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Old 01-07-2008, 10:04 AM   #1
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Aero on a Prelude

I have a 4th gen Prelude, it's lowered and has a wings west RS lip around front and sides. I'm planning on putting underbody panels on.

My question is this: the lip kit already redirects a good deal of air that would normally hit the tires, should I put a flat underbody on or should i try to continue that shape along the length of the car??

for example:

its hard to see, but the lip is lower in front of the wheels on both sides.

should i try to basically make a tunnel to keep wind away from the tires, or just put a flat piece of sheet metal over the entire bottom? also, do diffusers at the back help to minimize drag as well as create downforce? I'm not a ricer, trust me. i want to get 35+mpg out of my H-series... 26hwy doesn't cut it for me.

lastly, would my car's shape benefit from vortex generators or is it already low drag?

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Old 01-07-2008, 01:08 PM   #2
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doubt your car woudl benefit much from vortex gennies with the low-angle hatch it looks to have. We can hypothesize here all day but to find out, buy a roll of yarn and go for a drive with a friend and a camera. tape 6" long strands of yarn to he windows and trunk area. read up on seperation points and laminar flow stuff. as for the much ground clearance do you have? don't want to grind your diverters of on the asphault. I woudln't worry about it too much. smooth for now, you can always add them later.

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Old 01-07-2008, 05:47 PM   #3
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Welcome severedgein!
Yea, most fastbacks won't really benefit from VGs since the laminar flow stays attached longer anyway. It looks like you have a slightly obtrusive wing on the back there...ever think of removing it? If there are mounting holes left behind you could plug them with rubber plugs...just an idea.
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Old 01-08-2008, 04:34 AM   #4
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my ground clearance is not much, maybe 4 finger widths, at most 4". I drag the front fairly often coming out of gas stations and such. which is also another thing, should i mount panels to the stock bumper to keep from scraping? it would leave a decent gap though, if i did that.
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Old 01-08-2008, 05:06 AM   #5
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I have an odd question, can those side skirts be swapped side to side? Because it looks right now like they start large (and block a bunch of air), then taper to nothing by the time they get to the rear (leaving the rear tire exposed to airflow). If they could be reversed they would guide the air away from the rear tire much more effectively.

Ditto on ditching the rear wing, it's ruining a very aero shape.

I'd attached your underbody panels to the lip, but you don't have to attach them to the bottommost edge. Use the underside of the grille opening to attach them, that'll give you decent airflow, but might keep it up high enough to avoid scraping.

Since the car is so low, you might benefit from some taller skinnier tires. This would reduce your RPMs on the highway, which will help FE, and skinnier tires will have less rolling resistance. And since you're scraping on driveways, adding a total 2" to tire height might give you the clearance you need, without negatively affecting your frontal area too badly. You will have to re-adjust the speedo for the taller tires though.

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Old 01-08-2008, 07:46 AM   #6
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unfortunately, the skirts are not reversable, they attach to the door sill, which is not equidistant front and back.

kames: can you describe what i'll be looking for with the yarn? from pictures and the other info on this site, i'm guessing i should see the yarn caked to the top of the car until a certain point where it starts flapping around, and that will be the separation point?? if that is very high up on my rear window, then might my car benefit from the VG's? can the same be done with my rear bumper to find out how long my wake is?

what CAN be done about a car's wake? aside from boat-tailing the rear end... can installing anything small disrupt flow enough to cut the wake down??
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Old 01-08-2008, 01:57 PM   #7
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With the yarn : Look for minimal movement to show smooth airflow and "dancing" yarn to show disrupted air flow. If at all possible have someone else drive your car while you sit in another car as passenger and take photos. The still photos will help with the aero analysis later as the smooth airflow yarn appears clear and sharp while the disrupted aero yard appears blurred.
On a dark car like your use a strong coloured yarn like fluoro green or yellow.

WAKE: Short answer is yes. There is a type of rear end design called a KAMM tail which has a very angular shape to help airflow.
Most add on spoilers do little if anything to help aero.
They add stability by forming parasitic drag which makes the car feel more stable to the driver.

Have a look at some of the European low drag production cars for ideas of what to do at the rear end.
Many have a blade lip on the roof panel or on the hatch trailing edge which are very effective.

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Old 01-09-2008, 04:08 AM   #8
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has anyone ever tried something like a little scoop on both sides of the rear to funnel air into the wake? would the drag created by the scoops be more parasitic than helpful?
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Old 01-09-2008, 07:54 AM   #9
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i'm not sure if scoops are the right approach, but if they don't add any frontal area it might work.

on a google picture search i came accross of a tuned mercedes (might have been a bmw) wich had ducts leading from the rear wheelwells to openings in the rear bumper... looked a bit like inverse scoops... these where there to "reduce the turbulence in the rear wheelwell at high speeds"

so in order to reduce the wake it might be a good idea to have a rear bumper that's not to low or even has a defuser and have rear wheelwells that are not to restrictive at the back so that air has an easy way out towards the middle of the car.

another interesting idea i once saw in the parking lot, a mercedes as well, wich had a custom trunk lid that incorporated a spoiler. the spoiler was in place of the original trailing edge of the trunk and the surface of the lid sloped down underneath this spoiler. so in theory the spoiler did not add any frontal area but of course the interior volume of the trunk was reduced....

of course these things require some surgury to impliment and are not as easy to reverse as add-on features.

i'm considdering getting some thick tape, maybe with a foam layer to create some turbulators om my rear window and maybe the sides of the car...not sure if i can find the right spot for them though

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Old 01-09-2008, 11:44 PM   #10
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I'm also a Prelude (gen 4) owner and I have been doing some FE experiments the last months. I have the 2 liter engine with manual gearbox.
The easiest thing you can do is to tuck in the side mirrors. This seems to have a dramatic effect on aerodynamics and improved FE by about 5% at highway speeds for me.

I also switched to 195/60 16" wheels which is a 5% increase in circumference. This increased FE by another aproximate 5%.
I currently get about 40 mpg at highway speeds with a bit of Pulse & Glide and other tricks. However, this is in the Swedish winter with quite a lot of rain, snow and wind. I expect figures to improve when it gets warmer.

I have noticed from looking at raindrops that the airflow isn't attached on the rear window, which is a little surprising looking at the shape of the car. I haven't done tuft-testing yet but I suspect this has something to do with the turbulence from the seals around the sunroof.

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