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Brainstorming Aero ideas for Windtunnel II

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With another wind tunnel test approaching it might be helpful to throw out some ideas that the guys can test; this thread will provide a place to share those ideas.


I've had a couple ideas rolling around in the back of my brain for a while, things like splitters with airdams on the leading edge, trying to increase airspeed on a flat floor, etc. All have more to do with the underside of the car.


Here are my ideas: 

1. Splitter with airdam on the leading edge. I think this will work better for autox types of situations where the speed differential of the air going under the splitter won't be as significant. Since the windtunnel air speeds are pretty low where you guys are going, I think this would be a good spot to test the theory. I think Cameron actually has this setup so that all he has to do is bolt it on. Not sure if he will be part of this though. This was recently tested in Race Car Engineering on a Porsche, but the airdam they used was still 3-4 inches off the ground and not surprisingly it produced worse results than the splitter alone. I'd like to see it tried with an airdam 1-2 inches off the ground. A variation would be adding dams to the sides of the splitter. This could be fairly easily tested by taping a cardboard strip to the edge of a splitter, with some triangular cardboard braces on the backside to keep it from folding under pressure.


2. Splitter with diffuser. The NASCAR COT's used a diffuser after their splitters. This diffuser was about as simple as it gets. It was just a piece of aluminum mounted at an upward angle at the end of their splitter. Their splitters were very short, and the diffusers were too. If there is room, it would be interesting to try a 12 or 18 inch long piece of cardboard at the back of the splitter, just tape to the splitter and then tilt up towards the rear. The COT had a very rough bottom so you wouldn't think that a diffuser would work, but they were talking about 80 lbs of downforce difference based on the angle of the diffuser in a video I watched. Variations on the angle of the diffuser could be checked.


3. Ground effects. It's really tough to do an aero bottom on a Z because of exhaust and drivetrain clearance, but it might be possible for the guys with larger diameter wheels to do it, particularly if you have very stiff springs. I know 74_5.0L_Z is running at about 2.75" from the bottom of the front crossmember to the ground and not bottoming with spring rates in the 400s IIRC, so if you have 5 or 6 inches to the lowest hanging piece, you could drop a floor down a few inches and still not scrape. Flat bottoms are usually mandated by rules, but aren't the best option. Contoured floors are better and obviously more difficult to fabricate.


3a. Contoured floor. Splitter with diffuser into a second venturi into a rear diffuser. Basically we're talking about aping a LeMans car, mulsannescorner.com has plenty on this if someone wants to try it.


3b. Flat floor. Much easier to make is the pro here, and theoretically much better than the rough bottom with the drivetrain and exhaust hanging down. It would be awesome just to test the flat floor/diffuser setup, but I have a couple variations. The idea behind the contoured floor is that you're creating a venturi by getting the floor closer to the ground and narrowing the opening for airflow, causing the air to speed up. If you have a flat floor, what is to prevent someone from using fences on the bottom to narrow the airflow passage on the bottom of the car and get the same effect? Essentially I'm talking about more airdam material, and making a venturi shape on the bottom of the flat floor. This could be tested with aluminum 1/2 x 1/2 x 1/16 thick angle (easy to bend to shape) double sided taped to the floor and then the cardboard fences could be taped to the angle. A further variation would be to add air from in front of the rear tires to the venturi. I think there would be a decent amount of airflow to tap at that spot, particularly if there were dams pushing air out to the side of the car behind the front wheels, as I know Mike Kelly and others either have or were planning to have.


All of the above can be tested pretty cheaply and easily. These drawings are not to any scale or anything, but I think they might help to conceptualize the overall ideas. Red or blue is a fence that could be tested with cardboard and actually be put on the car with airdam material (plastic lawn edging is what I would use).


4. Spoiler/wing combos. It is pretty well known that a spoiler and wing can be more effective than a spoiler or a wing alone, but I don't think there is a hard fast rule about what size spoiler makes the best result, so that could be tried, in particular testing the amount of increase in AoA on the wing before you get separation on the bottom side with and without the spoiler would be helpful. This would probably be useful only on the car that was tested, but if my car were going in there I would definitely want to do this test.


5. Front fender contour. I doubt anyone will do this, but I'd like to see a Z with the front fenders cut and folded in behind the wheel to the wheel well, imitating what you seen in Spec Racer Fords, Can Am cars, etc. It's not a new idea and we know it works, but I'd like to see what the result is. Would require cutting up a set of fenders and making a fender liner so that air didn't get past the opening and trapped in the back of the fender, but fenders could be grossly cut and cardboard used to push in behind the wheel and make a crude fender liner as well.



6. Fender top vents over the front wheels. Preferably alligator tooth style.


7. CFD. Anyone ever considered putting this money into a CFD model? Not sure how much that costs to do, but if you had a CFD model, you could test infinite variations without going back to the tunnel.






Edited by JMortensen
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Yeah, I was thinking of putting a little piece of stainless on that part, or just painting more Rustoleum over the scratches from time to time. The way mine will be set up with the tires hanging out 6" past the doors, the whole side of the car is probably going to get sandblasted.

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  • 3 weeks later...

Here's another idea, since the first ones went over so well. Some Miata guys are running a vertical piece of plastic bent around the front end with a hole cut for the rad duct. This super simple idea removes all of the hassles with the shape of the valance and the turn signals etc where they (don't) cover the front wheels. It also would give a chance to try different heights for the rad duct, which I think would change the pressure on a splitter. Higher the hole for the rad, more pressure on splitter.


The Miata guys did some CFD and found this to be advantageous as compared to the stock front end plus airdam. It would also give Mike Kelly a chance to play with some more Kydex.


Pic here:


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Kydex is a plastic that you can heat up and bend. Popular with gun guys who make holsters out of it. I don't know that it's necessarily the best material to use for this, but I know Mike likes it and uses it all over his car and it would be easy to form.

Edited by JMortensen
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  • 2 months later...
  • 2 weeks later...
  • 2 weeks later...

Love it. Can't wait to see the installed pictures. What thickness did you use, and what is the weight of the completed dam?

Honored... Thank you..


I think it is .125" ABS.  Maybe 3-5 lbs worth?  Not much.  I made an Aluminum former (1/16" angle aluminum with 3/4" legs) to reinforce the front and follow the profile of the hood, then relaxed it all with the heat gun.

I also left the fiberglass airdam as a structural support.    I wanted it to be strong enough to hold up, but still able to break-away should the need arise.


I was having problems with my hood latches popping before (and over cooling) but now I have much less intake air and hopefully less aero-lift.


Leaving Thursday for NASA Competition school at Inde, Willcox AZ so it will get the full trial by fire.


Right now I have it about 3" off the ground and it is rubbing on some bumps even with my 425lb front springs.


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Great ideas so far.


What about finding the optimal place for a hood vent? If you could locate a good low pressure zone on the hood, a proper extractor could be very helpful in reducing lift. If the location ended up being behind the radiator, a nice shrouded exit behind it under the hood feeding the extraction vent would be a nice addition. Should be easy enough to whip up with the kydex or abs. Thinking something a la c6r.




As far as the flat under tray goes, have you seen this DIY attempt? With the plastic rain gutter he ended up adding it was very effective, simple by design, and the wear edges were easily replaceable. A great home brew IMO.






I agree a nice wing with spoiler would be nice. Hard to beat a nice straight trailing edge with a gurney flap. I would start with a simple piece of aluminum on the trailing edge of the car attached by hinges with turn buckles to support the edge of the spoiler. This way the AOA is easily tested and changed. Different lengths could be pretty easily swapped out as well. I assume due to the gas tank location on a s30 it would be best to have the diffuser hanging out past the rear of the bumper by a foot or two to allow a decent extension and room for the proper AOA. This would also dictate a longer spoiler, and for the wing up rights to be longer and laid back some.


Also, have vortex generators been tested for flow separation over the hatch? This could really help the aforementioned wing + spoiler + diffuser combination. Knowing where the separation occurs would be also benefit where to mount the spoiler. If the air is already separating by the end of the window on the hatch, mounting the spoiler right under the windows edge should be better than at the end of the hatch.

Edited by 1vicissitude
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On track this past weekend the new spoiler thing worked great.
Felt much less aero lift and understeer.  3" off the ground worked just fine even with me hammering across burms and over rocks.


I am still getting warm coolant after 10+ minutes at "War Power" even with that huge aluminum radiator.

Once I slow down the temps come down SUPER fast. 

  • Going to try a restrictor instead of T-stat.
  • Going to vent MORE from underhood via additional hood vent and bigger fender vents.
  • Maybe a front / bottom plate thing?
  • Looked like i was up around 210f with an 18psi cap and 192 T-stat.  About a cup of coolant in overflow from entire weekend.
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What have you done to get the air out from under the hood? I see the turbo hood scoop which typically lets air in, not out. Any other venting?


That DIY VW undertray is right up my alley. Pinch the center of the side fences in a bit, and you essentially have my underbody venturi idea realized.

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What have you done to get the air out from under the hood? I see the turbo hood scoop which typically lets air in, not out. Any other venting?



Jon my turbo hood scoop is installed backwards with a little kicker at the front of the naca duct. 

It should only be acting to vent OUT. there is no "catch" to pull air in.

I have 3" holes in each fender, probably need more like 1 square foot on each side?

Was going to add another vent to right side of hood too.


Interestingly, I am still popping hood latch, and also the 2 Quik-Latch Minis on each side.

I do find cooling seems better once I crash over a curb and the hood finally "blows" open to the manual catch.  There must still be significant pressure there.


The tunnel feels hotter now also, hotter than I remember.


Edited by duragg
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You might try just removing the vent trim off of the hood and letting the large hole do its thing and see if you can tell a difference. I think the gurney would work better at the front of the trim, BTW. If you had a whole lot of pressure under the hood you wouldn't need to hit a bump in order to get it to lift up. Stock hood only weighs 45 lbs or so IIRC. Maybe just relatively stagnant air is the issue, although you would think that stagnant air = pressure and that would push the hood against the stop. Regardless, a rad exhaust duct would probably help a lot. Is that possible for you with your rule restrictions?

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I think NASA ST3 I can do about anything I want, which is why I chose it.  "To showcase my ideas".


You are saying pull that whole black business off and see how that works.


Was trying to dig up previous wind-tunnel data:  What is pressure under car?  (Low / High / just plain Angry)

  • Anything going INTO inlet duct needs a strong and proper motivation to LEAVE?
  • Sides and Top seem better than bottom?
  • Ideally we would want low pressure under hood trying to draw through radiator versus backing it up.
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