996 TURBO AWD Question
11-10-2014, 04:54 PM
#1
996 TURBO AWD Question
I have been reading about how the front differential can get damaged from tire size difference, but have a fundamental question about the system if differing wheel speeds affect the operating soundness of the system. When the car is driven in the snow the front and rear wheels are no longer spinning at the same rate as they would on dry pavement. Would it be safe to say that driving in any weather other than dry will wear out the awd system? How about when the PSM kicks in does that hurt the differential too? How can Porsche create such a poor system in its top end car? Wouldn't the center differential compensate for the differences in front and rear wheel speeds? Or is the center a fixed differential. I thought that the center was the viscous differential with the fluid that got thicker as it heated increasing torque to the front wheels. Or is my thinking flawed somewhere.
11-10-2014, 06:39 PM
#2
Rennlist Member
Its flawed, as the rear wheels spin the front viscous clutch becomes more solidified and has more of a driving force. If both front and rear tires are spinning then there is no traction and less heat will build up because there will be less resistive force on the clutch system.
11-10-2014, 07:16 PM
#4
Three Wheelin'
The awd system can only transfer a specific amount of the available torque. It never truly locks up. So a difference in front to rear rpm, though both turning, is not detrimental. That said locking the front wheels, say on a tow truck, and dragging the back, over a long distance, could damage the viscous coupling and or front differential.
11-10-2014, 07:53 PM
#5
RL Community Team
Rennlist Member
Rennlist Member
What DaveCarrera4 wrote sums it up. Vicious coupling damage will only occur if the tire sizes are massively mismatched or if the front or rear wheels are locked in place while their opposites are allowed to spin at extreme speeds for an extended period of time.
There is a huge variety of tire brands and tire types that match up front/rear just fine. Stop worrying and enjoy your Turbo.
There is a huge variety of tire brands and tire types that match up front/rear just fine. Stop worrying and enjoy your Turbo.
11-10-2014, 08:44 PM
#6
Don't get me wrong, I'm enjoying it, just trying to understand how the awd works. There are a lot of 997 wheel sets for sale and I'm always tempted to buy a set but the tires obviously don't work.
11-10-2014, 08:51 PM
#7
Trending Topics
11-11-2014, 12:44 AM
#10
sorry if my reply was blunt, seems to be a ton a AWD/VC threads lately
11-11-2014, 11:41 AM
#11
RL Community Team
Rennlist Member
Rennlist Member
The wheels themselves will work fine but you are correct that factory 997TT tires sizes will not work well on a 996TT. But there are many tires in 996TT correct sizes you can have mounted on the 997TT wheels that will work perfectly.
Last edited by Carlo_Carrera; 11-11-2014 at 01:31 PM.
11-11-2014, 11:43 AM
#12
Race Director
FWIW, I drove my 03 Turbo in the snow, for about 40 miles, with summer tires on the car and with no ill effects to the AWD. I would not recommend others attempt this, not out of concern for the diff but simply because summer tires just don't work very well in the snow. I was lucky the snow was not on ice and that I had a big rig (empty) going slow to follow which helped make a path for my car's tires.
I might add a few times during the drive I gave the engine extra throttle enough to break the rear tires loose wanting to "experience" the expected AWD feature and feeling the front tires pull the car forward but I never felt a thing.
Despite the fact the AWD system doesn't render the car a mountain goat, or an all terrain vehicle, the AWD system is a very good system, and does what it is intended to do and does it reliably.
It is intended to direct anywhere from 5% to 40% of the engine's torque to the front wheels. The amount is based upon the vehicle's speed. At low speeds (I do not know the threshold) just 5% torque is transmitted to the front wheels. The max of 40% is directed to the front wheels (based on my info) at 150+mph.
The AWD system is not intended to be something like IIRC used in the VW Golf R32 (at one time I considered buying one of these) which can direct 100% torque to the axle with the most grip.
IOW, the Turbo AWD system is not a system intended for use in marginal traction conditions where the front and rear tires can turn at excessively different speeds, over and above the speed difference that arises from the differences between rear and front tire diameters.
The car can of course be used in marginal traction conditions but the expectation is the car will be fitted with proper tires -- snow tires -- so as the car moves forward the front tires roll and turn at the same speed as they would on dry pavement.
The "center differential" is a viscous fluid coupling. It is a housing packed with discs. Every other disc is connected to and turns with the rear diff. The other set of discs is connected to and turns with the front diff. The housing is also filled with a special silicon based fluid (GKN made by a company in the UK is my info).
Might also mention that the coupling does allow the two axles to turn at different speeds which occurs during a turn.
As the car is driven and the two sets of plates spin at different speeds, courtesy of the tire diameter differences between the 2 axles, this creates fluid friction which heats up the fluid. This fluid as it becomes hotter (IIRC the fluid's working temperature can exceed 160C (320F)) becomes more viscous and and through friction with the rear diff discs transmits torque to the front diff discs. However, the coupling is still a fluid coupling and the two sets of discs never rotate at the same speed.
It is I think impressive to consider that with just a few percentage speed difference the fluid can transmit 40% of the engine's torque. That is some special fluid. It is also impressive that at speeds of over 150mph, in order to transmit that 40% of the engine's torque to the front diff the amount of heat generated by that viscous coupling must be huge and it requires the air flow of 150mph to cool it.
Every time I delve into the details of Porsche cars, and in this case the Turbo, the more impressed I am at the level of technology.
I might add a few times during the drive I gave the engine extra throttle enough to break the rear tires loose wanting to "experience" the expected AWD feature and feeling the front tires pull the car forward but I never felt a thing.
Despite the fact the AWD system doesn't render the car a mountain goat, or an all terrain vehicle, the AWD system is a very good system, and does what it is intended to do and does it reliably.
It is intended to direct anywhere from 5% to 40% of the engine's torque to the front wheels. The amount is based upon the vehicle's speed. At low speeds (I do not know the threshold) just 5% torque is transmitted to the front wheels. The max of 40% is directed to the front wheels (based on my info) at 150+mph.
The AWD system is not intended to be something like IIRC used in the VW Golf R32 (at one time I considered buying one of these) which can direct 100% torque to the axle with the most grip.
IOW, the Turbo AWD system is not a system intended for use in marginal traction conditions where the front and rear tires can turn at excessively different speeds, over and above the speed difference that arises from the differences between rear and front tire diameters.
The car can of course be used in marginal traction conditions but the expectation is the car will be fitted with proper tires -- snow tires -- so as the car moves forward the front tires roll and turn at the same speed as they would on dry pavement.
The "center differential" is a viscous fluid coupling. It is a housing packed with discs. Every other disc is connected to and turns with the rear diff. The other set of discs is connected to and turns with the front diff. The housing is also filled with a special silicon based fluid (GKN made by a company in the UK is my info).
Might also mention that the coupling does allow the two axles to turn at different speeds which occurs during a turn.
As the car is driven and the two sets of plates spin at different speeds, courtesy of the tire diameter differences between the 2 axles, this creates fluid friction which heats up the fluid. This fluid as it becomes hotter (IIRC the fluid's working temperature can exceed 160C (320F)) becomes more viscous and and through friction with the rear diff discs transmits torque to the front diff discs. However, the coupling is still a fluid coupling and the two sets of discs never rotate at the same speed.
It is I think impressive to consider that with just a few percentage speed difference the fluid can transmit 40% of the engine's torque. That is some special fluid. It is also impressive that at speeds of over 150mph, in order to transmit that 40% of the engine's torque to the front diff the amount of heat generated by that viscous coupling must be huge and it requires the air flow of 150mph to cool it.
Every time I delve into the details of Porsche cars, and in this case the Turbo, the more impressed I am at the level of technology.
11-11-2014, 11:57 AM
#13
Drifting
The viscous coupling in the 996 awd system is a dumb and purely reactive component, it has no idea whether the vehicle is static or moving. It contains a dilatant, shear thickening, fluid and a large number thin clutch plates connected alternately to the input and output side of the coupling. Any difference between the rotation speeds of the input and output sides causes shear of the fluid between the plates and consequential thickening of the fluid which allows torque to be transmitted from the input of the coupling to the output of the coupling. The greater the difference in speed between input and output the greater the torque transmitted from input to output.
On all 996 awd cars the OEM tyre are specified with the rear tyres having a smaller rev per mile rating than the front tyres, with the exception of the C4 on Pirellis where the rev per mile rating is the same front and rear. This means that the rear tyres rotate faster than the fronts and some degree of torque is always transmitted rear to front, hence Porsche claims of a nominal minimum split of 5%FR 95%RR. The torque transmitted to the front wheels will increase with speed as the differential speed across the viscous coupling also increase with speed. The nominal split assigned by Porsche may be questionable since OEM Bridgestones on a tt have have a rev per mile difference of 14 front to rear whilst with OEM Michelins the rev per mile difference is only 3 which means there should also be a difference in torque transmission. This also suggests that Porsche feel that a 3.8% (14/837) difference in the rev per mile rating rev rating is OK for the components of the awd system. Note that the rev per mile rating is determined experimentally and is not necessarily directly related to the nominal tyre diameters. There seems to be no reference in any Porsche literature to the actual limits for the allowed rev per mile difference. Slippage of either of the rear wheels causes in an increase in prop shaft speed relative to the viscous coupling output and hence a further increase the torque transmitted to front wheels.
So far, so good. In the case of the car on the flat or a slight incline with front wheels on the dry and rear wheels on ice/ snow; the spinning rear wheels should transmit enough torque to the front axle to power the car forward. In fact one of the simple tests used by Volkswagen to test the efficacy of a viscous coupling is to place the vehicle with its driven wheels in a rolling brake tester in freewheel mode; if the viscous coupling is functional the wheels powered from the viscous coupling should pull the vehicle out of the rollers whilst the rears are spinning, if it doesn't the coupling is shot...and a lot of them are.
Next, non standard wheels: If you are running a GT2 or 997 turbo 19" setup on your 996 awd then your rear tyres have a smaller rev per mile rating than the fronts and are hence running slower than the fronts. This means that the input of the viscous coupling is running slower than the output; a torque will still be generated but it is effectively a braking torque rather than a driving torque. If rear grip is lost at speed, torque to the front wheels will be reduced due to a reduced speed differential between input and output at the viscous coupling. The reasons for this: the GT2 is 2wd and awd tyre compliance is irrelevant and the 997 final drives are different front and rear which compensates for the larger tyres at the rear of the 997.
I needed that!
On all 996 awd cars the OEM tyre are specified with the rear tyres having a smaller rev per mile rating than the front tyres, with the exception of the C4 on Pirellis where the rev per mile rating is the same front and rear. This means that the rear tyres rotate faster than the fronts and some degree of torque is always transmitted rear to front, hence Porsche claims of a nominal minimum split of 5%FR 95%RR. The torque transmitted to the front wheels will increase with speed as the differential speed across the viscous coupling also increase with speed. The nominal split assigned by Porsche may be questionable since OEM Bridgestones on a tt have have a rev per mile difference of 14 front to rear whilst with OEM Michelins the rev per mile difference is only 3 which means there should also be a difference in torque transmission. This also suggests that Porsche feel that a 3.8% (14/837) difference in the rev per mile rating rev rating is OK for the components of the awd system. Note that the rev per mile rating is determined experimentally and is not necessarily directly related to the nominal tyre diameters. There seems to be no reference in any Porsche literature to the actual limits for the allowed rev per mile difference. Slippage of either of the rear wheels causes in an increase in prop shaft speed relative to the viscous coupling output and hence a further increase the torque transmitted to front wheels.
So far, so good. In the case of the car on the flat or a slight incline with front wheels on the dry and rear wheels on ice/ snow; the spinning rear wheels should transmit enough torque to the front axle to power the car forward. In fact one of the simple tests used by Volkswagen to test the efficacy of a viscous coupling is to place the vehicle with its driven wheels in a rolling brake tester in freewheel mode; if the viscous coupling is functional the wheels powered from the viscous coupling should pull the vehicle out of the rollers whilst the rears are spinning, if it doesn't the coupling is shot...and a lot of them are.
Next, non standard wheels: If you are running a GT2 or 997 turbo 19" setup on your 996 awd then your rear tyres have a smaller rev per mile rating than the fronts and are hence running slower than the fronts. This means that the input of the viscous coupling is running slower than the output; a torque will still be generated but it is effectively a braking torque rather than a driving torque. If rear grip is lost at speed, torque to the front wheels will be reduced due to a reduced speed differential between input and output at the viscous coupling. The reasons for this: the GT2 is 2wd and awd tyre compliance is irrelevant and the 997 final drives are different front and rear which compensates for the larger tyres at the rear of the 997.
I needed that!
11-11-2014, 01:27 PM
#14
^ the revs per mile are greater in the rear. Not smaller. (on stock 996tt spec size)
higher revs per mile = smaller overall diamter = spinning faster.
Interestingly the spec for winter tires (from porsche 225/40 and 265/35) has the rear overall diameter slightly bigger than the factory summer tire spec. So the rear will run slower than the front. We are talking a handful of revolutions per mile here.
higher revs per mile = smaller overall diamter = spinning faster.
Interestingly the spec for winter tires (from porsche 225/40 and 265/35) has the rear overall diameter slightly bigger than the factory summer tire spec. So the rear will run slower than the front. We are talking a handful of revolutions per mile here.
11-11-2014, 01:28 PM
#15
The viscous coupling in the 996 awd system is a dumb and purely reactive component, it has no idea whether the vehicle is static or moving. It contains a dilatant, shear thickening, fluid and a large number thin clutch plates connected alternately to the input and output side of the coupling. Any difference between the rotation speeds of the input and output sides causes shear of the fluid between the plates and consequential thickening of the fluid which allows torque to be transmitted from the input of the coupling to the output of the coupling. The greater the difference in speed between input and output the greater the torque transmitted from input to output.
On all 996 awd cars the OEM tyre are specified with the rear tyres having a smaller rev per mile rating than the front tyres, with the exception of the C4 on Pirellis where the rev per mile rating is the same front and rear. This means that the rear tyres rotate faster than the fronts and some degree of torque is always transmitted rear to front, hence Porsche claims of a nominal minimum split of 5%FR 95%RR. The torque transmitted to the front wheels will increase with speed as the differential speed across the viscous coupling also increase with speed. The nominal split assigned by Porsche may be questionable since OEM Bridgestones on a tt have have a rev per mile difference of 14 front to rear whilst with OEM Michelins the rev per mile difference is only 3 which means there should also be a difference in torque transmission. This also suggests that Porsche feel that a 3.8% (14/837) difference in the rev per mile rating rev rating is OK for the components of the awd system. Note that the rev per mile rating is determined experimentally and is not necessarily directly related to the nominal tyre diameters. There seems to be no reference in any Porsche literature to the actual limits for the allowed rev per mile difference. Slippage of either of the rear wheels causes in an increase in prop shaft speed relative to the viscous coupling output and hence a further increase the torque transmitted to front wheels.
So far, so good. In the case of the car on the flat or a slight incline with front wheels on the dry and rear wheels on ice/ snow; the spinning rear wheels should transmit enough torque to the front axle to power the car forward. In fact one of the simple tests used by Volkswagen to test the efficacy of a viscous coupling is to place the vehicle with its driven wheels in a rolling brake tester in freewheel mode; if the viscous coupling is functional the wheels powered from the viscous coupling should pull the vehicle out of the rollers whilst the rears are spinning, if it doesn't the coupling is shot...and a lot of them are.
Next, non standard wheels: If you are running a GT2 or 997 turbo 19" setup on your 996 awd then your rear tyres have a smaller rev per mile rating than the fronts and are hence running slower than the fronts. This means that the input of the viscous coupling is running slower than the output; a torque will still be generated but it is effectively a braking torque rather than a driving torque. If rear grip is lost at speed, torque to the front wheels will be reduced due to a reduced speed differential between input and output at the viscous coupling. The reasons for this: the GT2 is 2wd and awd tyre compliance is irrelevant and the 997 final drives are different front and rear which compensates for the larger tyres at the rear of the 997.
I needed that!
On all 996 awd cars the OEM tyre are specified with the rear tyres having a smaller rev per mile rating than the front tyres, with the exception of the C4 on Pirellis where the rev per mile rating is the same front and rear. This means that the rear tyres rotate faster than the fronts and some degree of torque is always transmitted rear to front, hence Porsche claims of a nominal minimum split of 5%FR 95%RR. The torque transmitted to the front wheels will increase with speed as the differential speed across the viscous coupling also increase with speed. The nominal split assigned by Porsche may be questionable since OEM Bridgestones on a tt have have a rev per mile difference of 14 front to rear whilst with OEM Michelins the rev per mile difference is only 3 which means there should also be a difference in torque transmission. This also suggests that Porsche feel that a 3.8% (14/837) difference in the rev per mile rating rev rating is OK for the components of the awd system. Note that the rev per mile rating is determined experimentally and is not necessarily directly related to the nominal tyre diameters. There seems to be no reference in any Porsche literature to the actual limits for the allowed rev per mile difference. Slippage of either of the rear wheels causes in an increase in prop shaft speed relative to the viscous coupling output and hence a further increase the torque transmitted to front wheels.
So far, so good. In the case of the car on the flat or a slight incline with front wheels on the dry and rear wheels on ice/ snow; the spinning rear wheels should transmit enough torque to the front axle to power the car forward. In fact one of the simple tests used by Volkswagen to test the efficacy of a viscous coupling is to place the vehicle with its driven wheels in a rolling brake tester in freewheel mode; if the viscous coupling is functional the wheels powered from the viscous coupling should pull the vehicle out of the rollers whilst the rears are spinning, if it doesn't the coupling is shot...and a lot of them are.
Next, non standard wheels: If you are running a GT2 or 997 turbo 19" setup on your 996 awd then your rear tyres have a smaller rev per mile rating than the fronts and are hence running slower than the fronts. This means that the input of the viscous coupling is running slower than the output; a torque will still be generated but it is effectively a braking torque rather than a driving torque. If rear grip is lost at speed, torque to the front wheels will be reduced due to a reduced speed differential between input and output at the viscous coupling. The reasons for this: the GT2 is 2wd and awd tyre compliance is irrelevant and the 997 final drives are different front and rear which compensates for the larger tyres at the rear of the 997.
I needed that!