I just read the article that Jack reff'd to... Good reading (specially for a simpleton like me :D)

Basically, it states that any smoothing of the bottom of the car will reduce drag and will provide some downforce (Hayden also already mentioned this)
So if it works for cars ranging from a street Audi A4 to a track NSX, why wouldn't it be an improvement for ours?

If you change the diagonally positioned muffler for 2 seperate units that mount lenghtwise, you would have a good clearing to add a venturi/diffusor

My guess is that Ed is right. that is, Ed has information that you're on a path to learn by experimentation. The tricks you are considering are known tricks. And the smooth underbelly idea is good, but the air dam idea is better, with skirting on the sides to maximize the negative pressure under the car. The good news is that we know what maximizes a track car's aerodynamicity (did I just invent a new word?). The bad news is that you must make your car look like a brick to achieve it. Your car is much more beautiful with the "fuselage" sides. The brick shape is different and ugly. In my humble opinion.

Board: A diffuser allows the air traveling underneath the car a place to expand and decelerate back to road speed as well as providing wake infill. As the air enters towards the front of the car it accelerates and reduces pressure. There is a second suction peak at the transition of the flat bottom and diffuser. The diffuser then eases this "high velocity" air back to normal velocity and also helps fill in the area behind the race car making the whole underbody a more efficient downforce producing device by reducing drag and increasing downforce.

Understand that a true flat bottomed car (one without a diffuser) will produce downforce in and of itself when run in rake. Essentially the entire flat bottom becomes one large diffuser. It too has two suction peaks, one upon entrance, the second at the trailing edge of the flat undertray. A diffuser acts to enhance this underside suction, it acts like a pump, encouraging better flow under the car.

One thing to note is that the rear wing interacts with the diffuser "driving" it. The proximity of the low pressure side of the rear wing encourages better flow through for the underbody
http://forums.pelicanparts.com/uploa...1095175090.jpg

Yellowbird:
OK....so the platform has to be incredibly stable for this to work ( 2000 lb/in springs or more !) ....no pitch and no roll allowed....and certainly no 4" ground clearance...more like 1/2".

All things you said are true...but I take exception that a grounds effect car reduces drag...if the car is sucked-down onto the road...."drag" will increase....

--Wil

To motor racing engineers, wing might be a good solution to lift, but still far away from what they really want. A typical formula one racing car corners at around 4g lateral acceleration, that requires substantial downforce to keep the tyres firmly on track. Install a huge wing with high angle can satisfy this requirement, but also deteriorates the drag coefficient.
In the 70s, Collin Chapman (again) invented a completely new concept to provide downforce without altering drag - Ground Effect. He incorporated an air channel into the bottom of his Lotus 72 racer. The channel is relatively narrow in front and expand towards the tail. Since the bottom is nearly touching the ground, the combination of channel and ground forms virtually a closed tunnel. When the car is running, air enters the tunnel in the nose and then expands linearly towards the tail. Apparently, air pressure is reducing towards the tail so that downforce will be generated.
Ground Effect is so superior than wing that it was soon banned in Formual One. In 1978, Brabham's Gordon Murray tried again with different means - instead of expansion channel, he used a powerful fan to create low pressure near the tail. Of course FIA banned it again.

Ground effect is not too suitable for road cars. It requires the bottom to be very close to the ground to form a closed tunnel. For racing car, this is no problem. But road cars should have much higher ground clearance to suit different rough roads, up hill and down hill etc. This greatly reduce the effectiveness of Ground Effect. McLaren F1 road car followed Brabham's trick by using 2 electric fans to create ground effect, but honestly speaking, no tester had ever praised its down force. Dauer 962, a so-called "road car" but it is actually a road-legal Porsche 962 endurance racing car, use conventional air-channel ground effect as the race car. Adjustable ride height allow it to run in rough road (slowly) and make good use of Ground Effect in Germany's Autobahn. Nevertheless, it can barely generate 40% downforce of the racing car
http://forums.pelicanparts.com/uploa...1095176286.jpg

undertray
 

We can also reduce the influence of underside airflow by covering the car's bottom by a smooth undertray, as shown in this Ferrari F355. This avoid turbulence and lift.
http://forums.pelicanparts.com/uploa...1095176420.jpg

Jack,

Good thread, bringing all sorts of opinions forward.

I am sure you and Tyson are sufficiently independently minded to run the idea to conclusion, however you choose to do it.

Even complex solution begin with a single thought - here is an example of one of mine, from another lifetime, when I had significant resources at my disposal, but it did grow from a single thought and a bunch of unanswered questions.

http://www.mulsannescorner.com/allardj2x.html

One other test procedure that you might consider - a car like yours is rpm limited at Willow Springs, perhaps try the experiment on a car with less h.p., one where Vmax will be aero driven, drag-v-hp.

The day we were there in Jan, my car would pull the limiter into T8, and about 800 rpm less into T1, despite being in 5th since well before the pit in. A reduction in drag will be far more obvious on the top speed of a car that is drag limited. The influence in that case was the wind - a variable I know, but with about 150 - 170 hp the test should show some results.

You could then have some expectation of the same parts installed on your own car, plus end up with discrete 2 data points, a luxury many experiments never enjoy!

Alternatively reduce your rpm limit (electronically) until your Vmax is drag limited and perform a back to back with the parts.

However you do it, perservere and enjoy the process, you will for sure learn something, either to use or avoid.

Regards

Hayden

I say try it. What have you got to lose? Some cool fab time? You already have a lipstick cam, right? Put it under the rear and observe how the wool strips flutter w/ and w/o the contraption. Smooth *upturned* strips will be your friend. Keep us posted and good luck.

Counter-intuitive, perhaps...in light of what I'm saying here...but look at the Ford GT web page or similar data...and notice that this car uses added-on diffusers at the very rear of the car.

If you can overcome the blocking effect these may have on the exit cooling air from the fan blowing air over the fins....you may have something here......

---Wil

i've posted this before on this subject:

here is a well respected book on race car aerodynamics.

<A HREF="http://www.amazon.com/exec/obidos/ASIN/0837601428/piombocom-20"><IMG SRC="http://images.amazon.com/images/P/0837601428.01.MZZZZZZZ.jpg" border="0" alt="cover" hspace="3" vspace="3"></A>

i believe ground effects were banned in f1 due to the danger of the sliding side skirts losing their seal with the ground. this would cause an immediate loss of downforce from ground effect!

The more posts, the more mis-information.... I don't know where to start, but one thing I can say is that any smoothing of the underbody will help certain figures of merit at least somewhat.

Jack -

Please post these pics of the front of your car:

Head on
Head on and low
Underneath

---------------------------------------------------------------------


Having spent time at a wind tunnel (Boeing, 747, 777) and working in the aerospace field (airplanes) as an engineer, I can weigh in on this for you --

I can virtually guarantee you will accomplish but one thing by using sheeting - reduction of turbulence in the front and middle portions of the underbody. This is not a bad thing, just don't expect measurable results.

*Your main goal must be this - reduce the amount of air going UNDER the body.* This is critical.

Reducing turbulence in the rear will be a hard trick; creating downforce will be nearly impossible. You just do not have room for tunnels which you would need -- these increase in height towards the rear to promote that all desirable 'suction' gives you the mechanical grip.

On a 911 we have the additional 'problem' of air cooling - air is coming DOWN off the engine, effectively trashing many good ideas at reducing lift and turbulence in the rear.

So, I'd like to see those 3 pictures:

Head on
Head on and low
Underneath


and we can go from there.
We can come up with something that truly works!:p

What doubly stinks is that a rear diffuser would be nearly impossible due to the exhaust location - bummer!

your solution sir...
http://forums.pelicanparts.com/uploa...1095182910.jpg
;)

I know from the 964 that I had - the bottom was virtually smooth all the way back. I would try and modify the underbody panels to try and fit as much as possible. And the engine tray that I had on mine had two small NACA ducts. My car had no problem keeping cool at speed - but stop and go was a whole different issue!

Great information, here. Thanks.

To reiterate, this is a no-resources project, pretty much. I'm looking to see what a couple of trips to the hardware store could do to improve my laptimes.

For our baseline, here's what I've got now.

Head-on, 16 inches up:

http://forums.pelicanparts.com/uploa...1095186469.jpg

Head-on, ground level:

http://forums.pelicanparts.com/uploa...1095186525.jpg

Here's as good a picture as I can manage of the belly-pan, right now. I added approximate measurements of ground clearance at the beginning of the sheet, the front axle line, and the rear axle line. (Keep in mind, my belly pan has an AC condenser that lowers -- and flattens, somewhat -- a normal 911's profile.)

http://forums.pelicanparts.com/uploa...1095186595.jpg

In terms of blocking air in front, I have one of these, that I could modify to fit my bumper. Removeable aluminum sideskirts are also a possibility.

http://forums.pelicanparts.com/uploa...1095187268.jpg

From what I'm reading here, it seems that I could go in two directions, initially. One would be to continue my flattening of the underbelly with a little (maybe-helpful-maybe-not) diffuser in the back. The other would be to drop down my front splittler and do some skirts to control air coming in (and going out) on the sides.

My guess is that the splitter-and-skirts solution would do more for me, but would increase my frontal area and slow me down somewhat in a straight line?

More specifics. Here's a look at the rear of the car, with the pieces made more visible.

http://forums.pelicanparts.com/uploa...1095189841.jpg

And here are some measurments, in a (not-to-scale) drawing.

http://forums.pelicanparts.com/uploa...1095189904.jpg

Jack, I think this is a great idea. Maybe use your DL-90 with RPM input to record RPM's/GPS speed at a known point on one of the tracks you run. There's a new Beta version of Trackvision out, you could couple the output of the DL-90 with the video feed from a low-light lipstick cam mounted under the car with some yarn tufts. This would allow you to visualize the flow as you move around the track.

By the way, before you fellas cut a NACA duct anywhere on your car, there's an interesting article by Peter Garrison in the recent issue of FLYING magazine that basically says that "submerged ducts" were NOT recommended by the original guys at NACA for oil coolers, carburetor air intakes, or anything requiring a high velocity airstream.

These are the original references: Charles W. Frick, Wallace F. Davis, Lauros M. Randall and Emmet A. Mossman, An Experimental Investigation of NACA Submerged-Duct Entrances, NACA ACR-5I20, November, 1945;

Emmet A. Mossman and Lauros M. Randall, An experimental investigation of the design variables for NACA submerged duct entrances, NACA RM-A7I30, January, 1948;

Alvin H. Sacks and John R. Spreiter, Theoretical investigation of submerged inlets at low speeds, NACA TN-2323, August, 1951, pp. 49. http://naca.larc.nasa.gov/reports/1951/naca-tn-2323/

Excellent information provided, Jack - here is my answer.

#1 priority is to *reduce* the ground clearance of the front end, ie
the splitter you have needs to be 'lower to the ground' by about at least 1 inch or ideally (but not realistically for street use) 2 inches.

You must reduce the air getting under the car - I see some turbulence inducing elements that are not taken care of - ie the A/C nerf bar with torsion attach points.

http://forums.pelicanparts.com/uploads3/Underbelly1095186595.jpg

Fantastic job of smoothing the undertray! Well done.
I propose that nothing else should or could be done in the rear - you are left with opposing forces in this area, ie:

1) Air flow over & through engine for cooling vs. airflow mgmt under engine. These are at odds w/each other - what to do?

2) No room for a diffuser - and a diffuser just cannot work in this above mentioned (1) environment.

Bring that front lip/splitter down in such a way so that it is the lowest point of the front 1/3 of the vehicle. Keep the air from getting under the car - then you have optimized your 911 and will have one hell of a great setup - the best you can do. You are already 80% of the way to this point -

Nice to have those dims to play with. So if I were going to try something I would not try the blocking all air route- the downforce you get from this is very sensitive to ride height, and I believe you would need stiffer springs to make this work. It is very effective at making downforce, but I've found it can be too effective- in my case the car would suck the front down and take the travel out of the suspension (increasing downforce as it did and moving the center of pressure forwards- the car was very scary). Thus I think you’d want to make a number of other changes to make this work right.

Looking at your car I would change the height of the belly pan about a foot behind your jack point to about 3” from the current 4” or 4.25”, probably with an aluminum plate. I’d then connect a second aluminum plate to this (the diffuser) going up at the max angle you can get away with- you’ll probably basically touch both the heat exchanger and the heat pipe, and end up with about 5 degrees. I’d put about 1.5” side skirts, bugger in the diffuser.

I believe this setup would create real downforce right where you want it (just behind the CG) without making the car un-drivable as the ride height changes. It would be banned under most modern rules, as they often don’t let the diffuser start forwards of the rear axle. Maybe that’s an indication that it’s probably fast?

Perhaps Hayden or someone else that’s tried this could weigh in with an opinion on this, but like I said I’ve tried the basic low splitter/ diffuser approach others are advocating, and I think that would require you to stiffen the suspension up in order to insure the car doesn’t get scary as the ride height changes. I’d clean up the entrance to the underbody at the front while I was at it, btw.