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    Gearing - 997 Turbo versus 996 Turbo

    There is no question that VTG technology improves the 997 Turbo's acceleration performance versus the 996 Turbo. But the gearing changes Porsche made to the 997 Turbo also add to the acceleration mix.

    To show the importance of gearing, consider a 996 Turbo with VTG, and with the same torque as an SC equipped 997 Turbo...the only difference being the gearing differences between the two cars. What's the impact of just the different gearing?

    The driving force at the wheels is equal to flywheel torque x the overall gear ratio (assuming a rolling radius of 1 for the wheel, and disregarding transmission losses...both of which can be assumed to be equal between the two cars).

    Porsche changed some of the gear ratios in the 997 Turbo, and it's interesting to see what that means in terms of available turning force at the wheels. For a point of reference I've used maximum available flywheel torque (505 lb-ft) for both cars so the transmission differences can be isolated...so here are the numbers for both cars with equal transmissions (6-speed) and torque. The turning force is in pounds...

    Overall gear ratio/turning force/advantage/percentage difference

    996 Turbo

    1st gear - 13.14/6636/equal/equal
    2nd gear - 7.05/3560/-157/-4.4%
    3rd gear - 4.85/2449/-121/-4.9%
    4th gear - 3.85/1944/-106/-5.5%
    5th gear - 3.16/1596/-86/-5.4%
    6th gear - 2.58/1303/-71/-5.4%

    997 Turbo

    1st gear - 13.14/6636/equal/equal
    2nd gear - 7.36/3717/+157/+4.4%
    3rd gear - 5.09/2570/+121/+4.9%
    4th gear - 4.06/2050/+106/+5.5%
    5th gear - 3.33/1682/+86/+5.4%
    6th gear - 2.72/1374/+71/+5.4%

    The numbers above present what the impact of gearing (in isolation) has in terms of the turning force at the wheels. There are obviously other considerations in terms of a car's acceleration numbers...traction, weight, the shape of the power curves, etc. But gearing has a big impact.

    The 997 Turbo's acceleration advantage over the 996 Turbo is better because of VTG, but it's also better because of the improved gearing.

    Re: Gearing - 997 Turbo versus 996 Turbo

    Dock - I believe that the 997TT's gearing is about 5% lower simply to compensate for the larger diameter 19" tires. I think the net change is negligible...

    In fact, if you take that into account, the 996 version has more favorable gearing in 1st gear, since they share the same gearing (but larger tire on the 997 makes the effective gearing taller).

    Re: Gearing - 997 Turbo versus 996 Turbo

    Quote:
    Grant said:
    Dock - I believe that the 997TT's gearing is about 5% lower simply to compensate for the larger diameter 19" tires.



    Great point about the 19" wheels. I thought the 19" wheels with the 305/30's would have the same (or very close to the same) rolling radius of the 18" with 295/30's. In fact the 997's rolling radius is 1.09' and the 996's rolling radius is 1.04'. The overall gear ratio x torque has to be divided by the rolling radius, and when you do that the 997's advantage in 2-6th gears averages ~10 lbs (really neglegable).

    In terms of acceleration, it seems the 997 Turbo would be better off with the 996 Turbos wheels/tires.

    Re: Gearing - 997 Turbo versus 996 Turbo

    Quote:
    Dock (Atlanta) said:
    In terms of acceleration, it seems the 997 Turbo would be better off with the 996 Turbos wheels/tires.


    Maybe, but I don't think they'd fit over the 380mm PCCB's...

    Re: Gearing - 997 Turbo versus 996 Turbo

    Quote:
    Grant said:
    Quote:
    Dock (Atlanta) said:
    In terms of acceleration, it seems the 997 Turbo would be better off with the 996 Turbos wheels/tires.


    Maybe, but I don't think they'd fit over the 380mm PCCB's...



    18" is the minumum wheels that can fit over 380mm and caliper.

    Re: Gearing - 997 Turbo versus 996 Turbo

    Quote:
    svtrader1 said:
    Quote:
    Grant said:
    Quote:
    Dock (Atlanta) said:
    In terms of acceleration, it seems the 997 Turbo would be better off with the 996 Turbos wheels/tires.


    Maybe, but I don't think they'd fit over the 380mm PCCB's...



    18" is the minumum wheels that can fit over 380mm and caliper.


    Really? The only cars that use the 380mm version of PCCB are the CGT, 997TT, and 997 GT3 and they all use 19" or larger wheels...

    Re: Gearing - 997 Turbo versus 996 Turbo

    Quote:
    svtrader1 said:
    Quote:
    Grant said:
    Quote:
    Dock (Atlanta) said:
    In terms of acceleration, it seems the 997 Turbo would be better off with the 996 Turbos wheels/tires.


    Maybe, but I don't think they'd fit over the 380mm PCCB's...



    18" is the minumum wheels that can fit over 380mm and caliper.



    The 18'' don't fit. This is why the Cayenne Turbo Powerkit and the 997 Turbo/GT3 with PCCB require 19'' wheels, incl. the winter wheels. Expensive stuff, a set of 997 Turbo winter wheels is over 5000 Euro or so, don't remember anymore.

    Re: Gearing - 997 Turbo versus 996 Turbo

    Quote:
    Grant said:
    Quote:
    svtrader1 said:
    Quote:
    Grant said:
    Quote:
    Dock (Atlanta) said:
    In terms of acceleration, it seems the 997 Turbo would be better off with the 996 Turbos wheels/tires.


    Maybe, but I don't think they'd fit over the 380mm PCCB's...



    18" is the minumum wheels that can fit over 380mm and caliper.


    Really? The only cars that use the 380mm version of PCCB are the CGT, 997TT, and 997 GT3 and they all use 19" or larger wheels...



    LP640 has 18" wheels with 380mm ceramics.

    Re: Gearing - 997 Turbo versus 996 Turbo

    Quote:
    svtrader1 said:
    LP640 has 18" wheels with 380mm ceramics.



    I thought we're talking about Porsche cars?
    Maybe the LP640 has a different rims design, different front rim offsets, etc., don't have a clue.

    Re: Gearing - 997 Turbo versus 996 Turbo

    Quote:
    RC said:I thought we're talking about Porsche cars?
    Maybe the LP640 has a different rims design, different front rim offsets, etc., don't have a clue.


    Or maybe the calipers on the Porsches are larger too...

    Re: Gearing - 997 Turbo versus 996 Turbo

    Quote:
    Grant said:
    Or maybe the calipers on the Porsches are larger too...



    They definetely look larger.

    Re: Gearing - 997 Turbo versus 996 Turbo

    LP640 use 380mm ceramics both front and rear, 6calipers front and rear also!

     
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