orangecurry

What exactly am I trying to learn?

How it works - I thought the title gave that away.

There is a lot of mostly WRONG info out on the internet which has been reported here, and every unique car responds differently is a rather odd answer.

vincer77

The VC is comprised of rotating plates that are attached to the drive shaft and the differential, but not to each other, and submersed in a viscous fluid. When the driveshaft and the diffential are spinning at the same speed, the fluid is also spinning at the same speed and no torque is transferred. When there is a differential in the rotational speed of the plates, viscous forces from the fluid impart a drag force between the plates. Now torque is being transferred.

see http://www.awd.ee/viscous.html

The 5% torque transfer is due to difference in the rotational speeds between front and rear wheels, as proposed earlier, but the difference is more likely due to gear ratio in the diff as opposed to tire size. The VW article states that this is to help the reaction time of the system but causes premature tire wear.

In a Volvo write-up, there is a description stating that when there is so much slippage that the friction heats the fluid to much, the coupling mechanically locks up. So the 35 or 39% torque transfer is probably the maximum torque transferred through the fluid, and the 50% is the maximum for mechanical lock-up.

see http://www.autoworld.com/news/volvo/awd.htm

I am still a little confused as the Volvo description claims that up to 95% can be transferred to the rears (opposite of our cars).

So, due the 996s use the VC or a Haldex system?

Cheers,

Vince

orangecurry

yes - happy with that

OK but we know on a 993 this is not down to the tyres.The difference in rolling radius of say the 17" wheels is 0.5mm when the tyres are new. As each tyre can lose up to 7mm off of the rolling radius as it wears out, the AWD cannot be dictated by this tiny difference.... so does anyone know details on the diff?

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

So is the 993 also the ZF design, or is the 5% due to a gear ratio in the diff?

Zuffenhausen :O)

According to the technical data in Porsche's own 993 repair manual the drive ratio's front and rear are the same at 3.444:1, so this confirms Streathers data, provided it is as simple as that.

A point of interest also appears in the manual on page 39-205/206, this relates to the testing of the VC, it appears the VC is temperature and speed sensitive(as you'd expect) to quite a narrow margin, now how this relates to the 5% permanent awd transfer I'm not sure as I'm no mechanical engineering expert, but this does yet again point to the possibility of sensitivity to tire profiles and or drag in the system especially as there's no purported difference in drive ratios. Help!!

Porsche's claims no doubt will have been savaged at release if indeed there's little truth regarding the operation of it's permanent AWD system, so there is a valid explanation somewhere amongst the fog of it all.

orangecurry

Good stuff. Thanks for that.

So almost certainly front/rear ratios are equal, tyres are equal (or vary too little and tyre wear would alter the ratio too much), so these are not the source of the 5%.

I have also read somewhere that the percentage of drive to the front INCREASES with increasing speed (but I can't find that info again.... yet)

Zuffenhausen :O)

Thanks Chris, this is looking like getting my vote....................

"Viscous diffs can be tuned to suit a particular application by altering the way it is put together. Plates with different inside and outside diameters, different numbers of plates, different spacing between the plates, the arrangement of the plates, the grade of the viscous fluid, all affect how much slip the diff will allow."

http://www.fastfordmag.co.uk/resourc...80.lowdown.pdf

As the above info alludes to, it's quite possible to design the VC to a Porsche specification in a number of ways, even down to the silicon fluid viscosity, this then takes into account all the other issues such a tire profiles, wear, pressure etc having little effect on the VC, as long as the axle speed difference isn't too radical of course.

bart1

Transfer would likely increase with speed. If the tires are different rolling radii, the RPM difference would grow greater as the RPM increased. Keep in mind that with 17s, the F & R pressures are recommended to be the same, where the 18s are higher in the rear, lower in the front, probably affecting rolling radius.

orangecurry

Thanks all - I did read Chris's explanation, but it seemed to be contrary to Streather (and other quotes) saying that VC (if all things are equal) transmits no power to the front.

I guess that as no-one else has jumped-in with anything from a workshop manual, we'll have to make that assumption, because the different rolling radius-thing doesn't really fly, certainly not to give 5% front to every C4 with whichever wheel-size and whatever tyrewear is on whichever axle.... I'm sure Porsche of the 1990s would have engineered a precise solution.

Thanks Chris.

Robert993TT

Watch four minutes into the video. They compare four-wheel drive on the 993tt and 997tt.

http://www.youtube.com/watch?v=ZxqcE...ayer_embedded#

scott watkins

While in IMHO, I believe the 993 to be the most beautiful car ever built.

The new 997 Turbo is truly an engineering marvel at any price!

Magdaddy

In my 964 C4 vs 993 C4 winter use comparison research, it seems that the 964 system is better. Well that's where the evidence is leaning so far.

There are a few members here who have had the oppourtunity for real world usage, what are your comments.

Is the 993 C4 system a confident, sporting performer, when equipted with dedicated winter tires?

Does the VC get the drive to the fronts enough?

I miss my old Audi(90 coupe Quattro-torsen diff's)so much in the winter. That car was just so much fun, absolutely damn near break the law of physics with Nokians on it in the winter. Will I be happy with a 993 C4?

steve porter

I Think it works like the government ( its only there to spoil you`re FUN)

orangecurry

Thanks for the video link Robert - shame the test driver doesn't state how much power is going to the fronts BEFORE the fluid heats up due to the rears spinning.... it could be 5%, or nothing!