Fuel efficiency
09-15-2014, 01:12 PM
#17
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On paper, GT3's MPG looks worse than it really is. For example, it is rated same as Cayenne TT, but in real life gets about 25% better MPG. It's still poor but not as bad as spec numbers make it look. I guess the main cause of poor mileage is gearing. Most cars have 1/2 RPM at the same speed.
The flip side is that GT3 gets much smaller MPG penalty for being driven hard. I see 15% difference between granny mode (or driving back from a track day filled with Ibuprofen because I tweaked my back mode) and a moderate fun drive. On other cars the difference was much bigger (25%-50%). MPG also improved a lot over the first 1000 miles. Now I'm seeing almost 24mpg @ 75MPH average when on Ibuprofen ;-)
The flip side is that GT3 gets much smaller MPG penalty for being driven hard. I see 15% difference between granny mode (or driving back from a track day filled with Ibuprofen because I tweaked my back mode) and a moderate fun drive. On other cars the difference was much bigger (25%-50%). MPG also improved a lot over the first 1000 miles. Now I'm seeing almost 24mpg @ 75MPH average when on Ibuprofen ;-)
09-15-2014, 01:15 PM
#18
Rennlist Member
Panda5: There are some valid points above, but really it's mostly due to the extreme difference in the compression ratios of the 2 engines. The 991 Turbo S has a 9.8:1 compression ratio and the 991 GT3 has 12.9:1. As I'm sure you know, the compression ratio is the ratio of the volume of the cylinder and the combustion chamber when the piston is at the bottom, and the volume of the combustion chamber when the piston is at the top. Automotive engineers can GENERALLY improve fuel efficiency and fuel economy by designing engines with high compression ratios. However, when taken to the extreme limits like you see w/ the GT3's ratio of >10x, what happens is that the engine is able to create more power when running hard because it's getting a longer power stroke, but there's more drag imposed on the car when you're running around town at slower speeds. You'll notice that high compression ratio cars have lots of engine drag when you try to slow the car down w/ downshifts and that's effectively smashing the air/gas mixture down to extremely high pressures.
My other car (like some on the forum) is an Audi RS4 (2008) and it also has a super-high compression ratio of 12.5:1. That's why it's a real dog around town in terms of efficiency. Your effectively burdened with gaseous drag inside the engine. The upside is that your maximum horsepower is higher than it would be w/ a lower ratio. Everything in life is a compromise of some sort and the Porsche engineers skew towards higher performance in the GT3. Now, the regular 991 Carrera base model and S also have 12.5:1 ratios (almost as high), but the difference of 0.4:1 actually makes a big difference and the ECU mapping of the GT3 is set more toward performance than efficiency (drives fuel & air volumes & mix). All that makes for a lower-economy setup, but still not bad in light of the level of performance you're getting.
My other car (like some on the forum) is an Audi RS4 (2008) and it also has a super-high compression ratio of 12.5:1. That's why it's a real dog around town in terms of efficiency. Your effectively burdened with gaseous drag inside the engine. The upside is that your maximum horsepower is higher than it would be w/ a lower ratio. Everything in life is a compromise of some sort and the Porsche engineers skew towards higher performance in the GT3. Now, the regular 991 Carrera base model and S also have 12.5:1 ratios (almost as high), but the difference of 0.4:1 actually makes a big difference and the ECU mapping of the GT3 is set more toward performance than efficiency (drives fuel & air volumes & mix). All that makes for a lower-economy setup, but still not bad in light of the level of performance you're getting.
09-15-2014, 04:25 PM
#20
Race Director
The Porsche website says turbo s is rated 12.2l/100km while gt3 is 18.9l/100km, roughly one third more, for city driving...a stark difference. Don't get me wrong, anyone who can afford a gt3 will not care to much, that I agree, but the mark of engineering genius is to provide a vehicle with amazing performance at less fuel cost. The gt3 still is much better than rivals such as the 12c, etc., in terms of fuel consumption but surely Porsche is interested in improving and should continue to strive for that IMHO
