Headlights and AWD...
10-30-2014, 11:09 PM
#31
Rennlist Member
Macster appears to be of the opinion that any slippage within the vc will kill it, when in fact it is slippage which makes it function by design. He was shown a cooked vc from a vehicle which was towed with the front wheels held stationary and the rears driven by the tow vehicle the distance not stated but could have been many miles and at speeds up to 65mph who knows. He has then jumped to the conclusion that any turning of the rears without the fronts turning results in instant destruction of the vc. 993GT lives in a snow belt as do I and the forecast for Sunday is 20>30cm of white stuff. I will try the Turbo as I said I would but I know what it will do thanks to 993GT's description of my electro mechanical clutch which locks up 100% rear and front on take off and is the same set up as in my 2010 RX350 only reversed ie front driver with the rear clutched in on a detected difference of rotation speed front to rear. One final point on his concern about the wheels all turning freely under power on the hoist, from the owners manual "Balancing the wheels on the vehicle" >> During finish balancing of the wheels, the vehicle must be hoisted and all wheels able to rotate freely.
Last edited by paul29; 10-31-2014 at 12:11 AM.
10-31-2014, 11:23 PM
#32
Race Car
Exactly. This statement is in contention:
1) Does anyone think it's true that if the rear wheels start to spin on ice with the car stationary and the front wheels are on dry pavement that there will not be enough power transferred to move the car at all, not even a little? If it does, after a few seconds the car will be out of the ice. Is that all it takes to fry the front diff? But wait, see 2)
2) The rear wheels will not spin anyway because the PSM will stop them, so even if there will not be enough power transferred to the front wheels in order to move the car (say if you are on a 40% incline or something), you just won't go anywhere
Another quote:
How is extended rear wheel spinning/fronts not moving possible without PSM intervening? Yeah, if the car is being towed on two wheels, but not when you are inside trying to get up an incline on the car's own power.
So, there is NO WAY you can damage your front diff in that scenario.
Dan
If you ever encounter a situation in which the front wheels/tires are on dry pavement, such as might be the case when pulling into a garage with a sloped driveway, and the front tires get on the dry garage pavement while the rear tres remain in the snow and spin as you try to make the last few feet into the garage, do not continue with this. This will cause the front diff/viscous coupling to possibly get damaged.
2) The rear wheels will not spin anyway because the PSM will stop them, so even if there will not be enough power transferred to the front wheels in order to move the car (say if you are on a 40% incline or something), you just won't go anywhere
Another quote:
as said, slipping for a short period is fine, extended rear wheel spinning/fronts not moving, and vise-versa, is what cooks the VC.
The VC needs some speed variation for it to transfer power, but big speed variation for an extended time will overheat and kill it
Use of PSM when stuck will lead you nowhere, but at the same time letting the rear-wheels spin excessively will kill the VC....993/6/early7 VC system is a 'handling' aid and not also a true traction device like 964, 997TT, 997.2+
The VC needs some speed variation for it to transfer power, but big speed variation for an extended time will overheat and kill it
Use of PSM when stuck will lead you nowhere, but at the same time letting the rear-wheels spin excessively will kill the VC....993/6/early7 VC system is a 'handling' aid and not also a true traction device like 964, 997TT, 997.2+
So, there is NO WAY you can damage your front diff in that scenario.
Dan
11-01-2014, 10:44 AM
#33
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where is anyone going to find a 40% incline?! lol. max for a public road is about an 8% grade with *some* rural roads allowed up to 12.5 MAX!
11-01-2014, 01:51 PM
#34
Because we are talking PSM off....
With PSM on, there is virtually 0 power sent to the front...
There are many scenarios where the front end has traction and rear doesn't yet the front won't pull the car out....here's the classic 996 on grass vid...
THIS SYSTEM WORKS BY HAVING A SPEED VARIATION IN FRONT-REAR DRIVES, NO SPINNING=NO POWER TRANSFER, EXCESSIVE SPINNING=DEAD VC
With PSM on, there is virtually 0 power sent to the front...
There are many scenarios where the front end has traction and rear doesn't yet the front won't pull the car out....here's the classic 996 on grass vid...
Exactly. This statement is in contention:
Another quote:
How is extended rear wheel spinning/fronts not moving possible without PSM intervening? Yeah, if the car is being towed on two wheels, but not when you are inside trying to get up an incline on the car's own power.
So, there is NO WAY you can damage your front diff in that scenario.
Dan
Another quote:
How is extended rear wheel spinning/fronts not moving possible without PSM intervening? Yeah, if the car is being towed on two wheels, but not when you are inside trying to get up an incline on the car's own power.
So, there is NO WAY you can damage your front diff in that scenario.
Dan
11-02-2014, 01:56 PM
#35
Race Director
Thread Starter
Macster appears to be of the opinion that any slippage within the vc will kill it, when in fact it is slippage which makes it function by design. He was shown a cooked vc from a vehicle which was towed with the front wheels held stationary and the rears driven by the tow vehicle the distance not stated but could have been many miles and at speeds up to 65mph who knows. He has then jumped to the conclusion that any turning of the rears without the fronts turning results in instant destruction of the vc. 993GT lives in a snow belt as do I and the forecast for Sunday is 20>30cm of white stuff. I will try the Turbo as I said I would but I know what it will do thanks to 993GT's description of my electro mechanical clutch which locks up 100% rear and front on take off and is the same set up as in my 2010 RX350 only reversed ie front driver with the rear clutched in on a detected difference of rotation speed front to rear. One final point on his concern about the wheels all turning freely under power on the hoist, from the owners manual "Balancing the wheels on the vehicle" >> During finish balancing of the wheels, the vehicle must be hoisted and all wheels able to rotate freely.
Porsche techs pick up things when in training with other techs from other areas of the country or other regions of the world. The word is this VC issue comes up in cars that spend time up north or in areas where there considerable snow.
I thought Turbo owners would be interested. I myself had believed (assumed) the AWD could help a car that was stuck but I'm glad to have learned it won't and if I ever find myself in such a position I'll avoid spinning the rear tires.
If you feel my info is bogus then ignore it.
11-03-2014, 10:14 AM
#36
Race Car
I was unaware that we were talking PSM off... I did not see that anywhere on the first page of this thread, sorry.
Referencing the above video, is it really true that so little power is transferred to the front wheels that they don't even start to spin in such low traction/low speed conditions? Or could it be that that specific car already had a damaged front diff from before, considering how that guy was treating the car? I guess I will just have to try this out for myself when I get a chance.
Dan
Referencing the above video, is it really true that so little power is transferred to the front wheels that they don't even start to spin in such low traction/low speed conditions? Or could it be that that specific car already had a damaged front diff from before, considering how that guy was treating the car? I guess I will just have to try this out for myself when I get a chance.
Dan
11-03-2014, 12:09 PM
#37
Rennlist Member
Had the opportunity to try a 2013 turbo with LSD and OEM tires on a sloped driveway with front tires on concrete pavers and rear tires on packed 4" of fresh snow, temp 35F. With the PSM on, the car moved forward the with rear brakes clicking . Next tried with PSM off and a little more accelerator. The rear tires spun and the right front actually spun on the concrete pavers so lots of power being transferred to the front. So it appears this model of Turbo will easily move up a slope with just the front having traction. This does not make this car a good driver in snow as it is just too low.
11-03-2014, 12:22 PM
#38
Had the opportunity to try a 2013 turbo with LSD and OEM tires on a sloped driveway with front tires on concrete pavers and rear tires on packed 4" of fresh snow, temp 35F. With the PSM on, the car moved forward the with rear brakes clicking . Next tried with PSM off and a little more accelerator. The rear tires spun and the right front actually spun on the concrete pavers so lots of power being transferred to the front. So it appears this model of Turbo will easily move up a slope with just the front having traction. This does not make this car a good driver in snow as it is just too low.
11-03-2014, 12:45 PM
#39
Just fry it and youll have an excuse to remove it.
but seriously this wont be an issue unless its abused. Getting stuck and trying to unstuck yourself for a brief period isnt abusive to me.
granted you arent in 2nd or 3rd gear with rear wheels spinning for a somewhat lengthy period of time.
the pulling into the garage scenario should not damage it / unless you sit there for 2 mins with your rear spinning.
The system works by having the speed variance.
and a used STi is usually cheaper than even the most ratty 996.
but seriously this wont be an issue unless its abused. Getting stuck and trying to unstuck yourself for a brief period isnt abusive to me.
granted you arent in 2nd or 3rd gear with rear wheels spinning for a somewhat lengthy period of time.
the pulling into the garage scenario should not damage it / unless you sit there for 2 mins with your rear spinning.
The system works by having the speed variance.
and a used STi is usually cheaper than even the most ratty 996.
11-04-2014, 04:28 PM
#40
I'd imagine the VC would "burn out" at different rates depending on specific conditions, I don't think you could put a specific time frame of the front wheels locked and rears spinning that would kill the VC. Too many factors at play. Here's some more info that helped me understand how the 996 automatic AWD system works and has also made me happy that I have Quattro for the winter instead of the 996's setup:
996 AWD
Automatic AWD
Viscous Coupling
996 AWD
Automatic AWD
Viscous Coupling
11-04-2014, 11:43 PM
#41
I was unaware that we were talking PSM off... I did not see that anywhere on the first page of this thread, sorry.
Referencing the above video, is it really true that so little power is transferred to the front wheels that they don't even start to spin in such low traction/low speed conditions?
Dan
Referencing the above video, is it really true that so little power is transferred to the front wheels that they don't even start to spin in such low traction/low speed conditions?
Dan
11-22-2014, 11:42 PM
#42
Exactly what should happen on a 996 awd with an intact VC.