ca993twin

My understanding is that the viscous fluid warms up VERY FAST under wheel spin conditions... way less then a second, if I recall correctly.

LVDell

So then there are 2 situations where the front diff increases the percentage of power?

1. When heated up
2. When wheels slip in the rear (front doesn't need to be heated up)

ca993twin

I think there is only one condition where the percentage of power to the front increases... when the fluid heats up. This happens gradually as straight line speed increases (because the fluid gets warmer due to the basic offset that yields a 5% transfer), or VERY QUICKLY when the rear tires spin (rapidly increasing the temp of the fluid).

<edit> I was already wrong once... i assumed that the 996 coupler was at the nose of the tranny, like the 993, but was informed that you guys have the coupler attached to the front diff... prolly for weight distribution benefits. But, AFAIK, there is a single coupler, regardless of its position, and it works pretty much the same. Its the coupler that will "connect" the rear drive to the front drive with varying degrees of connectivity, based on the heat of the fluid. IMHO.

doubleurx

I can't speak for 1, because this is the first I have heard of more than 5% of the power being transferred under static load. I am surprised that the fuid will thicken with heat as generally that is the opposite of what happens with petroleum based fluids. Number 2 is absolutely correct with any AWD system.

LVDell

Ah, that makes more sense. Just trying to grasp how fast the fluid heats up. Sounds like the initial part of a slip in DD is where PSM kicks in and the AWD follows shortly (less than 1 sec) after.

It also sounds that when taking the car to the track the max increase in the front is achieved rather quickly so essential the car becomes a full AWD car for the entire session.

Thanks for the education!

blockhed

it all seems to be a mystery and until we have a Porsche Tech who actually knows how it works, it all seems to be a game of assumptions. None of us can tell when 5% kick in or 10% we're just assuming.
I believe its a rwd car with front wheel drive help when in panic situation. I wish we could get someone on here who built these or engineered them to get a full and final explenation once and for all.

ca993twin

doubleurx wrote: "I am surprised that the fuid will thicken with heat as generally that is the opposite of what happens with petroleum based fluids."

Its not petroleum-based. Its silicon fluid, and it has the property of thickening when warmed. Like those radiator fan gizmos that would idle the fan when the radiator was cool, but would get solid as the radiator warmed.

ca993twin

blokhed wrote: "it all seems to be a mystery and until we have a Porsche Tech who actually knows how it works, it all seems to be a game of assumptions."

Yes, very true. But I think we can kick this around and with enough input can come to some reasonable conclusions, until some Porsche expert chimes in. But the bottom line is that with any viscous coupler, the heat of the fluid is critical, and the heat is determined by the difference in front/rear rotational speeds. So that is why it is critical to be very careful when selecting tire sizes.

Land Jet

This is where I got my information from as well. The book says that at 155mph there was a 30% transfer to the front. It also says that if both rear wheels are spinning, the maximum drive transfer forwards is limited by the viscous coupling design to 40%.

The viscous drive contains a unique silicon fluid that is extremely sensitive to heat. Rotating plates in the fluid generate heat which causes the fluid to harden and that causes the plates to lock together and transfer torque to the wheels. The ZF design ensures that the friction generated within the coupling under normal driving conditions is sufficient to provide 5% permanent front drive which is achieved approximately 45 seconds after driving off.

doubleurx

Thank you. I always knew what the viscous coupling provided, but not exactly how it worked.

timmhaines

Gt3 wheels and adman neiva 08's.
225/40. 18. 295/30. 18
25 inch 831 revolution
Have just installed 5 set on car Oem tires only lasted 4000 miles last set saw 14000 and I have x74 x50 options new tires are 08 model last set where 07
The new ones are so much quoted and grip seems better
Highly recommend them

Sent from mobile app

Signalmtb

here's good online calculator that also tells you which tires are comparable sizes with +/- % tolerances
https://tiresize.com/calculator/

Freddie Two Bs

I think the OP must have figured things out at some point in the last 6 years...

Signalmtb

I certainly hope so. And for all the other forum members and web searchers who read these threads for related time-independent information who may be experiencing a similar situation, hope that calculator helps.

"02996ttx50

well put.

pertinent and valuable new sources of information are neither date nor time sensitive.