto.howstuffworks.com/porsche-911-history27.htm

Hi_Fi_Guy

This is one I had to learn the hard way. Before starting my search I read 3 different books that state LSD and ABD are STANDARD on all US 993's. So the first Carrera I'm looking at the owner is explaining to me why his car is worth a premium because it has LSD and ABD and I'm thinking, "This guy must think I'm stupid. Doesn't he know all 993's have that stuff?".

Yes, I was the one who was stupid!

Zuffenhausen :O)

Here's a quote from Porsche themselves.

"In comparison with a viscous multi-plate clutch, where clutch regulation is not initiated until there is a difference in speed between the front and rear axles"

http://www.porsche.com/microsite/tec...lectedVariant=

Although the above quote is related to the modern Carrera's the point of how a VC works cannot be mistaken, also doesn't the traction control system ABD, and ABS system work independently from the VC? I smell a rat with the above data, it simply doesn't ring true.

Also in Porsche very own advertising brochure for the 993 c4 it quotes torque transfer up to a maximum of 50%, shown below.

vincer77

Yes, I believe that the VC is a complete mechanical device, so no electronic control. The ABD would have an effect on the rear wheel spin which would then affect the operation of the VC. If the difference in rotational speed between the front and rear tires tires is what splits the torque, then tire size changes may affect handling.

For sake of arguement, using the tire data for NB cars presented earlier, the fronts have smaller circumference than rears. Going to 18s, the rears become smaller than the fronts. How does this effect the VC operation? Or is it simply the difference in sizes all that is needed to properly distribute the torque?

Cheers,

Vince

orangecurry

OK... so the Porsche-sourced info casts doubt on the 'howstuffworks' articles accuracy...... nearly back to square one.

Thanks Zuffenhausen - great find.

But we still don't know how it works, though we do now know the 5%-35% then 50%... why did they phrase it like that? Under what circumstances does it go to 50%... when BOTH rear wheels are slipping?

cgfen

No

every unique car responds differently.
there has been a lot of mostly correct info here regarding general design, (and a bit of marketing hype )
We are merely reporters of what we've read and experienced.
We are not the design engineers.
what exactly are you trying to learn?

I doubt you'll get a better explanation then Adrian S. provides in his book.

Cheers