C2 Cabrio not secure at high Speed ?

Today I finnally managed to take my C2 to full speed (290 km/h) and it was with surprise that above 220 km/h I notice the front of the car all over the place (expecially with side wind). Is this normal ? All of my other cars don�t have this problem, neither my older cars (Maserati 3200, BMW Z3 M ...). Is this normal ? It really fels unsecure ... ---------------------------------- 2004 Porsche Cayenne Turbo 2003 Porsche 996 C2 Cabrio 1999 Jaguar Xk8 Cabrio

It's probly bc of the rear weight bias. Are the wheels/tires in good condition? Any looseness in steering? I doubt it's due to convertible since the cab is pretty aerodynamic.

The wheels and tires are in perfect conditions and yes, I think it�s because of the rear weight bias. Insteed of a back spoiler Porsche should put a front automatic spoiler (don�t ask me how, i�m not engeneer :-) BTW, the rear spoiler doesn�t help, it makes things worse, because it lowers the back of the car and makes the front more loose. ---------------------------------- 2004 Porsche Cayenne Turbo 2003 Porsche 996 C2 Cabrio 1999 Jaguar Xk8 Cabrio

There are so many things that will cause the front end to get light at high speeds. What size tires are you running? Stock suspension? Any lip on the front bumper? Spare tre in the trunk? Top up or down?

19 wheels, 225/40 front and 285/30 rear, stock suspension, no lip at front bumper, spare tire at the trunk and top up (I don�t think it�s a good idea to drive with the top down at 290 km/h :-) ---------------------------------- 2004 Porsche Cayenne Turbo 2003 Porsche 996 C2 Cabrio 1999 Jaguar Xk8 Cabrio

Well, the slightly lower back end due to tire sizes is not that big, but a front spoiler would sure help keep the air from going under and lifting the front end.

Upps, it�s not 19 wheels it�s 18, sorry for that. I just think that if the car needs a front spoiler in order to be safe at high speeds, Porsche should set that as standard in 996. ---------------------------------- 2004 Porsche Cayenne Turbo 2003 Porsche 996 C2 Cabrio 1999 Jaguar Xk8 Cabrio

Your back end is more than 1/4 inch lower than it would be with the 265/35/18 tires. Doesn't seem like alot, but at speed, it makes alot of difference because of the change in angle between the bottom of the car and the road surface. Changing the fronts (when you replace the tires) to a 235/35/18 will bring the front end down about 1/4 inch. I agree with putting a skirt or lip on the bottom of the front bumper.

Yes, you are right, but Porsche installation size approved are the following: - 225/40 and 285/30 - 225/40 and 295/30 Next time I�ll change my back tires to 295/30 (it will be lower than front, but less than it is now) Here in Portugal if you use different tires dimension that the car was approved, you�re in big trouble next time your car goes to inspection (2 in 2 years). Stupid laws ... ---------------------------------- 2004 Porsche Cayenne Turbo 2003 Porsche 996 C2 Cabrio 1999 Jaguar Xk8 Cabrio

I have a C4 cab and I know that the car got MUCH more stable at high speeds when I installed PSS9 coilovers and lowered the car....I also have the Turbo front bumper so I am thinking there is much more downforce on the front end than the stock front at stock height.

290 klicks is how many MPH?

Assuming "290 klicks" = "290 km/hr" it'd be 180 mph. If not, then 290 kilo-licks must be something nasty ...

Wow, 180 with the top down, that's fast!

our cars can reach 180????

must be getting tail wind.

He said top up. Also, maybe he pushed his 911 off a really big cliff to get to 180mph? J/K...

I don�t know why are you soo admired, my car has manual transmition and has been upgraded by superchips. But even if it was stock, if you look at Porsche manual it reads like this: ---------------------------------- 2004 Porsche Cayenne Turbo 2003 Porsche 996 C2 Cabrio 1999 Jaguar Xk8 Cabrio

1 mile = 1.61 km => 290 km/h = 180 mph Usually speedometers lie. I remember taking my Yamaha R1 to 196mph, while my GPS read-out was 176mph.

i took my C4 to 250km last friday on a trip back from Poland. Steady as a rock, i guess the 4 feels more stable at high speeds, like you are on rails.

At that speed it really doesn't matter.

I think GPS's are wrong since they can be up to 10 feet off. Non the less my speedo vs GPS differences have been no more than 1.5 mph at low and very high speeds.

i too find my 02 cab a bit unstable at 250+ i have the identical setup as you.

I think all cars get a little more unstable at high speeds! That is why F1 cars have the huge wings and and special bottom under car to create stability. I drove my P up to 145MPH before I lowered it and it was a little scarry...now after lowering it feel much better......a couple of years ago I had a 928s.....and this P was solid as a rock..I worked in Germany a lot and 270-280 KMH was like driving 100KMH that stable..I think that weight distribution is also a large factor!

------ trust the GPS speed. The up to "30 feet" off (10 meters) is due to alot of factors. The speed is calculated relative to the timing which is set using Cesium clocks (accuracy of 2 nanoseconds per day or one second in 1,400,000 years). I'd go with the GPS speed to "calibrate" the on-board speedometer. JMHO Dan

The C4 is heavier in the front and therefore quite stable at high speed. I have tried 280 Km/t = 174 Mph with mine and it was quite steady. A turbo front spoiler will probably do the rest.