orangecurry

I have read about the 4wd system on the 993, but I remain confused.

Can anyone sum it up for me please? Or point me to the FAQ?

How is 5% of power transmitted to the front in normal driving?

Is it true that MORE power is transmitted to the front as the cars SPEED increases? If so, how? (if no wheels are slipping)

I have read:
- v/c means that no power is transmitted to the front if the rears are both spinning?

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

When all 4 wheels are rolling at the same speed the fluid in the VC stays cool. If the rear starts turning faster (think burn out) it heats this fluid up by friction and starts to lock up the front drive train.

put a 993 4S or TT on a lift and start it up, it is almost instant.

orangecurry

but if the rears are NOT slipping, why is there 5% at the front?

CalvinC4S

I understand it to be friction. Don't take this as fact as I have never seen one apart and and dissmiss it's function as FM.

The fronts will turn by hand on a lift as well so this would lead me to there is more then 0% transfer at room temp.

Edit to add, hydrolic friction not like clutch discs untill at temp.

orangecurry

but not 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

I'll look back through the factory info I have. I always understtod it is 5%, may be wrong.

orangecurry

where's the s****** smilie?

I'm sure that IS what happens (on a lift) - I just would like to understand HOW it works, and DOES the percentage split to the front increase with speed (no wheels slipping) again we could assume due to increased friction.... ?

I don't like assume

CalvinC4S

Ok, hold tight... I have a glossy white "Teknic" book in the garage some where on this.



Edit:

No luck finding the info. There is a factory service book on this system that goes into great detail on the subject. I found some genral data but nothing beyond what a basic viscus coupling is.

"DOES the percentage split to the front increase with speed" No the system is based off speed diffrences ie.. back side faster then front side. With 4 wheels turning at the same speed no matter what the speed the VC wouldn't tighten up. A lot like a LSD for example.

The 5% is built in from hydrolic drag/friction from the viscosity of the oil/discs. <---- ASSumtion so take it for what it's worth

Hope someone can jump in that has the TecniK book on this with facts as mine is burried.

If you want to complicate it further, bring in what the LSD and ABS do for the AWD system.

scott watkins

http://www.autozine.org/911/911_9.htm

orangecurry

Thanks Scott - I've seen this and I'm afraid it makes no sense....

'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, which was much less than the 964’s system'

...because the tyres ARE the same diameter, or rather they are 0.5mm smaller.

205/50/17 = 318.4 mm rolling radius
255/40/17 = 317.9

Similar on 18s....

225/40/18 = 318.6
265/35/18 = 321.35 (still only a 0.86% difference, and more importantly LARGER)
and WB
285/30/18 = 314.1 (1.41%)

As Bogart said, 'I must have been misinformed'

CalvinC4S

Good stuff, This is new to me and would be the writers data on the 5%..

"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. "

CalvinC4S

2 minutes apart.. lol

orangecurry

Thanks Scott - I've seen this and I'm afraid it makes no sense....

'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, which was much less than the 964’s system'

...because the tyres ARE the same diameter, or rather they are only 0.5mm smaller in radius, 1mm in diameter.

205/50/17 = 318.4 mm rolling radius
255/40/17 = 317.9

i.e. a very good match.

Same on 18s....

225/40/18 = 318.6
265/35/18 = 321.35 (still only a 0.86% difference)
and WB
285/30/18 = 314.1 (1.41%)

As Bogart said, 'I must have been misinformed'

orangecurry

apologies - I can't delete or edit existing posts for some reason?

Anyway my point is that the wheels are the same diameter, or are so close in diameter that normal tyre-wear differences would have a pronounced effect on the awd effectiveness, which seems rather unlikely

chris walrod

The static amount of front drive (as a percentage) the 993 AWD has is essentially viscous drag through the VC. A VC can never 'disconnect the input from the output 100%. This is why the 5% or so front drive without any 'action'