Jack, I have a brilliant solution for you.

Find a way to "reverse" the engine cooling fan so that it sucks air out from under the rear of the car and exhausts it through the engine lid grille. Combined with your side skirts, you will now have pukka ground effects just like the F1 "fan car", plus reduce the drag from the sloping rear of the car.

Until your engine overheats, anyway.

Will it overheat? I would think so, what with air being pulled past the exhaust system before it hits the cylinders. If not, then maybe this could actually be a serious suggestion . . . maybe if you wrapped or shielded or ducted air around the headers or something.

I'm assuming you don't have enough time on your hands to figure out a way to reverse the cylinders so that the intakes are on the bottom and the exhausts on the top, together with the reversed fan. Would make for nice long intake runners, plus think of all the weird underbody "ram air" you could use.

I'd better stop drinking.

According to Frere the engine tray of the 964 not only helped with aero because it smoothed the bottom of the car, but also because instead of the air from the engine fan blowing out the bottom of the car it was now exhausted out the back of the car which reduced the pressure under the rear and also helped with some other aero thing that I can't remember right now.

To further develop Banjomike's idea...

http://forums.pelicanparts.com/uploa...1098372001.jpg

If the two rails running under the center of the car flared out that may further some of that effect.

"According to Frere the engine tray of the 964 not only helped with aero because it smoothed the bottom of the car, but also because instead of the air from the engine fan blowing out the bottom of the car it was now exhausted out the back of the car which reduced the pressure under the rear and also helped with some other aero thing that I can't remember right now."



That also would help tremendously with managing air for a diffuser in the rear - air coming down from the engine messes up the concept of air exiting horizontally through a diffuser -

I was think about all these long aero threads the other day....

What we really need is a real smart student to adopt something like this as a grad. thesis/dissertation or as (more likely) an undergrad. project. A kid at Cal Tech would be perfect.

Trying one thing after another on a full size can help (JO's lap time decrease = case in point). But we really need is something more comprehensive (and I don't see PAG investing $$ in a 30 year old design).

Conceptual thoughts are fine but can never get you to where a physical model in a flow tank would or some real equations and numeric finitie element modelling. Somebody has to know somebody who ... knows a smart student at an engineering school. Your tax dollars at work....

You will never see a Cup car or RS w/ the engine tray in place

Exactly. Helps with the 'aero' -- kills the engine.

Not neccesarly. You can install ducting that will help suck air out of the engine comparment and create more air to go over hotter areas.

I haven't read all 7 pages, so please forgive me if this has come up before, but i remember a "story" about the POC banning all ground effects becuase of a local guy, Franz. Apperntly the guy gets contested as soon as his car rolls off the trailer because he finds any way possible to exploite loop holes in the rules (just like Porsche and everyother race manufacture/group has done). Back to the story at hand, he studied up on ground effects, and made a complete fiberglass underbelly for his 70's era 911 racer. I think it had a pumped up 2.7L, and during the first race with the ground effects, he was passing 3.6L 911's with obviously way more power. Now he was kicking ass, and the big boys with all the money were butt hurt, so it was banned.

Anything is possible, placing NACA ducts and luvers in the approperiate places will keep your motor cool, but for the speeds your going to be working at on the street, its kind of over kill, although its more noticable, the 3.8 RS wing should do the same trick, and not cost you a motor if you make an oopsy with the cooling.

Jack - I love the work you are doing to your car. I am planning some winter projects to get more speed, brake and safety in my 930. My mechanic has already put together an incredibly well sorted, fast car but I am a sponge for all ideas. Can you describe the fabrication of, material used, and method for mounting your aids.

Cool thread.

I was going to suggest reversing the engine fan, but John L beat me to it. Maybe some cold air ducts brought in under the engine and directed up between the cylinders along with a horizontal fan on top of the motor. Could going back to a stock decklid increase the speed of air across the decklid and create a venturi effect to suck air out of the engine compartment? It would have to be combined with a monster tall wing so you didn't create more lift at the rear. Might require some seriously insulated exhaust headers.

The rally drivers use some slick plastic/teflon type material under their cars for scrub strips. You could try some of that and get your skirts even lower. You could also add front spoiler and skirt extensions of a thin, stiff rubber or equivalent material with less friction that extends almost all the way to the ground. Something that was stiff, flexible, and maybe low coefficient of friction would be ideal. Maybe something like they used on the "sucker cars" in A-mod Solo competition a few years back.

I probably haven't added anything to the thread, but thanks for the brainstorming opportunity. Keep us posted.

I think one of the benefits of the cooling fan blowing down is that the engine stays clean, having the thing suck from the bottom would get it really dirty, and that dirt would not only cover the whole engine, top and bottom, but also the entire engine compartment and the trunk and back bumper. Just imagine if you got an oil leak at the mess a fine spray going straight out of the trunk grill at speed. Besides the whole back of the car being covered in oil, you'd really piss off everyone that was ever behind you because you'd cover their car with oil too. Good idea, but probably not very practical.

You definatly don't want to suck air up, but sucking it down wouldn't be difficult at all. Just use NACA ducts, they are the ones that Lambirginni (con't speel) used on all the old countash's. They start out narrow/ shallow and hour glass out and get deep. Depending on which way the air flowing over them creates a vacuum; facing forward (like an arrow) pulling air in, or facing backwards pulling air out. This done on the underside of the ground effects would be very efficient, expecially if you were able to make the underbelly low in the front and higher in the rear, creating vacuum in the rear of the car. That vacuum going over the NACA ducts should really suck... in a good way.

Jack

I see you had the skirts back in action at the alfa event. Any further observations or conclusions?

http://www.autobahnextremist.com/gallery2/fc6.jpg
These are huge -

I don't have hard data from the Alfa event, since my data logger appears to be broken. (I had the fastest time of day, though.) I tore off the front piece again, which is getting expensive.

Here's version 3.0 of the chin piece. I still won't have any good data, since the data logger will still be out of commission. But this version moves the ABS splitter up high, with an aluminum skirt down below.

The advantages of the new design include a shape that will encourage folding back under impact (as opposed to tearing off, with the splitter being so low), and also low replacement cost. Three pieces of 6"x24" .025 aluminum. $15 at the hardware store, and significantly less at a sheet metal shop.

http://forums.pelicanparts.com/uploa...1099948831.jpg

Looks good Jack, now carry it out to the whele well then up deflecting flow from the well area.
sort of like this
http://forums.pelicanparts.com/uploa...1099952278.jpg

That's actually the idea, down the line. The thing I cobbled together for tommorrow could be turned into essentially a one piece unit, all aluminum, with it going a little wider in front of each front tire.

The next iteration of my sideskirts will also extend out a couple of inches farther than the current ones do.

This picture shows how the side pieces are still a little short:

http://forums.pelicanparts.com/uploa...1098738472.jpg

I think that you are on track:) ;)

At least 3 of my wheels are on it, at least. ;)

most of the time any way

Once you get it all fixed up, you should remove it and see how far down your lap speeds fall....

There's a mag. article in this somewhere.

A good example!
 

http://forums.pelicanparts.com/uploa...1099980125.jpg
http://forums.pelicanparts.com/uploa...1099980148.jpg
http://forums.pelicanparts.com/uploa...1099980168.jpg
http://forums.pelicanparts.com/uploa...1099980198.jpg
http://forums.pelicanparts.com/uploa...1099980244.jpg

What's the function of the 'side curls' on each corner of the front? It's a popular part of 993 fiberglass kits.

For the very same reason endplates are used on spoilers. It prevents the boundary layer air from simply rolling around the car. They force the air to stop at a certain point and move along the side of the car in a more uniform manner. Most of the time the lower spoiler section will rise upwards along the length of the "fence".

Yellowbird, did those pics come from the Kinesis website? That car looks vaguely familiar.

Jack,

To elaborate on Porschekid962's accurate analysis, we use a similar design on our splitter. If the goal of said splitter is to create some downforce at the front, those vertical "end plates" force the air to remain on the splitter slightly longer, instead of spilling off to each side.
(More drag - more downforce.)

The horizontal parts of ours are angled up toward the rear at ~ 10 degrees, becoming dive planes. If anyone ever doubts the effects of SMALL aero changes, implement something like this, and then alter their angles, say from 40 degrees inclination to 5 degrees. To an experienced driver, it is like a different car!

Ed LoPresti

Porschekid, Yes they are from the kinesis WS, I was looking for a new set of wheels and found the box rockers and front splitter and think JO need to see it.

winglets, vortexes, and bears. Oh my!
 

Because of this thread I've been noticing that most 911 front air dams are lower in the middle and curve up at the sides. This would allow you to get as low as possible without dragging the corners while... cornering. The splitter, or the horizontal piece, would prevent lots of air from rolling off the dam and spilling under the dam. They also serve the purpose of preventing the air that is now flowing along the dam from rolling under the car. Finally any time you have something like a wing tip the air flow over the tip will form a drag inducing vortex that is proportional in size to the length of the wing. That is why air planes and F1 cars have the the winglets on the ends of the wings. The vortex off the large wing surface is blocked by the winglet and the vortexes off the winglets are much smaller. In the case of the front air dam with the folded up corners, without them the vortex might be directing spilled air under the car and generating drag. The vertical winglets also aim the vortex to flow over the roatating wheels which might even reduce drag from the wheels.

I wonder if the addition of skirts and dams would eliminate the need for the underbody tray? If there isn't much air under there to begin with, smoothing out the bottom might not matter as much. How much does it weigh?

Of course I may be completely wrong. I am remembering by aerodynamics class from 10 years ago, and those memories have been dragged through the mud of time and brain cell abuse.

banjomike's explanation of parastic vortices shed to the sides is what I have seen (and not on the Internet, either). They may serve multiple purposes. Only some serious testing can tell you what is the dominant effect.

I found a basic STL model of an SC on the web and thought it would be fun to do a quick flow analysis on it, given all the discussion in this thread. Unfortunately, the bottom of it is completely flat (Tyson would love it ;)), as I haven't taken the time to develop an accurate representation of the undercarriage. I also didn't put a "road" in the simulation, so the car is essentially floating in space in the analysis you see. If I have time, I'll make some changes and add "a road".

Nevertheless, it's interesting to see the low pressure areas (color coded, per the legend on the top left) and what not. As previously mentioned, the underside won't be very accurate since it's completely flat, and there is no road.

http://forums.pelicanparts.com/uploa...1100790968.jpg

http://forums.pelicanparts.com/uploa...1100790978.jpg

http://forums.pelicanparts.com/uploa...1100790986.jpg

What S/W are you using for this?

SolidWorks 2005 with CosmosFloWorks

Add vortices from the rotating wheels.....