I recently tore the front of my car off, partly to make lots of tasteful modifications but mostly to get rid of the last of the rust. Behold, my artistically meritorious exercise in art brute.
I count pink fiberglass, red sealant, white factory paint, a brown fender, black hood and trunk and bare metal. That's six colors!
More pictures and such here.
http://bengarrido.com/2013/11/15/highly-questionable-modifications-made-to-the-car-i-drive-every-day/
Nice, I've been thinking about flat bottom aero too. Good for those of us who don't have the spare power to plow wings and canards through the atmosphere.
LOL best mistranslation ever! 
I'm thinking there is going to be a "Poisonous and Evil Rubbish" stencil on my car ... 
The thing I'm really excited about is coming when the car is officially taken off the road. I want to build an aero tunnel opening through the floor behind the seats, extending through the trunk and opening out where the license plate used to be. I figure this might make pretty good downforce and cut the drag coefficient without changing the external appearance of the car very much at all.
A bigger diffuser is better, just keep in mind that they make most of their downforce near the leading edge.
An idea I've had is to use flexible (sheet ABS?) strakes so they can safely scrape the ground, so you don't have to remove it on the street, I think the outermost ones might bend inwards with speed due to the lower pressure inside though, so you'd probably have to splay these out a bit to compensate (also good for ground clearance).
You sure a diffuser would work better?
http://www.speedhunters.com/2012/06/for-my-ally-is-the-force/
As for flexible aero stuff to scrape the ground, I like the Rauh Welt path.
That air dam is entirely made from flexible rubber.
DaewooOfDeath wrote: You sure a diffuser would work better?
Well you said an "aero tunnel," which I interpreted as meaning a venturi tunnel. A diffuser is just a set of venturi tunnels, although I suppose it could mean a single big one too.
Edit: BTW I think a single big tunnel like that Evo has is absolutely the wrong configuration for your car. The downforce from one big tunnel is very high at high speed, but is much weaker at lower speeds, and more unstable than the downforce from many smaller tunnels (or a tunnel divided by many strakes if you prefer...same thing).
You want many small tunnels, because while the downforce doesn't ramp up so greatly with speed, it's more stable and will come in at lower speeds. Check out the one on loosecannon's wacky MG, that's closer to what you want.
What is a single big one called, like on that Evo? I honestly don't know.
It's interesting that the most downforce comes at the leading edge. One reason I wanted a completely enclosed tunnel was to make sure the center of downforce wasn't too far back on my car.
DaewooOfDeath wrote: What is a single big one called, like on that Evo? I honestly don't know.
Edited post above.
Here's loosecannon's diffuser. I'd say your tunnels should be no wider than these.
More pics here:
http://grassrootsmotorsports.com/forum/grm/scca-emod-mgb-gt-v12-build/26822/page2/
I just saw you meant flexible diffuser. Whoops. Yeah, no ideas for flexibility there if you want to avoid bending at speed. Maybe some sweet magnesium strikers. 
So just adding strakes makes the downforce start earlier? Interesting. There's really no reason I couldn't do that inside a tunnel like the one on that Evo, right?
I don't want to put a diffuser under the car because the rear suspension would be in the way.
Yeah you could do something like that Evo has and just add strakes to change the aero properties. You don't want the slope of the diffuser to change but flexible strakes should be fine, so that if they scrape a speedbump or something they'll just flex harmlessly instead of getting mangled permanently. You just might have to adjust the "camber" of the outermost ones (the two that any tunnel must have to work at all) so that they have a decent shape at all speeds.
Most diffusers start just behind the rear suspension.
GameboyRMH wrote: Most diffusers start just behind the rear suspension.
Aren't those typically the not very functional diffusers?
I don't think there's more than about 60 centimeters between the back edge of my suspension and the bumper. At a 7% incline that's, what? 4 cm rise? Would that actually make a difference?
You do want them to be as long as possible, 60cm isn't terrible but it's not great either...I'd go for an 8-9deg angle. The one on that RX7 is more of a show diffuser, but that's more because of the shape than the size.
Take what space you can get. Is lowering the whole diffuser a bit to try to get it under the rear suspension an option? (as long as you don't dip it below the car's floor level.)
Remember you can make the center tunnel(s) different and longer too, if they clear the suspension.
'Nother idea: You can have it stick out the back of the car
You've already gone down the wacky looks rabbit hole 
(You will lose some "kammback" effect with this though...not sure if that's the right effect, but what I mean is that air from the diffuser won't blend with low-pressure air behind the car. You want that because it reduces drag and boosts downforce. Ideally you want a smooth transition from the rear of the diffuser to the bumper / back end of the car, and even onto the underside of a spoiler if possible.)
GameboyRMH wrote: Nice, I've been thinking about flat bottom aero too. Good for those of us who don't have the spare power to plow wings and canards through the atmosphere.
That would be a good name for a race team.
Since youre already comfortable with it. make your diffuser from Fiberglass...Make up a simple mold so you can make a new one WHEN you smash it up. Fiberglass is relatively cheap, and somewhat flexible, so it may have enough give to avoid destruction on minor impacts, and is cheap enough to replace when you hit it on the curb while hardparking at White Castle.
Check this out. This is an excellent setup. Venturi tunnel exits flush with the rear of the car, near the underside of a wing.
There's an LMS-looking Jaguar race car that has a similar but even better setup but I can't find it. It has a spoiler integrated with the trailing edge of the car plus a high-mounted wing above it.
More goodies: Check out this article. VERY relevant.
http://www.motoiq.com/MagazineArticles/ID/3294/Project-FR-S-Rocking-Functional-Aero-from-Kognition-Afterhours-Automtoive-and-Seibon.aspx
I like that prototype, I was thinking of doing something similar with the trunk exit. I imagine a drag style flat trunk extension spoiler with a gurney flap a the trailing edge integrated with the end of the tunnels.
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