trebor_quitman

Hello all,

My understanding is as follows, say I were to go crazy and cut out my entire drivers side floor pan on the 944, and weld in a cavity, a box open to the road. That would produce downforce as long as the amount of air passing under the car is relatively small, and the size of the cavity proportionally large, Am I right? Or woulld the cavity require a method of venting back out from under the vehicle? Is that what difusers do? Let me quickly say that I have no intention of doing this to my 944, I just think allot and like to get a broader view point on "what if", and the way the world works. I do have an application in mind that I want to apply this knowledge to. Thanks,

Geo

You would have to vent the box to produce downforce. Otherwise it could conceivable build pressure eventually and casue lift.

An open box at the rear like some sports racers have will produce downforce because the air going underneath the car will have to expand to fill the void. This creates a low pressure area below a relatively higher pressure area (above the car) and thus downforce.

A tunnel or venturi is a much cleaner way of accomplishing this, but it's done the same way. A void is created in the underbody and the same amount of air must expand to fill this void causing an area of low pressure. As the air expands it must flow faster. The venturi works better than the open box at the end because the airflow is cleaner.

Now, related to a 944.....

If you installed an undertray up to the firewall and then cut the floorboard and raised the floor say 4", the air traveling under the car will have to expand to fill the void and you will create downforce. It will work even better if you create a undertray under the raised floorboard and under the suspension. Or you could create tunnels. Or you could simply create an undertray the length of the chassis and install a diffuser at the rear (ala F1) which is essentially a very short tunnel.

Florida951

A venturi works by reducing the area the air has to go through and increasing its velocity according to Bernoulli’s incompressible flow (bellow Mach 0.3). Then the diffuser is placed at the end of the venturi, increasing the area for which the now accelerated air goes through at a fast enough rate to cause the pressure to down without decelerating the air too much and creating high pressure, but also not too fast that it will cause the diffuser to stall.

RedlineMan

Hey;

If you look at the underbody of a Turbo, Turbo S, or 968, you will see some of what Geo is talking about. There are undertrays everywhere that reduce drag, which is essentially the same as keeping air flowing faster. You will also see the brake duct "venturis" in the lower bumper filler panel. These small depressions in the plastic give high pressure air a relatively easy place to escape to, flow it in the direction of the air fences on the struts, and thus into the rotors for brake cooling. The Turbo and S2 also have the rear lower spoiler. Far from being merely cosmetic, it actually cleans up and accelerates air flow out from under the rear of the car, helping to cool the trans a mite while also reducing lift at higher speeds.

In the old days (like the 956/962), cars were designed with full length venturi tunnels. Nowadays, most prototype race applications emphasize gaining front downforce by managing air going over the nose and reducing air going under it. In the rear most cars have a venturi that accelerates and a diffuser that cleans the flow of air exiting the rear of the car. This creates low pressure relative the air passing over the rear wing. With this setup, the wing can be designed with FAR less drag, and in conjunction with the venturi underneath, great amounts of downforce can be made with minimal drag.

Watch a Euro Masters Touring Car race sometime. Look at the rear bumpers underneath. Venturi City!!! WAY cool stuff.

mossy

Hi Roberto,
I read somewhere that the plastic undertray for the 944 engine compartment employs 'ground-effect' aerodynamics (the same technology that makes F1 and CART's stick to the track). I don't know if it's true or not, but I figure that one of the aspects of developement for a manufacturer such as Porsche is aerodynamic stability, so if you were to alter the balance of airflow under (or over) the car, you would have to compensate somewhere else to maintain the cars handling balance while improving high speed grip.
Having said all that, I'm intriged to know what would happen......

Mossy

Geo

OK so you explained in more detail what I explained in 8 words.

Actually though, ground effects work somewhat like an upside down airplane wing where the lift occurs because the airflow over the top of the wing is faster than that under the wing causing a low pressure above the wing. Reverse for a race car wing.

inactiveuser92616

ground effect and tunels are nearly useless on a street car. These technologies require ridiculously low ground clearance. Here is a link to a very informative article on it aerodynamics

Going for a perfectly smooth belly might help by eliminating the lift though.

Geo

There are all sorts of ways to generate downforce. A wing works as described above.

Ground effects, OTOH, can be achieved several ways. As I said, some cars (I think S2000 race cars) simply have an open box at the rear and suddenly the air passing under the car must expand to fill the open box. This creates low pressure.

You can shape the underside of a car like the top of an airplane wing. This will work as well. The Lotus 77 was designed this way. What the aero experts at Lotus found out was that the wing shape wasn't the way to go with a race car. It was too sensitive to pitch as well as other matters and it also created more drag.

After some wind tunnel work, they found that venturi tunnels under the bodywork created more downforce with less drag, and it was less sensitive to angle of attack to boot.

At the front of a car you can attach a splitter. Splitters are very effective. You can tune the aero balance with the length of the splitter as cars respond to them quite well. I have first-hand experience with this in my last race when our previously light understeering Sentra SE-R suddenly acquired snap oversteer after the addition of an airdam with a 3" splitter.

If you look closely at the Team RTR Nissan Sentra SE-R from Speed Touring Cars, you will see a deep splitter with an undercut front bumper. I've only seen this on the Team RTR cars and the Nissan BTCC winning Primeras. I strongly suspect that Nissan has uncoverd some serious benefit for doing this. Exactly what this is I can only speculate.

Anyhow, if you are thinking about undercar downforce, think in terms of pressure differentials and the things that influence them.

Geo

I wouldn't take that site as gospel. In glancing through it I was able to spot several glaring errors.

mossy

Still very interesting, though.
With the undertray, the way that I interpretted the information was that by directing air down from the engine compartment makes it occupy a greater volume (with the added bonus of reducing the engine temp ). I now understand that ground effects is (are?) a different thing, but I figure that altering the downforce or lift at the front of the car has a direct effect on the rear of the car as the dynamic balance is altered from the original design. This may give favorable results (such as better high speed grip at the front, rear or both), but also may have unwanted side effects (unpredictable handling at high speed, lower top speed, increased fuel consumption or even just increased wind noise).
Production cars are set up to be all things to all people, so I'm sure you can can change the design to fit your own specific needs.