First I'd like to welcome myself to the forums, as this is my first post. I joined this forum to gain some more information on a car that I've been interested in for some time now.. I've owned many sports cars over the past decade, and I've arrived at a narrow preference of balanced performance and refinement that I can only seem to find in Porsche and Bmw ( both fall in my budget, Italian is too expensive for me, and Japanese works well but with little character). I scrutinize engineering as an engineer myself, so I have been doing some heavy research on these cars. I am very passionate about design, and one thing I discovered when learning about porsche is that they came up with the design of the 911 back when the connection between aerodynamics and road holding may not have been as clear as it is now. The basic shape of the 911 has not changed all that dramatically since its inception nearly half a century ago.

I loved the way these cars looked from the time I was a little kid. However,
what bothers me about the 911 heritage is that the shape of the car looks like an airplane wing. Seems like it would cause there to be a lot of lift and instability at high speeds...I'm familiar with Bernoulli's principle so this just had me wondering a little bit.

Do any 997 owners experience high speed instability or light steering feel at high speeds? I've read about front splitters and rear wings...does this seem to help ?

 

Your intuition is right. The early Porsche profile had the seeds of an instability that plagued the designers right up through... well, perhaps the 993. I'm not sure honestly because I haven't kept up with the dates of the changes through the years. I'm an engineer also and I do remember reading of the discoveries they were making as the car evolved in parallel with our development of supercomputers that could be practical aids to aerodynamic design for consumer goods. In a nutshell, the earlier body had a sharp change in aero lift as the wind angle went from straight ahead to progressively further off the front. The lift in straight ahead flow was not the problem by then. They had added certain aids as seat-of-the-pants solutions, after watching the problems of cars like the Lamborghini Miura. (Let's face it. In the seventies, damn few cars ever got above 100, let alone the speeds a Porsche Turbo and its cousins had the horsepower to reach. Those early supercars were their own wind tunnels.)

That increase in lift when the wind angle moved off dead center caused unpredictable behavior in crosswinds and of course led to very light steering above ... 110 mph I think it was. Probably detectable earlier of course, but it was the high speed instability that worried people.

Basically, all that is past tense. They knew what they needed to do three or four models ago (around the early nineties I believe) but it took a pretty profitable decade or so before they could muster the engineering resources to create a car that was still a 911 but conquered the air -- so to speak.

I have read nothing bad about the 996 in this regard and the latest generation, the 997.2 that came out in 2009, is absolutely brilliant at track speeds. Exhilarating, but as controllable as my Formula Ford was. One aspect laymen don't see is the very well-executed underbody that contributes a lot at speeds above perhaps 75 mph. Of course, the aero effects are moot now in terms of safety. The in-car electronics in a car like Porsche are as good as the industry can create in production quantities, and the dynamics are monitored at very high real time rates so the computers can intervene if they must, so instability is no longer an issue. The important thing to owners is that the body lines are inherently stable these days and the computer just prevents disaster when people run out of talent and pavement at the same time. That means we get our sports car feel with... a safety net sounds trite, but I suppose the phrase is trite because it applies so often. I have never had the system intervene to save my ***, but I've read enough people commenting about it that I know you can still make mistakes with this car and have it recover without the driver personally having the experience I do.

These days, the car is delightful to drive at high speeds. It took me a couple of track days to get the feel of a modern Porsche's handling, but now that I have I find nothing to complain of at all. And you didn't ask, but the handling at 'mere' sub-100 speeds on public roads is very satisfying as well.

Gary

 

I have personally experienced a lift in my '03 Audit TT 225 when breaching tripple digits at around 140 and it was scary to say the least, I do not know how I recovered but letting of the gas and letting the car coast seemed to do the trick.

Since then I made myself a promise that I will not travel at triple digit speeds on public freeways no matter how empty of traffic they were. I have stuck to that promise pretty well. Every once in a while I am tempted and do a quick burst only to let the car slow down seconds after. I must say that doing 115 on a freeway in the 997 felt extremely natural. In fact, the faster the car went the more it seemed to be planted on the road. I know these cars are meant to travel at much higher speeds than that, and I am fairly confident they would do so without any lift.

If this is one of your concerns that sits in the back of your mind, you may consider getting a 997 with a factory Aerokit which adds significant downforce at high speeds. It was designed by Porsche with that specifically in mind:

http://www.porsche.com/microsite/tec...MT911Targa4All

 

Thanks for the input guys. I figured it wouldn't be too much of a problem under 100mph, but for track days it worried me just a little bit. That factory aero kit looks very nice and functional, I bet its a pretty penny. I'll have to save up for that

 

I had my 05 911S up to 150 quite a few times and it was stable as can be. No aero kit.

 

It is as an option on a new car, but on a used vehicle it shouldn't increase the price all that much, it will just take a little bit of looking around.

 

my 996 at 130 is stable like doing 50mph...So much weight behind, plus rear wing = perfect result.

 

I've driven 911's since 1993 at numerous tracks and on the autobahn at high speeds. You needn't worry about high speed aero instability. I've never come across an incident where this has happened. That said, I wouldn't try running 125+ on the autobahn in the rain!

Most track incidents I have seen where to unproper technique in corners usually a result of too high an entry speed compounded later by abrupt throttle lift at turn in when the driver realized has going to fast. The technique with a 911 is "slow in - fast out" of corners. The 911 also responds to a bit of trail braking (still slightly on the brakes at turn in) to keep some of the weight transferred under braking on the front axle to increase front grip and reduce under steer.

If you use your head, you'll probably run out of courage before you run out of car. Let me repeat myself, use your head. Most incidents with novice drivers are that they drive way over their heads and assume their Porsche with all it's technical wizardry will rewrite the laws of physics and Newton's laws of motion just for them.

Aero kits look hot to some (not me) and are just not usefull unless your running periods of sustained triple digit speeds. Save your money and spend it driving schools with good instructors. They don't run aero kits at the cars at the Prosche Sport Driving School at Barber Motorsports Park. They're just not necessary at those speeds. Knowing how your car handles and feels when driven properly provides much more stability than an aero kit.


PS - retired Mechanical Engineer. 36 years at Big Yellow Tractor Co. Driving Porsches since 1983

 

That gives me a lot of confidence about the issue, I told myself that these cars are some of the most successful race cars ever built...I can't imagine it being a problem. I'm also not a huge fan of aero kits, I much prefer clean lines - but that rear lip spoiler on the car next to your username looks very nice. Thanks again for the replies !

 

The small deployable spoilers used on the 911 since the 964 generation in 1993, don't actually produce "downforce" in the truest sense of the word. What they due is "spoil" the air stream as it comes down the rear window and off the car. The backwards wing shape of a 911 that you noticed produces a low pressure area at the rear af the car. This combined with high pressure beneath the car prices lift and lightens the rear axle load. By breaking up the air flow, the low pressure area is reduced, reducing or negating the lift. So is that adding downforce? No, it's just reducing the lift affect.

Now, on the 911 race cars you mentioned, they do run very sizable rear wings to increase high speed down force and grip in fast corners. They also create a lot of drag. But the speed and stability gained in the high speed corners more than offsets the reduction in top speed on the straits.

Hope you find a 911 to your liking. They are fun, somewhat quirky cars with a ton of history. I recently traded a Ferrari F430 to get back in the dependable, properly engineered 911. "You can sleep with a Ferrari, but you will more likely marry a Porsche".

 

I had my 997 on the track for two days in September. I was getting to almost 140 on the back of Road Atlanta with no stability issues. What's unique about my setup is that I have the aerokit front, but the stock rear auto-deploy spoiler. My front lip is the Cup lip, but shaved down (DIY) to eliminate bottoming out. I have some suspension bits, listed in my sig, but you're asking about aero. Folks say that the car will become unstable with mis-matched aero - that's not my experience.

 

I can attest to the "flying " characteristics of the 70's cars. I owned and drove a new DeTomaso Pantera L 1972 in Europe and at speeds of around 200 kpm, it would get REALLY squiggly... Any change in wind / direction would also make you hold the steering wheel with TWO hands for sure!! Must say though that a 996 C2 will also feel quite light in the front. Its bobbing and weaving has been discussed over and over again. My 2Turbo with the factory aero wing package rides solid and seemingly unperturbed by all of it.

 

What it does, Jack, is become a test mule because it's not a situation tested in a wind tunnel or on track by the factory drivers. Of course, at race tracks we do lots of things the factory didn't test, but people who do must use judgment about the skills of their own test drivers. Which are... uh, usually themselves or a family member.

So mis-matched aero isn't necessarily unstable, just unknown. In your case, with the aerokit front but stock spoiler I would expect an increase in high-speed oversteer. Expect in the sense of 'maybe' not "bet my life on it" you understand. Aero forces from one end to the other interact and if it were easy to predict, we wouldn't need those supercomputers.

The opposite direction would actually worry me more. That is, putting on a big rear wing but not changing the front. Maybe that's just my personal assessment of my own driving skills. I can handle oversteer building with speed but I wouldn't be sure what was happening next if the rear was planted but the front end was getting alternately grippy and light as the aerodynamic forces varied. The word 'spooky' comes to mind.

Glad yours worked out well,

Gary

 

Having owned a 964, 993, 996 and a 997, I can say unequivocably that the 997 is light years ahead of the 964 in terms of high speed stability. That said, the heavy bias in rear weight means that with a 'normal' suspension setting, the front end floats a bit at speeds of 90+mph. You absolutely need the 'sport' setting to stabilize the front end unless you have gorilla-sized *****. Personally, when cruising at 75mph or less the suspension is fine in normal setting. Sport mode is stiff and I only engaged it when having some fun.

 

It would be more wing-like if you drove it backwards (fat end forward).

And that's why they have a wing or spoiler on them. Matching the airflows above and below would lead to some odd looking designs.