996 Four Wheel Drive vs 997

[laserman21]

I have a 996 C4S and want to put Turbo Wheels from a 997 on it. I was told I had to change tire size because the 997 uses different diameter tires front to back where mine uses same diameter tires all around. How does the 997 get away with this? I assumed the transfer case was one to one ratio. Thanks

[Zone7Rep(Larry Sharp)]

There could be a split between the front and rear to allow for different sizes .. I think they have a Viscous differential between the front and rear that allows for a torque split

[PAUL LARSON]

My 997 C2s has 235/19/35 on the fronts and 305/19/30
on the rears. I am pretty sure the C4s use these same tires.
Paul

[laserman21]

Thanks Larry and Paul. Paul, your tires are almost 3/4 inch different diameters front to back. I have been told my car needs identical diameter tires, front and back. Larry is probably right, there is a split between front and rear on the newer cars that my car does not have.

Thanks again.

[Springgeyser]

General concensus for 996 C4 C4s is to have no more than a 4% variation on the overall diameter.

[itacud]

The 997 Turbo's front differential is computer controlled, and is programmed to look for the wheel rotation speeds that are consistent with it's tire diameters. Since the 996 front differential is a passive viscous coupling, any difference in wheel diameters can cause partial lockup of the differential when less than normal wheel slip is occuring. For example, a front tire that is slightly larger than the rear causes the viscous coupling to lock up prematurely. Somewhere on the PCA national tech forum is a post that indicates lockup occurs at about a 7% difference in axel rotation. Keeping front/rear diameter differences as small as possible maintains the stock behaviour of the rear to front torque transfer.

I have a 996 C4S and want to put Turbo Wheels from a 997 on it. I was told I had to change tire size because the 997 uses different diameter tires front to back where mine uses same diameter tires all around. How does the 997 get away with this? I assumed the transfer case was one to one ratio. Thanks

[laserman21]

Thank you, that is the answer I was looking for.

[PAUL LARSON]

Thanks Larry and Paul. Paul, your tires are almost 3/4 inch different diameters front to back. I have been told my car needs identical diameter tires, front and back. Larry is probably right, there is a split between front and rear on the newer cars that my car does not have.

Thanks again.

The difference is 2 3/4". The fronts are 235 (9 1/4") and the
rears are 305 (12").
Paul

[Doug Clark]

You should listen to Paul - he is very fast (more than he should be)

[Zone7Rep(Larry Sharp)]

Paul it isnt the width that needs to be the same or within a certain size ,it is the diameter of the tires

[Dan Thompson]