Jul 5, 2006 4:13:46 PM
- Dock (Atlanta)
- Pilot
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- Loc: Atlanta, Georgia
- Posts: 386, Gallery
- Registered on: May 1, 2002
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.
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.
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. 
