Gravs

I hope neither. In particular I am a turbo cynic, believing that in real life they're no mor efficient than NA.

Please can everyone with either GT3 or TT post their average fuel consumption based on everyday driving (no good if it's all track or anything weird).

I get 22-23 mpg in my 991 GT3 (from section 2 in the trip computer, based on a few thousand miles).

Or even more interesting, what's the mpg in your 918 (assuming you don't plug it in).

FrstPorsche

To get what you are you have to be driving in "normal" mode 90% of the time. In my 4s I get around 16 but mostly drive in manual mode between 3-5000 rpms.

Gravs

Flib I must be driving like a *****!

NateOZ

UK petrol is a much better than US Gasoline, you can buy E0 and E5 - here almost 99.9% of it is E10.

MayorAdamWest

I get about 26-27mpg on the highway with the 991S. I'm not sure I understand why the GT3 is so much worse, other than gearing.

carcommander

My turbo average city driving some expressway 20.1 mpg. Why do we care. At 15000 miles a year 15mpg fuel cost is $3500. 20mpg saves $1200 a year. 125K plus car it's peanuts compared to depreciation.

Jimmy-D

Holly smokes - In my GT3 I am getting like 15 mpg, if I am lucky. I will check in the AM because you now have sparked my interest. Only 30% HWY but no way is the GT3 fuel efficient. It has a decent appetite and glad I got the extended tank. Now- my oil consumption is equal to my 991 C2S loaner which has surprised me. Meaning- I probably will not have to top it off until about 1500 miles and I have about 850 now. My 997.2 Turbo was around 18 mpg; that I knew which was pretty good.

All in all; I think Turbos are, by far, more fuel efficient

Gravs

The main reason I care is that I prefer NA engines and am not convinced turbos are more efficient in real world, rather just suited to government tests. That's why real data from everyone's trip computers would be interesting. We'd need a good sample size though to account for different driving styles.

disden

My mpg in the "real world" when comparing my new Panamera turbo 6 vs the old V8 are identical. Everyday I wonder why Porsche killed off the V8 for no real world mpg improvement. I know, I know, China and engine displacement taxes, blah blah blah .... By the way, love your guys GT3s, wish I was a track junky and could justify one! Before I ordered my new 991 turbo I went back and forth on the 2. I saw a few gT3s at the Zuffenhausen delivery center a few weeks ago when I picked up my Turbo; damn those GT3s are sick! Enjoy all.

Gravs

I think I have the answer. Keep the high revving V6 or V8, build in a extremely aggressive cylinder-cut technology, like the one Bentley does but cut down to 2 cylinders. Let the damn thing drive like a dog on the fuel test cycles. And then have a button to turn it off!

That's not too far from what a turbo does anyway!

Petevb

Are Turbos more efficient? No. And yes...

First realize that all modern gas engines have decent peak efficiency- they convert close to 35% of the energy in the gasoline into useful power to move the car forward when they are in their "sweet spot" for rpm and load. If 35% doesn't sound that hot, realize that physics imposes some upper limits in internal combustion engines that kick in as you near 50%, so 35% is actually doing reasonably well.

Next realize that this peak efficiency does not necessarily improve with turbocharging, and in fact it often goes down. The most efficient engines, like those in the Prius, are up at 38% and are not turbocharged.

Our unfortunate problem is that regardless of engine type we spend near zero time in our cars running the engine at peak efficiency. Peak efficiency is found near the low end of the peak torque RPM and roughly 75% load, meaning the throttle is open a long way. Each motor will vary, but the effieiency is measured with a BSFC (Brake Specific Fuel Consumption) chart like this one from a random Saturn:

Stare at this a minute, because it's the key to this. We have RPM on the bottom, torque on the right, and colored "islands" showing how efficient the engine is at converting fuel into motion. The hotter (closer to red) the area is, the more efficient the engine is running. You also have lines going across the graph showing how much HP the engine is making, and what load the engine is under.

So we can see that the motor above is most efficient making about 45 hp at 2500 rpm, and it's right at 75% load there (throttle around 75% open). Anywhere other than this rpm and load and efficiency drops...

Cruising down the highway at reasonable speed requires surprisingly little energy- something like 20 hp. Unfortunately this engine is about 10% less efficient making 20 hp, and to achieve that efficiency you need to be turning under 1500 rpm. Making 5 hp, just 4% of it's peak output, and the engine is unhappy- at 1500 rpm it's roughly half as efficient as it would be at peak. You don't want to spend time down in this part of the graph if efficiency is important.

Now comes our problem with powerful cars: You spend all your time down in this part of the graph when you're just cruising around. The GT3's graph will look similar to this, but multiplied by 4 because it makes 4x the power. So where the Saturn is falling off the map making just 5 hp, the GT3 is doing the same making 20 hp, which is as we said is close to where you cruise at on the highway. The problem is simple: the engine is to big. Make it 1/5th the size and you'd always be in your sweet spot cruising around, and you'd get great mileage.

So how do we deal with this? There are really just two good options- make the engine smaller, or change the way you drive.

We know from the chart that making the engine smaller works, but how do you make the engine smaller but maintain performance?

Cylinder cut is one way- drop half or more cylinders and the remaining cylinders run at their ideal load, improving efficiency. Of course you still have the friction from the remaining cylinders that hurts efficiency a little, but it helps a lot.

Another way is to turbocharging, because you can make the same peak power from a smaller engine. The BSFC graphs for turbo engines show wider efficiency islands- at low loads it behaves like a small motor, at higher loads it behaves like a big one. This is where the real world efficiency of a turbo comes from.

Another way of making downsizing the engine but maintaining performance is to lighten the car. 10% less weight means 10% less engine means the motor running in its sweet spot more of the time. Lower weight by itself doesn't help nearly as much as you'd think, but lower weight used to make the engine smaller helps a lot.

Adding hybrid is the final obvious way to make the motor smaller, simply because you've got reserve power for performance elsewhere.

That's about all the good options for the engine: sure direct injection, low friction coating, electric pumps, etc all help, but size is the big ****. And this is why comparing the Turbo to the GT3 might not show a significant efficiency improvement: the Turbo engine didn't get any smaller, so it's not likely to get much more efficient. And if you drive hard, using all the power it has, you should get worse mileage- it is, after all, a higher power and heavier car.

As for the other way to improve fuel economy, hybrids and PDK both address how we drive. The PDK can easily run a tall top gear and switch out of it the moment you need power, letting you cruise in the engine's most efficient area. The fact that it can downshift so quickly when you need power means the transmission can upshift sooner, keeping the engine in the sweet spot longer.

Hybrids go one better, disconnecting the engine from the load entirely. A 918's engine can be as big as you'd like and still run at peak efficiency all the time when you're cruising- simply shut the engine down, then switch is back on at the ideal RPM and load to charge the battery for a while. Never needing to run in the inefficient areas is a massive advantage, and is clearly the ultimate way to achieve both performance and efficiency short of full electric. Unfortunately it's far more expensive than making a the engine smaller, so it will be need to be reserved for the higher end for some time.

Very long explanation but hopefully helpful. Real world examples do bare this out, but as we know it all depends on driving style. Just as an example my 1M has virtually identical performance to the last V8 M3, but uses a smaller turbo motor to do it and does get ~20%+ better mpg in "normal" usage. So concept does in this case work in practice when you downsize the engine...

neanicu

This is one of the answers :



A motor working together with the engine and you can keep NA. Let's see what happens...

Gravs

Very thorough answer thanks Pete! Couple of questions if I may:

1. I can see that smaller engines are more efficient at the same power output as a larger one since they are working nearer their peak, but what causes this? Less friction relative to output at faster speeds?
2. How does non-turbo compression compare?

Petevb

It's largely the result of a very low effective compression hurting efficiency when a motor is run at part throttle. Run a gasoline engine at low throttle openings and you're starving the cylinders by letting less air past the throttle plate. So while your engine might have a 12:1 "nominal" compression ratio, if you've choked the air off you start the compression cycle with a vacuum in the cylinder and only a fraction of the air that would be there if you were at WOT. You mix in and burn less fuel, but the result is a engine whose combustion is behaving like it's perhaps 4:1 compression, and low compression motors are inherently much less efficient.

The other main issues with running a motor at low loads have to do with friction, heat loss and pumping losses. The big engine has more surface area to cause friction and lose heat inside the cylinder, both of which are sources of inefficiency. Finally it's not free to pull all that air across the throttle plate every stroke- each stroke takes energy, like running a vacuum pump, and the smaller motor running closer to WOT is both moving less air and pulling a smaller vacuum. This is all part of the "pumping losses", and these go up with motor size.

The lower the load you run a motor at the more these issues catch up with you. Consider that any motor running at idle is 0% efficient- it's making enough power to keep itself running, but no more. So simplistically the closer you get to idling the less efficient you get, and a big motor in a small car (think Viper) virtually idling down the road, with most of the motor power simply overcoming internal motor friction, heat loss and pumping losses.

We should discuss effective compression. Consider a modern direct injection turbo engine at 9:1 vs a 12:1 normally aspirated. Before the boost kicks in the higher compression of the NA engine will let it get hotter and hence run with more peak efficiency.

With 1 bar of boost, however, the turbo engine doubles the amount of air in the cylinder before the compression stroke, so you run a very high effective compression which creates very good efficiency.

Another way to think about is that both the NA and turbo engine need to move the same total amount of air to make the same power. However in effect the turbo allows the engine to add air on demand (quite efficiently), while the normally aspirated engine only has the option of subtracting air by closing the throttle plate, and as we said above that increases losses...