991 GT3 MPG
11-21-2013, 05:03 PM
#61
Race Director
I don't think anyone in the thread is discussing this from a frugal or eco-concern context; more from an engineering standpoint.
Electronic power steering removes parasitic loss from an engine, therefore should result in more efficiency. Same goes with drag coefficients, the PDK efficiency and so on.
It's just curious that the 991 GT3 has all these things over a 997 that an engineer would think causes better MPG as a byproduct, but clearly the contrary has occurred.
Electronic power steering removes parasitic loss from an engine, therefore should result in more efficiency. Same goes with drag coefficients, the PDK efficiency and so on.
It's just curious that the 991 GT3 has all these things over a 997 that an engineer would think causes better MPG as a byproduct, but clearly the contrary has occurred.
2. I'm pretty sure there is no significant difference in the coefficient of drag between the 991 and 997, and any that exists is offset by the improved 991 GT3 aero package which creates more downforce (and drag).
3. Main point: What makes a car with PDK more efficient than one with a MT is that it has a very tall top gear and an ECO mode with very early upshifts for economy. PDK-S has neither of those features. It's not so much about the mechanicals, it's about the programming that is possible and is of benefit in EPA testing. To say that PDK, in and of itself is more efficient, is misleading, IMO.
4. Just my $.02......
11-21-2013, 05:16 PM
#62
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Can't speak to EU vs US fuel differences. Test methodology is obviously different.
My numbers are from US Porsche website for both cars so as apples to apples as it gets. I highly doubt magazines spend much time replicating MPG testing methodology YoY to the extent they event do it.
Final drive ratio change is the most obvious contributor AFAIK. 3.44 vs 3.97; a big difference.
11-21-2013, 05:32 PM
#63
Race Director
Exactly.
Can't speak to EU vs US fuel differences. Test methodology is obviously different.
My numbers are from US Porsche website for both cars so as apples to apples as it gets. I highly doubt magazines spend much time replicating MPG testing methodology YoY to the extent they event do it.
Final drive ratio change is the most obvious contributor AFAIK. 3.44 vs 3.97; a big difference.
Can't speak to EU vs US fuel differences. Test methodology is obviously different.
My numbers are from US Porsche website for both cars so as apples to apples as it gets. I highly doubt magazines spend much time replicating MPG testing methodology YoY to the extent they event do it.
Final drive ratio change is the most obvious contributor AFAIK. 3.44 vs 3.97; a big difference.
The final drive ratio difference is mitigated by two factors. First, the 0.84 7th gear in the PDK-S box is taller than the 0.92 6th gear in the 997 GT3. That makes the effective final drive in the 991 GT3 a 3.31 versus 3.16 in the 997. Further, the 991 uses a 305x30x20 tire which has revs per mile that are 4% lower than the 305x30x19 tire of the 997. This wipes out the final drive ratio difference for freeway driving.
As I said, IMO, the biggest difference is the lack of an ultra tall 7th gear and ECO shifting mode with PDK-S, at least when compared to Carrera models with PDK and why relative to the 997 GT3, which has less power, the comparative mileage isn't that surprising to me.
11-21-2013, 11:47 PM
#65
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11-22-2013, 12:12 AM
#67
Race Director
11-22-2013, 12:41 AM
#68
1. I'm not entirely convinced we're comparing apples to apples, ie. whether testing procedures are identical.
2. I'm pretty sure there is no significant difference in the coefficient of drag between the 991 and 997, and any that exists is offset by the improved 991 GT3 aero package which creates more downforce (and drag).
3. Main point: What makes a car with PDK more efficient than one with a MT is that it has a very tall top gear and an ECO mode with very early upshifts for economy. PDK-S has neither of those features. It's not so much about the mechanicals, it's about the programming that is possible and is of benefit in EPA testing. To say that PDK, in and of itself is more efficient, is misleading, IMO.
4. Just my $.02......
2. I'm pretty sure there is no significant difference in the coefficient of drag between the 991 and 997, and any that exists is offset by the improved 991 GT3 aero package which creates more downforce (and drag).
3. Main point: What makes a car with PDK more efficient than one with a MT is that it has a very tall top gear and an ECO mode with very early upshifts for economy. PDK-S has neither of those features. It's not so much about the mechanicals, it's about the programming that is possible and is of benefit in EPA testing. To say that PDK, in and of itself is more efficient, is misleading, IMO.
4. Just my $.02......
The coefficient of drag on the 991 is better (even though downforce is improved), but the car is higher off the ground and has a larger front area which makes it probably a wash in aero drag.
Electric steering is good for 1-3%. Big factor for Porsche here is to reduce weight and complexity (no more PS hoses from rear of car, etc), not just efficiency - I bloody hope they got something out of it, the steering the 991 is terrible compared to the old world.
PDK helps, but probably not a factor in the City rating given how little opportunity there is to use the 7th gear.
11-22-2013, 02:47 AM
#69
Race Director
The Cd of the 991 is 0.33 compared to 0.32 on the 997.2 GT3 (the RS is 0.33). Frontal area of the 991 is slightly more than the old GT3 but less than the RS, and overall drag also has the 991 GT3 bracketed by the other two cars.
11-22-2013, 02:50 AM
#70
Rennlist Member
Mike. I also have a document that shows the front ramp brake angle of the 991 GT3 is beter than teh 997.2 GT3 by a few degrees although the ramp brake over angle (i.e. middle of the car over a mound) is less).
11-22-2013, 02:57 AM
#71
Race Director
Macca, agreed I have those numbers too. I'm sure the longer wheelbase is the culprit re: the ramp breakover angle.
11-22-2013, 03:21 AM
#72
Race Director
11-22-2013, 12:02 PM
#73
According to the Porsche product information guide that I have on the 991 GT3 which compares all the technical aspects and features of the 991 GT3 to it's 997.2 predecessors, both the 991 and 997 GT3 have an identical ground clearance of 93mm (3.66") although the 991 actually has a lower overall height at 1269mm (49.96") versus 1280mm (50.39") on the 997.
The Cd of the 991 is 0.33 compared to 0.32 on the 997.2 GT3 (the RS is 0.33). Frontal area of the 991 is slightly more than the old GT3 but less than the RS, and overall drag also has the 991 GT3 bracketed by the other two cars.
The Cd of the 991 is 0.33 compared to 0.32 on the 997.2 GT3 (the RS is 0.33). Frontal area of the 991 is slightly more than the old GT3 but less than the RS, and overall drag also has the 991 GT3 bracketed by the other two cars.
Last edited by ShakeNBake; 11-22-2013 at 12:17 PM.
11-22-2013, 02:07 PM
#74
Race Director
991 GT3 72.91"
997 GT3 71.18"
997 RS 72.91"
Width with mirrors
991 GT3 77.87"
997 GT3 76.85"
997 RS 76.85"
Width across front axle
991 GT3 70.67"
997 GT3 68.27"
997 RS 69.29"
Drag Coefficient Cd
991 GT3 0.33
997 GT3 0.32
997 RS 0.33
Frontal area A
991 GT3 2.036m2
997 GT3 2.013m2
997 RS 2.071m2
Drag Cd x A
991 GT3 0.672
997 GT3 0.644
997 RS 0.683
11-22-2013, 03:45 PM
#75
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It's interesting how such small differences add up to a car that looks so much larger in real life (based on my inspection of the 991S). Maybe its the combined effect of those dimensions along with lower roofline and more rake in the windshield.