TB993tt

The aerodynamics of Porsches is always of interest to me and the desire to keep the 993 shape planted at high speed without resorting to Motorsport appendages....

I recently put on a factory Turbo S rear spoiler (already have S front lip) and recently had the opportunity to try it out at up to 180mph in Germany - the car felt very stable. It is difficult to know how much is attributable to the spoiler and how much to the 4WD being hooked back up.

When the rain dried the 180mph blast left some interesting aero tell tale details on the car which I share for those of similar disposition

This first pic you can just make out the airflow coming off the top of the Ruf pillar infill and it heads at an angle of about 70 degrees to the middle of the rear screen -this is exactly the same the other side:

This next one shows the airflow coming off the top of the light and heading over the plastic arch and down towards the door sill:

This one show the air coming back up from the sill level and over the top of the arch then it tucks in under the spoiler:

This is my favoutite -you tell me what it is doing ? there are no other signs on the rest of the spoiler !!

Whilst on this subject, what do we think of this:
The guy has vision, but would this "mod" be effective in generating some downforce ?

Basal Skull

TB
I think those lines we get are neat too. If I ever get to drive at those speeds in my life I think I would take pics too! The air flowing and run-off of the wet spray over a Porsche when you follow one in the rain is quite different from other cars I've noticed. Usually a couple of small swirls near the wheels on each side. I have no knowledge of this type of thing though...

Ryojo

Jean

Interesting post TB.

I would think that the dirt sticks only on the hardest hit parts and the ones that are flowing upwards.. It would be scary to find out that there is no air flow going into the rear deck lid and grill, whereas they are supposed to be an area with good airflow.

The most interesting finding as far as I am concerned is the rear wing picture that TB posted of his car, where it shows the great high pressure zone where the air intakes of a GT2 wing would be, the amount of fresh air being ducted into the turbos from there could be substantial.. definitely my next mod.

LAT

TB; can you describe the difference if any between the original rear wing and the bi-plane both with AWD?

TB993tt

Thanks for replies
Jean, yes I see what you mean, the airflow into the GT2 spoiler must be fierce - why Porsche have carried on the rear spoiler intake principle since...

LAT
I think it is probably the 4WD which makes the difference in "feel" at the high speeds -Stummel suggested that is the case - we only touched ~160mph (it was very wet) so his conjecture was based on his experiences in other 2WD 911s.

With 2WD the 993tt is very responsive which at lower speeds (sub 120mph) is great fun but as the big numbers roll on (certainly on my mine) it felt bloody scary. Now with 4WD intact and the Turbo S aero it felt planted up to ~180mph.... I have been looking at loads of Porsche data recently and the one which really stands out is the CGT - the Sport Auto Supertest checked out the CGT aero in the wind tunnel and at 124mph it had 45kg downforce at the front and 40kg at the back, rising to 400kg total at 204mph ! By contrast the 993tt has 27kg of lift at the back at 124mph (they didn't measure front lift when the 993tt was tested )

One can really feel these numbers at high speed, the CGT at 193mph felt like it could take a decent corner whereas with the 993tt you are happy to keep it in a straight line +/- 3 metres

LAT

TB

The 993tt has more lift than I would have guessed, wonder what lift would be for the S version?

I have the Aerokit Ib kit on my C4 Cab and it is a one handed drive at 165 mph
(265 kph) . My other hand was holding the camera and taking pictures.

Sounds like the 993tt is much like my CTR-2. I had it up to 190 mph (306 kph) and it was +/- 3 metres as you describe and not looking for any corners. Mine has an optional adjustable Gurney wing which will assist in improving downforce at the expense of top speed. I have it set at 20 mm of lip and it can go to 60mm.

Geoffrey

Jean,

Don't worry about air not going into the engine via the air vents. On my racecar I have a thin lexan rear window and at about 100mph, there is enough force to push the window towards the inside of the car by about 2". There is about 15lbs of pressure (crudely measured with a bathroom scale) required to deform my rear window. The rear air vents are right in the path of this air stream so I don't think you have anything to worry about.

Jean

Geoffrey

You are absolutely right and I should have thought about it as I have the same phenomena in my car (rear lexan too). It is quite amazing to see how it flexes to the inside, does not even need very high speeds. Didn't think about the scale trick though

I will be adding air ducts through the air intakes on the GT2 wing into the turbos, and two scoops ala GT3 on the rear grill to add some fresh air into my engine, a trick that a good friend of mine saw on a race 993 GT2 in Italy. Geoffrey any good sources with many choices of 3 layer racing air ducts and their scoops?

TB the 993GT2 wing with the "banana" addition has been estimated to give the 993 around 200 Kgs of downforce at 200 kph in a magazine test (and then people worry about moving a few lbs from rear to front on 911 race cars!). When your rear aerofoil goes higher, you are increasing greatly the downforce, up to a certain level, and then it does not anymore, unless it has a huge angle of attack. The Turbo S wing in Macieg's picture might be doing some good at high speeds, but the instability that we feel at high speeds in our cars is more due to front lift and tramlining, than rear lift IMO.

pmf

Hi Jean, might tramlining be due to too wide front tyres? Top speed and handling requires completely different setups, IMHO. Pmf

Jean

A mix of things in my opinion, but the main reason for tramlining is suspension geometry characteristics in the 993. The 996 does not suffer from the same symptoms much despite fatter front tires.

As a rule of thumb, tramlining is increased by fatter tires, more toe out and more camber.. All of which are bad news for highway speed driving and reduce top speed performance....but on the other hand, great for track driving. The lighter front end at speed, due to the front lift coefficient of our cars, makes it even worse.

sfl993t4

I was looking at the modified turbo S wing that TB posted. Has the centre section of the standard S wing simply been moved upwards using the carbon end-plates?
As Jean says there could be some benefit from moving the wing higher into the airflow, but it may be that moving it a small amount (relatively) has little effect. I think that a more important factor is likely to be the efficiency of the aerofoil section which looks quite thick. It would be interesting to modify a TTS wing with a better section at some point.
Steve

bb993tt

This is something similar, and is from the 1995 Total 911. Wing is supposed to be a Ruf design, but I've never seen it in their catalog.

LAT

Steve Beddor was telling me that he has a strain gauge set up on his rear suspension to measure travel and thus downforce. He runs a double rear wing and extended front wing on his CTR-2 to get enough downforce at 180 mph on high speed tracks

Have any of the German race teams, Manthey etc, tried strain gauges on four corners with high speed autobahn runs we could of gotten a lot of information on different set ups. Unfortunately I don't think there is much research being done on the 993 series any more.

Geoffrey

I have suspension sensors on my car and can use a math channel to estimate downforce from the airflow across the car. One item of interest, there are a number of wind tunnels here in the southeast that will rent time with an engineeer. One place is about $680 for 2 hours with an engineer which is cheap. I'm in the process of making plans to take a customer car there over the winter.

Felix

tb993tt, I had much the same road grime traces on my car after returning home from Germany but didn't give them much thought other than enjoying how they followed the widebody's curves.

I under what the air pressure is over the sunroof? Or is it cabin pressure that causes the odd impromptu sunroof delete at 180+ mph ?!

Isn't there an Australian website that has some animated GIFs of the 993tt in a wind tunnel?