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Sport Auto magazine in Germany conducts aerodynamic evaluation as part of its SuperTests. The figures are taken at 200 kph, or 124 mph.
A 997.1 C2S had 20 kg of lift in front and 7 kg in the rear (44 and 15 lbs)
A 997.2 C2S had 24 and 14 kg (53 and 31 lbs)
A 991.1 C2S had 2 and 0 (4 and 0 lbs)
A 991.2 C2S had 12 and -4 kg (26 and -9 lbs, or actual downforce)
In contrast, a 991.2 GTS had -16 and -53 kg front and rear (-36 and -117 lbs)
a 991.1 GT3 RS had --50 and -94 kg (-110 and -207 lbs)
It looks as if the active spoiler definitely works to limit rear lift and keep the front-rear aerodynamics on the reasonable side of stability. That means relatively more downforce--or less lift--in the rear. That's why, in the Seventies, Porsche always recommended using the ducktail and front spoiler together and strictly prohibited using the front spoiler alone, as that would lead to high-speed instability.
I had an interesting experience this past Saturday before reading this thread. The '18 GTS Coupe I got a few weeks ago is my first 911. I've had Corvettes, and came directly from a 981 Cayman S, which has a much smaller deployable spoiler. I doubt very much it was just a decoration, but I can't definitively say I ever felt much of a difference when it deployed at speed. What I can say definitively is the GTS at speed was a "holy crap, what just happened" moment.
I was on I-75 (1st time on an an open highway), and the LEOs were apparently taking the day off, as the traffic was practically flying. At one point I pulled over to the far right lane, and was following along at 85MPH in the slow lane, but I digress. Driving along at even less than 100MPH, I was shocked by how the car felt almost pinned to the ground, or on rails. It was something I had never felt on any prior sports car. It was almost as if I could let go of the steering wheel, and everything would be OK.
I don't know, maybe that is a common feeling in these cars, but I had certainly never felt that pinned to the road at speed like it before. It was an eye opening, and welcome experience. Maybe the RAS contributed to the stable feeling, and it wasn't all the spoiler, don't know. I'm sort of anxious now to see what it feels like at least at the 124MPH noted in the post above.
Zero rear lift might seem insignificant, however, without the spoiler the rear lift would likely be substantial. And keep in mind that the 911's home country is not completely hamstrung by our benighted speed limits. Cars with significant rear lift, such as the early Audi TTs, suffered serious crashes due to high-speed instability.
And for anyone who takes back roads out West, there are plenty of opportunities to explore triple digit speed where the rear spoiler is likely earning its keep.
To piggy back on this post, I too remember the TT's having to add spoilers to prevent crashes due to lift in Europe (https://www.carsaddiction.com/articl...assics-audi-tt ). I come from the camp that Porsche won't add anything superfluously, but haven't read much in regards to figures on our spoilers. I think it's more about reducing lift than producing down force, but could be wrong. Here's a video straight from the source. No figures though, start at the 39 second mark.
I had an interesting experience this past Saturday before reading this thread. The '18 GTS Coupe I got a few weeks ago is my first 911. I've had Corvettes, and came directly from a 981 Cayman S, which has a much smaller deployable spoiler. I doubt very much it was just a decoration, but I can't definitively say I ever felt much of a difference when it deployed at speed. What I can say definitively is the GTS at speed was a "holy crap, what just happened" moment.
I was on I-75 (1st time on an an open highway), and the LEOs were apparently taking the day off, as the traffic was practically flying. At one point I pulled over to the far right lane, and was following along at 85MPH in the slow lane, but I digress. Driving along at even less than 100MPH, I was shocked by how the car felt almost pinned to the ground, or on rails. It was something I had never felt on any prior sports car. It was almost as if I could let go of the steering wheel, and everything would be OK.
I don't know, maybe that is a common feeling in these cars, but I had certainly never felt that pinned to the road at speed like it before. It was an eye opening, and welcome experience. Maybe the RAS contributed to the stable feeling, and it wasn't all the spoiler, don't know. I'm sort of anxious now to see what it feels like at least at the 124MPH noted in the post above.
It is a common feeling in these cars since roughly about 20 years now. Yes that would be about the time the car everyone loves to hate the 996 came out. But that was when I was instructing the most and believe me, I know.
First experience was a 996 that went way off line at about 120. T1 at PR (fka SIR) transitions from an upward sloping drag strip to a downward sloping entry to T2 with a peak right in the middle of T1. Not that big a deal if you are on-line, it is only a small drop. But this one student was WAY off line, like 20-30 feet, where the drop is dramatic. I had never been so far off-line there. It felt like the car dropped off a cliff! Like without the seat belts pulling me down my head would have been hitting the roof! It was also right at this particular moment my student had taken one hand off the wheel and was gesticulating about something or other. With his other hand he had at most two fingers on the wheel. It was in this precarious situation we were cornering half air-borne at 120 mph. Any previous 911, and I have driven a lot of them, we would have been lucky with the right corrections just to stay on the track. Instead the 996 went right down the road utterly unperturbed.
The next experience was a 2005 TT. Same turn, only with me driving and doing an easy 150+ in T1 lap after lap because the car is just that awesome. Memory could be hazy on this but I sure seem to recall AP saying the 991 Turbo (or maybe GT3?) was the first street 911 actually to generate downforce. Not merely reduce lift, but actually generate some downforce. Whatever. Lift was a HUGE problem back in the day. Applying the brakes at 120+ in those days called for carefully squeezing them on as you controlled the steering through the delicate phase of transitioning from acceleration to braking with all that lift complicating weight transfer. Well, whether or not the 996 TT generated downforce or merely reduced lift a lot I don't know, but it was a HUGE difference. You could apply the brakes in that car at 150, and I mean hard, even on the crappy irregular surface going into T2 where you had camber changes, patches, seams, and at least two different types of surface, yet that car would brake straight and true without any perceptible need for steering correction. At. All.
Now bear in mind: Even though a Turbo is pretty much the pinnacle of Porsche development, that was a decade and a generation ago. Every 991 of any build has engine, suspension, brakes, and aero all much better than that 2005 996 Turbo. Aero development is huge, but there is a lot more to it than aero. RAS for example, allows the car to emulate the behavior of a longer-wheelbase at high speed. RAS definitely contributes to high speed stability.
About aero though. It is NOT just a high-speed or track kind of thing. One last story to really drive the point home. One day I found myself driving two cars exactly the same except for the spoilers. The thing that blew me away was not how much more planted the spoilered car felt in T1. You expect that at high speed. Even those who think its all about marketing and looks usually admit there probably will be some difference at 200 mph or whatever. But no. What blew me away was how dramatically more planted it felt in turns 4-7, which are all 50-70 mph. Perfectly normal driving speed. Huge difference.
I had an interesting experience this past Saturday before reading this thread. The '18 GTS Coupe I got a few weeks ago is my first 911. I've had Corvettes, and came directly from a 981 Cayman S, which has a much smaller deployable spoiler. I doubt very much it was just a decoration, but I can't definitively say I ever felt much of a difference when it deployed at speed. What I can say definitively is the GTS at speed was a "holy crap, what just happened" moment.
I was on I-75 (1st time on an an open highway), and the LEOs were apparently taking the day off, as the traffic was practically flying. At one point I pulled over to the far right lane, and was following along at 85MPH in the slow lane, but I digress. Driving along at even less than 100MPH, I was shocked by how the car felt almost pinned to the ground, or on rails. It was something I had never felt on any prior sports car. It was almost as if I could let go of the steering wheel, and everything would be OK.
I don't know, maybe that is a common feeling in these cars, but I had certainly never felt that pinned to the road at speed like it before. It was an eye opening, and welcome experience. Maybe the RAS contributed to the stable feeling, and it wasn't all the spoiler, don't know. I'm sort of anxious now to see what it feels like at least at the 124MPH noted in the post above.
Breaking her in nicely! Cannot do speeds here like that other than on a track without getting arrested but SO looking forward to when mine comes in! Glad you are driving it like Dr Porsche meant them to be driven.
I think the comments regarding downforce vs. reduction of lift are important, especially for a street car. The function of a spoiler vs. a wing is also important.
Porsche engineers don’t typically design things purely for aesthetics or for marketing purposes. The design of the 911 has always taken a “function first” philosophy. The aero components in the 911 are designed for a specific purpose. Remember that these cars are meant to be driven on the track, and on the unlimited speed sections of the Autobahn. Aero matters in these places, moreso than others. One specific example that comes to mind is from the 996 series. The 996 C4S shares many components with the 996 turbo, including the front bumper cover. However, the front spoiler lip is different between the two. Porsche’s explanation for this is that the turbo’s front lip is specifically designed to balance the aerodynamics of the car at speed working in conjunction with the rear spoiler. It’s too aggressive for the C4S, so a different spoiler is used in that application.
I remember when Audi first launched the TT. I realize it’s not a Porsche and not a 911, but it’s applicable here. The initial design didn’t reduce lift enough at the rear of the vehicle, and there were some horrific crashes on the Autobahn because of lift at the rear fo the car at high speed. Lift at the rear of the vehicle resulted in instability and loss of control. The design was quickly changed.
I prefer to talk about aero rather than spoilers because there is so much more going on than just spoilers. Aerodynamics encompasses the entire spectrum of airflow management over, under, around and through the car. Yes, the size and angle and position of a spoiler does matter, and when it gets to the point you suspect Chris Harris just jizzed in his pants going on about an enormous RS wing everyone takes notice. But there is a lot more going on. Like, those cutouts in the wings (front fenders) of the RS's, they found positive air pressure under there, which by giving that air somewhere to go eliminated something like 100 lbs of lift. It stands to reason all that pressure represented a bottle-neck of sorts meaning reduced air flow, which is needed for cooling, not something I know for sure only Porsche knows but it just stands to reason. So now that air is flowing along the sides of the car. Where they can channel it to air intakes on the rear fenders. After making sure it passes along the sides. In a nice laminar flow. Without too much turbulence from the side view mirrors. The point being even just this one little aero tweak gets real complex real fast.
I also know its more than spoilers from having been around a lot of competition types, with real-world experience and knowledge. I remember one car, it looked totally stock, yet somehow generated over a hundred extra pounds of downforce. At street legal speeds. Again, to look at it, you would never suspect a thing. Even on a lift- even when they point out the parts!- you wouldn't believe it. Again, high speeds not necessary. This car was built for autocross! Autocross is mostly 50-70 mph, maybe 80 mph max!
Love it. One guy says "it's cosmetic and it does matter" and 4 pages later and over 50 post, it's still going strong. It was only ONE GUY who said "no".
I found the section in the 991.2 Techniks manual addressing this. Here is a cut and paste of that section:
Controlling the cooling air For thermodynamic support under all operating conditions, the rear spoiler can be extended to the aerodynamic nominal position at speeds below 75 mph (120 km/h) in order to guide the cooling air to the charge-air coolers if required. Due to the position of the rear spoiler and the cooling air duct intakes in front of it, a ram pressure is generated, through which the cooling performance of the charge-air coolers can be increased if necessary. With this additional function, the rear spoiler on the 911 Carrera model year 2017 models has not only an aerodynamic, but also a thermodynamic charge-air cooler function.
Thanks for that. Explains why the spoiler deploys at the relatively tame speed of 56 mph on my 991.2 GTS .
Might be useful to know for a roadside chat with a state trooper:
LEO: Of course you were speeding, the spoiler was deployed.
Me: But officer, on the 991.2 it goes up well under the speed limit!
Factory data below. Not for the 991, but let's assume that this is basically consistent with the current cars. And let's use 90MPH as the speed (the upper limit of US street driving). Rear spoiler vs no rear spoiler is a difference of 50 pounds at the rear. If you can detect that, you should quit whatever job you currently do and become a professional race driver -- that's how good you are.
PS this may be the dumbest debate in the world, and I can't believe I'm participating in it. There is hard data people. No need to speculate.
See below for values for 991 series vehicles (at 200kmh) Front/Rear
991GT3RS 50kg/90kg
991GT3II 16kg/53kg
991GT3I 18kg/29kg
981GT4 12kg/31kg
991.2S +8/8kg
991.2GTS +12kg/4kg
as a reference point the AMG GT S - +40kg/8kg
02-19-2018, 01:04 PM
Cannot do speeds here like that other than on a track without getting arrested but SO looking forward to when mine comes in! Glad you are driving it like Dr Porsche meant them to be driven. 
How much track driving do you do?
