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996 Four Wheel Drive vs 997

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laserman21
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Joined: Fri Oct 31, 2008 6:37 pm

996 Four Wheel Drive vs 997

Post by laserman21 » Sat Nov 01, 2008 5:28 pm

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)
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Joined: Thu May 12, 2005 7:49 am
Location: Livermore Ca

Re: 996 Four Wheel Drive vs 997

Post by Zone7Rep(Larry Sharp) » Mon Nov 03, 2008 12:11 pm

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
1993 RS America
Grand Prix White
Car #6

1974 911 Carrera (resides in Australia)
Light Yellow

1974 914-6 GT (resides in San Luis obispo)

1987 944 turbo (location unknown)

1979 911SC(Hummers have it)

1972 911T (location unknown)

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PAUL LARSON
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Re: 996 Four Wheel Drive vs 997

Post by PAUL LARSON » Tue Nov 04, 2008 10:30 pm

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
Posts: 5
Joined: Fri Oct 31, 2008 6:37 pm

Re: 996 Four Wheel Drive vs 997

Post by laserman21 » Wed Nov 05, 2008 8:29 pm

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
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Joined: Thu Feb 22, 2007 1:26 am
Location: San Mateo

Re: 996 Four Wheel Drive vs 997

Post by Springgeyser » Wed Nov 12, 2008 9:44 pm

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

996 C4. AX6

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itacud
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Location: norcal

Re: 996 Four Wheel Drive vs 997

Post by itacud » Fri Nov 28, 2008 3:39 am

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.
laserman21 wrote: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
Last edited by itacud on Mon Dec 01, 2008 4:48 pm, edited 1 time in total.
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laserman21
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Joined: Fri Oct 31, 2008 6:37 pm

Re: 996 Four Wheel Drive vs 997

Post by laserman21 » Mon Dec 01, 2008 9:40 am

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

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PAUL LARSON
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Re: 996 Four Wheel Drive vs 997

Post by PAUL LARSON » Mon Dec 01, 2008 4:35 pm

laserman21 wrote: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

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Doug Clark
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Re: 996 Four Wheel Drive vs 997

Post by Doug Clark » Mon Dec 01, 2008 6:34 pm

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

Zone7Rep(Larry Sharp)
Posts: 389
Joined: Thu May 12, 2005 7:49 am
Location: Livermore Ca

Re: 996 Four Wheel Drive vs 997

Post by Zone7Rep(Larry Sharp) » Mon Dec 01, 2008 8:12 pm

Paul it isnt the width that needs to be the same or within a certain size ,it is the diameter of the tires
1993 RS America
Grand Prix White
Car #6

1974 911 Carrera (resides in Australia)
Light Yellow

1974 914-6 GT (resides in San Luis obispo)

1987 944 turbo (location unknown)

1979 911SC(Hummers have it)

1972 911T (location unknown)

User avatar
Dan Thompson
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Location: Auburn, CA

Re: 996 Four Wheel Drive vs 997

Post by Dan Thompson » Mon Dec 01, 2008 9:07 pm

:lol:
Dan Thompson
GGR DE/TT/CR Racecontrol

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