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Discussion Starter #961

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Discussion Starter #962 (Edited)
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

sometimes dreams do come true! LOL

I got some decent work done on the diffuser fitment/install this weekend. I figured out the two main supports, hung the diffuser and started working on trimming out the end-fences. They are fairly tricky since I'm basically trying to have them go all the way up until they hit something across their whole length. Drivers side end-fence is trimmed out to the car nicely, and the passenger side is just started. Once I finish getting the passenger side trimmed to the car, I will have to back the car down the drive way to the street to trim the back end for ground clearance. Feels like I'm getting close here :)

I used bolt holes for the OEM diffuser bracket to hang the two main support rods. Unfortunately they are not equal distance from the center of the car though :tdown: I'm planning to add a support rod to each end fence though, so it shouldn't matter.


You can see here that the diffuser is going to stick out a bit further than the V1 setup.


As a result I'm going to mount a bar at the end painted in neon yellow just to add some visibility and draw attention to the big-ol-thing hanging off the car (which you can kind of see in this pic)


Working on the drivers side end-fence:




And one more shot with a decent view of the whole underbody to date:
 

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Discussion Starter #964
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

Car is at this shop this afternoon getting the new fuel pump and filter. About 30 minutes after I drop the car off, they send me this picture:


What kind of jackwagon uses a half inch of tubing and a single hose clamp?? :wtf:

I'm lucky that thing hasn't come off! At least its getting fixed now...
 

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Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

:eek:

Geez. Quite some professionals in the buisness.
Good thing it was found and is being fixed.
 

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Discussion Starter #967
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

Got a little bit more work done during the week:




Added rubber trim to the top side of the endfences:


Then last night I test fitted it so I could mark where to cut for ground clearance:




And where I need to cut:
 

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Discussion Starter #968 (Edited)
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

revisiting the NACA Duct discussion relative to the underbody. I pulled out my aero book and it does have some info on naca duct design and usage but nothing specific about application in an underbody. I did, however, find an interesting CFD model of a NACA duct that I think we can extrapolate some information from. This picture is from an article comparing airflow through a scoop vs. a NACA duct but its still relevant here.

This is a profile shot of a NACA duct showing airflow through the duct as well as outside on the "exterior" of the vehicle. The far left of the vehicle falls away so that part isn't useful, but the "exterior" panel just past the entrance to the duct shows the disruption that the duct creates:


You can see a small turbulent airflow bubble beginning at the end of the duct, which appears to stabilize pretty quickly. So there is a disturbance in the airflow but it is small. However, the "exterior" flow velocity immediately drops at, and past, the naca duct which is the opposite of the desired effect of "ground effects" on a car. The whole point is to accelerate the airflow through a "venturi" shaped underbody to lower the air pressure. If you lower the velocity of the airflow by drawing some out that would yield a reduction in the venturi effect you are designing for.

Granted this is a lot of extrapolation based on one picture and the use of "mach number" (what I'm interpreting to basically show flow velocity) could be misleading. If this was a plot of pressure rather than flow, it would appear that the naca duct would be very beneficial for an underbody, but thats not what is being shown here.

What do you think? Does that make sense or do you think I'm way off here?

EDIT:
After doing more reading, I realized I misunderstood the relationship to pressure/velocity as it pertains the rear diffuser. The velocity under the car (before the diffuser) is accelerated, producing downforce, but as the airflow enters the diffuser and starts to expand, it actually slows down. Q=A1*V1=A2*V2.

In this scenario with the NACA duct immediatly before the diffuser, I'm not quite as clear how this would effect the overall aero performance, because it would be slowing the airflow down before the diffuser, where the air is still supposed to be high velocity, but at the same time, it could amplify the effect of the airflow through the diffuser itself. It could even be a wash with a (+) and a (-)...

So basically, this is a lot of words to say I don't know. And also, this may be a complete waste of all our time... haha
 

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Discussion Starter #969 (Edited)
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

Finally got the diffuser on the car! Its still not done of course (needs strakes and more painting).

This weekend I added the rear "reflector bar" for visibility, an exhaust heat shield/deflector made out of stainless, added the side support rods, painted the end fences and top of the diffuser, added rubber skirts to the end fences and installed it!







And one more picture from this morning. Turns out its kinda hard to see without direct sunlight since the whole thing is black... :lol:
 

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Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

Looks sick, Aaron!

You should reroute your exhaust to blow the diffusor for more downforce like they did a couple of years ago in Formula 1 :D

for my personal taste it's a tad too long but since this form follows function, this is totally legit :bow::cool:
 

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Discussion Starter #971
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

You should reroute your exhaust to blow the diffusor for more downforce like they did a couple of years ago in Formula 1 :D
Believe me, I have thought about trying to do that. haha! If i wasn't keeping the muffler I could do something like that.


for my personal taste it's a tad too long but since this form follows function, this is totally legit :bow::cool:
It is a little longer than I wanted it to stick out but this is definitely function>form. I added that bright painted visibility thing on the back just because of how far it extends.
 

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Discussion Starter #972
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

Another smallish update/proof of concept.

Its raining and I just went to lunch driving through the rain down the highway. Lets just say, its easy to SEE this thing is working :lol:

Adding the strakes will help to stabilize the airflow a bit as i was seeing some turbulence, but the airflow was still carrying a LOT of water up behind the car.

At one point I pulled in front of this little Geo Metro that had its headlights off and it just disappeared in the mist... I mean it was GONE. And I was only 5-7ish or so car lengths ahead when I started merging. Poor thing probably couldn't see at all for a second... :lol:
 

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Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

Too bad a bunch of your pic links are broken, but this looks interesting. Get some track time on the setup next season and get some data. I'm curious on how this will perform.

Damn it sticks out far!
 

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Discussion Starter #974 (Edited)
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

Too bad a bunch of your pic links are broken, but this looks interesting. Get some track time on the setup next season and get some data. I'm curious on how this will perform.

Damn it sticks out far!
Yeah, I'm not sure whats going on with the photo hosting. It looks like its something Flikr knows about but I'm not sure what I need to do...

its probably 6" past the edge of the bumper, but these pics do make it seem a little more...


EDIT: Got in touch with someone at Flickr and trying to resolve pic issues. May have to re-link in a different way...:notsure:
 

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Discussion Starter #975
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

65* afternoon and overcast? Perfect afternoon for working in the driveway on the underbody! Worked on some fitment and details and hope to trim the first strake to size tomorrow afternoon :tup:
 

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Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

Nice work.

Just a bit too big for the rest of aero but I guess it's function over form.
 

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Discussion Starter #977
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

Yeah it looks a little out of place on an otherwise “relatively” stockish looking car... but having it extend past the bumper is great for the airflow
 

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Discussion Starter #979
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

can i quit my job and become a full-time garage/track rat and still get my paychecks? That'd be great...

Sitting here at my desk wishing I was home building stuff :(
 

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Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

I feel you Aaron!
Same here. This 9-5 IT Job is something I'm not planning on doing the rest of my life for.
 
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