Not sure where to put this, but since it pertains to body I put it here.

I got a new datalogger which has a drag coefficient and rolling resistance measurement capability and I decided to test it out on the Z today. The logger uses a rudimentary coast down formula and measures the time it takes you to coast from 68mph down to 62 mph, then from 12 mph down to 6. Since I live in Illinois, I have a large selection of really flat roads to perform the test. The test does not control for variables very much, so I plan to combat that with a large sample of runs. I'm going to be making these runs daily and recording temp, humidity, wind etc for about two weeks. That should give me a pretty good baseline to start making changes.

The 5 runs I did today showed the Z coming in with a coefficient of drag around .34-.35. (I don't fully understand rolling resistance yet so I'll post more on that once I understand it a little better) I don't know how accurate the measurement is, but it should give me some feedback on reducing or increasing drag when I make changes. There are a couple of things I want to change and try out aerodynamically.

1 - Right now I have a C-West bumper and GT3 style wing but I will be going back to stock in a couple of weeks. I definitely want to see the before and after of that.
2 - I want to test the rear track diffusers. I'd like to do a same day, before and after test of the diffusers to see if they actually show any advantage.
3 - Undertray. Right now I don't have the engine undertray on because the C-West bumper does not allow it. That will go back on once the stock bumper is on.
4 - Also, I'm making a very basic rear diffuser to cover the hole left by aftermarket exhausts. And I want to test it out to see if it helps or hurts.
5 - After taking some video today, I'd like to try some vortex generators at the top part of the hatch to see if I can't get some more downforce.

Along with the datalogging, I decided to take some tuft videos.

The roof displays a very nice high pressure laminar flow pattern. http://home.comcast.net/~geoffbrown/aroof.wmv


The hood could be better, but it definitely has a pretty decent laminar flow. Interesting that the flow is disrupted when I come upon some cars. Will be interesting to see if the stock bumper produces a strong stream of air over the tufts.http://home.comcast.net/~geoffbrown/ahood.wmv


The back of the hatch however, shows quite a bit of pressure drop and some turbulence. This leads me to believe that some more downforce and reduced drag could be had with a set of vortex generators on the highest part of the rear window. I'm going to take the wing off next to see if that doesn't help the flow first. http://home.comcast.net/~geoffbrown/ahatch.wmv


Here's a pic of the vortex generator I'm working on. Its based on the Mitsu ones that the guys did the experimentation on. Still a work in progress. They should install on the window near the highest point on the hatch. When its done, this one will be primed painted and used for the mold. Then I can pop them out pretty quickly. I'll make 7 to try at first.

 

keep us updated on the vortex generator im curious to see the results.

 

Very interesting. If I recall correctly, the Z is supposed to have a drag coefficient of 0.30, while the track z has a coefficient of 0.29 due to the underbody defusers. Do you think you will see a decrease in drag when you put it back to stock? Also, what purpose do the vortex generators have?

 

I'll post regular updates as I get info.

The vortex generators (VG) should allow the air to follow the hatch down better by delaying the separation of the airstream from the car. It delays separation by creating a stream of air that stays together, like a tornado. That high pressure stream of air is "sucked" into low pressure areas. (Meaning it follows the contour of the car better) The delayed separation can reduce turbulence in low pressure areas (Over the hatch apparently) and help the airflow to adhere to the car. The less turbulent airflow over the body should reduced drag, although it will be partially offset by the drag of the VG's and the smoother airflow should produce more downforce because the pressure on the hatch will be increased. Although, if it works, it would probably be beneficial to add a lip spoiler or canard to the front to offset the newly added downforce on the rear of the car.

http://www.mitsubishi-motors.com/cor...004/16E_03.pdf

 

Great research. Keep us posted.

 

+1. Great work man, keep it up.

 

and thanks for sharing with the forum members! Very intresting!

/Roger

 

I've got an update on the VG's. They worked great on the hatch. Here is some before and after footage. You can also compare them to previous vid of the hatch.

Before vortex generators


After vortex generators

Vids were taken same day and the same speed. (I was in different gear though) There was a strong cross wind for both videos.

I found a place that sells highly optimized vgs for $2.50 a piece. So I just used those instead. I found the flow over the hatch to be greatly improved especially the further down the hatch you go, the smoother the flow. The picture is not the greatest of the lower tufts, but you can see a market improvment in the airflow of all of the tufts.

 

Intresting indeed, clearly noticable improvement!

I guess you havent yet tried this on the track yet, but with that wing and the vg's i can imagine you might notice the improvement in rear downforce and high speed stability!

Please keep us updated!

BTW it looks quite cool as well! Would be really cool to have them in CF

/Roger

 

good to see another person into downforce, however you may be in it hardcore, i plan on utilizing a small amount of downforce on the rear with the strosek wing, 85 Nm to be exact on the third setting, i will be purchasing the STOCK track diffusers as well! Only difference is that i am using the italian speed racer rear end, however the under trray maches perffectly with the muffler housing, and the sides are vented so i do not think they are going to be a major disadvantage! The evacuation of the udnercar airflow isnt affected, the important parts that needs to be carefully worked are the nose of the car, and the upper portion! ofcourse im sure you know the nose of the car accounts for the aerodynamics of the car. the rear section really doesnt mean much, This is the reasoning behind the mustangs rear flat rear end! Anyways keep up the good work buddy! and if i find any usefull information ill be sure to let you know, perhaps itll assist in your quest!

 

That's thoroughly impressive! It's crazy how such a small little piece made that much of a difference. Any recommendations for underbody diffusers that would cut down on drag?

 

Thanks. I'm working on a full body package to increase downforce. This is just part. Its really hard to reduce drag while gaining downforce. I rarely drive my car past 120 or so, so downforce is more important to me than drag reduction at this point. But I do have a rear diffuser in the works, and an underbody tray which theoretically should reduce drag. But its all a pot shot really.

The problem with underbody stuff is

1. that we can't really make tunnels under the car
2. in order to get suction out of the rear without tunnels you need really low side skirts and front splitter. That reduces driveability and increases drag.

However, I'm working on another underbody diffuser idea which is used on some Lexus models. I have a ways to go on all of this stuff, but I hope to be able to at least source all of this stuff if people want to put together their own package.

 

try this. go out on a semi-cold and humid night. go for a long drive and come home and park the car. come out a while later when the condensation has formed on the car. you can see on the back part of the roof some turbulent areas that appear in uniform patters on the roof. this is probably the same place Mitsu put their vortex generators on the Evo.

 

hey zillnois, would you mind letting me know where you bought those vortex pieces?

 

I'll definitely give that a look. On a similar note, yesterday, I had a small dusting of snow on the back window of my car and when I got up over about 35mph there was a clear pattern of stripes where the snow was cleared downstream of the VG's. Pretty cool.

You can read some of the research behind the EVO VG's here.

http://www.mitsubishi-motors.com/cor...004/16E_03.pdf

 

They are $2.50 each.

www.airtabs.com
www.buyairtabs.com

 

Wow, thumbs up on this. Very interesting.

 

Links don't work.

 

Sorry about that. Went off memory. Memory didn't work.

www.airtab.com
www.buyairtab.com

 

interesting. according the PDF file, the Mitsubishi engineers only reduced lift and drag by 0.006.

you'd probably get that by removing the GT3 wing, or slamming your car closer to the ground...

can you perhaps film the Flow Yarn after you remove the GT3 wing? or maybe i could find a stock Z to film it.. but it'll be interesting to see the flow characteristics over the rear hatch with out the wing..

also, put yarn on the surface of your wing, i'd be interested to see where your 'stall' speed it due to the high angle of attack your deck is at... is it adjustable?

EDIT:

i consulted the oracle (Race Car Aerodynamics Designing for Speed / Joseph Katz) this is what it states for VGs:


"...these can resemble small wing shapes or have more complex geometries as shown by thefour typical shapes (shown are: curved, delta, pyramid, and wedge tailed like the ones you got) a typical VG is a bit taller than the local boundary layer thickness, and the swirl of the cortices it creates helps to add fresh momentum (from the free air stream) into the boundary layer. if such VGs are placed near the EXPECTED SEPARATION LINE, the added momentum can DELAY flow separation. the overall effect is usually a gain in the maximum lift and a reduction in drag at the higher lift coefficient (as a result of the smaller separated flow regions) At lower lift coefficients, thought, the VGs may increase the drag. Most VGs are constructed from simple sheet metal and some are commerically avilable as a glue on-strip. "

the rest of chapter goes onto to describe the other options to delay flow seperation, including gurney flaps and built in channels into wings and surfaces.