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Discussion Starter #381
Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

^FYI, most of the stuff in that book isn't going to be as applicable on a street car due to the constantly changing gap between car bottom and the road. Unless you have a super stiff suspension setup.

Edit: mainly the underbody is what I'm meaning. Your route should be helpful though, get as flat as possible
Yeah everything in there is subject to a lot of variables, but the principles in design are valid anyway. I just won't see like 1000lbs of downforce like you might with a really low sealed floor flat bottom car... There is a lot of interesting data on how various factors impacted downforce in a CFD model and in a wind tunnel, and some of it is actually tested on road cars and very applicable to my car (even though my suspension isn't "race" stiff). Even down to the number of strakes on the rear diffuser! Its a good read either way though :tup:
 

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Discussion Starter #382 (Edited)
BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

Took a first pass at fitting the rear diffuser. Clearly a lot of work left on fitment before I worry about the strakes. Also need to weld these two pieces (see under the muffler for the seam)
 

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Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

Spotted you at Lowes and parked next to you.

My son said "Park next to the blue race car!"


Cars lookin dope. Keep it up.
Lol
 

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Discussion Starter #385
Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

Spotted you at Lowes and parked next to you.



My son said "Park next to the blue race car!"





Cars lookin dope. Keep it up.

Lol

Haha, I thought that was you! Saw your donut tire on :(
 

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Discussion Starter #387 (Edited)
Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

Spent the afternoon working on fitment and trimming and such. I don't know how much of a difference the pictures show, but its much better already!
 

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Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

Yeah my tire of same :-(

No funds right now to do the set up I want so it's going to be there for a Lil bit. I don't drive much so it's OK
 

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Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

Looking good man! I like those fender vents. What are you going to do about the fender liners?
 

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Discussion Starter #390
Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

Looking good man! I like those fender vents. What are you going to do about the fender liners?
I figured I would just cut some vent slits in the rear portion of the liner. I don't want to remove it all together, so I'll need to figure out how to get the air to the vent. I may try to do a semi-ducted setup from the liner to the actual vent.

Ofcourse I don't want to cutup my nice striaght fenders, so this all kind of hinges on me finding a second set of fenders to play with... :cool:
 

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Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

Sounds good man. I'm interested to see what you come up with. I'll post a pic for you when I get my solution sorted. Keep up the good work!
 

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Discussion Starter #392 (Edited)
Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

Spent a little more time on the diffuser this weekend. Decided to change up the plans a little bit. Initially I was going to have the entire rear bumper covered by diffuser (as shown in previous pictures) but I realized this was not ideal. Having the diffuser stretch over behind the wheel was going to catch high pressure air off the rear wheels and that's not what I want.

So I decided to cut the ends of the diffuser flush with the width of the phase 3 panels which makes a lot more sense. Next I started working on the end-fences to block out that high pressure air around the wheels and such. I got the driver's side one pretty much taken care of. I'm going to try to do more inside the wheel-well further forward as well, but I didn't get that far.

I went ahead and removed the rear mudflaps as well. They aren't really doing anything besides creating drag on the roadways anyhow. Front mudflaps will remain for the sake of the paint down the sides of the car.

Once I'm finished with the other end fence I'm going to head off to see a friend with a welder to stitch all these seams together permanently :tup:





Also went karting with the local subie club this weekend. I may have won the pole in both races :D
 

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Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

Looks sick already :eek::tup:
Makes sense with the hot air getting trapped, good thinking!

Congrats on winning the races :D:tup:
 

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Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

Well done! That's the way I plan on going when I make mine. I think I'm going to chop the rear bumper to the line that runs around, remove the sheet metal in the wheel well up to that point and make a flat bottom for the bumper underneath that meets the unibody to vent the rear wells.
 

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Discussion Starter #396
Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

Well done! That's the way I plan on going when I make mine. I think I'm going to chop the rear bumper to the line that runs around, remove the sheet metal in the wheel well up to that point and make a flat bottom for the bumper underneath that meets the unibody to vent the rear wells.
man that car is low! I wonder if they changed the suspensoin geometry/pickup points or if they just did body work on a slammed car...

I like the idea, although I wouldn't want to make a cut that big on a DD like mine... If it was a pure track car, I'd do it :) I have this internal battle with wanting a low-key DD, but wanting to do all the race-car aero stuff that attracts a ton of attention (for the function, not the attention), and its pretty much impossible to do both. haha

I wouldn't cut anything out of the middle part of the bumper though. Reason being, even with the bumper uncut and my diffuser ending an inch or two below that, the angle on it is still borderline to steep... Notice how the diffuser on that car doesn't come up that high (although that's probably more of a regulation issue than a design constraint).

On a road car 10-12 degrees is really about the max you want to run without having a lot of wing above to allow for higher angles (7-10 is normally recommended). You can do higher angles, but you start running into flow separating, less downforce generation, and downforce/drag efficiency dropping. I'm at about 14 degrees now which, hopefully with the rear wing, isn't far off of ideal. Even if it is, the difference will be minimal since downforce only tapers off with increased angle.

When I'm done with the diffuser and its all welded up, I'm probably going to get it powder coated flat black so people don't notice it as easily ;)
 

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Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

I'm fairly certain there's not a lot stock about the gt cars There's no way they could affectively be that low with the stock geometry.

As for the line that I want to cut from, i'm referring to the lower not the upper part of the bumper like the cusco race car. It won't be too dramatic and if it is.. whatever. The front end will already be wild enough (i'll share with you when its done). Also, not to stray too far off track but I found a 3" naca that will fit in the fender so I'll also let ya know what my iat's are once it's fit.

Thanks for the info though. I certainly need to do my research on the technical side of things before I get into splitter and diffuser fabrication. I'll likely wind up with an apr 300 wing with singular endplates and I'd like to brace from the inside of the trunk to better utilize it. Lots to still learn about the black art of aero ;).
 

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Discussion Starter #398
Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

I'm fairly certain there's not a lot stock about the gt cars There's no way they could affectively be that low with the stock geometry.

As for the line that I want to cut from, i'm referring to the lower not the upper part of the bumper like the cusco race car. It won't be too dramatic and if it is.. whatever. The front end will already be wild enough (i'll share with you when its done). Also, not to stray too far off track but I found a 3" naca that will fit in the fender so I'll also let ya know what my iat's are once it's fit.

Thanks for the info though. I certainly need to do my research on the technical side of things before I get into splitter and diffuser fabrication. I'll likely wind up with an apr 300 wing with singular endplates and I'd like to brace from the inside of the trunk to better utilize it. Lots to still learn about the black art of aero ;).
Sounds awesome, I can't wait to see some pics :tup:
 

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Discussion Starter #399 (Edited)
Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

So I found myself in the garage last night for a bit. Found out that 1) I have a coolant leak from the thermostat housing gasket, and 2) my passenger front axel has a torn boot and no grease :( which might explain the noises I've been hearing occasionally...

After deciding to do nothing about those last night (since it was already late), i decided to spend a few minutes on some of the minor aero improvements I want to add.

First off, I added a flexible foam "garage door" gasket to the bottom of the front bumper. I would've taken a pic of it back on the car for better perspective, but I didn't put it back on yet...


Then I added some some rubber the the rear wheel air deflector thingys. The material for this one is from commercial floor molding that goes over carpet. I just cut it up to fit and riveted it on.


While I had the car up, I noticed something interesting. There was some neat "water patterns" under the car. Yesterday it was raining all day so the car was soaked. By time I got under the car last night it was partially drying off. The interesting bit is where the water was under the car. Under the majority of the front part of the car, it was all DRY. But the "phase 3" panels and the rear diffuser were still SOAKED. The drivers side had much more water on it than the passenger side, and that is the side that has the end fence installed on the diffuser. Even though the diffuser still has a lot of work left, and will increase in efficiency from were it is, the water pattern seems to suggest that it is all working! This is probably about as close to a wind tunnel as I will ever see...

If you look close you can see the rear panels are wet and the front are dry :D
 

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Re: BlueScooby's 06 STI - Underbody Panels Phase 4 (rear diffuser)

If you wanted to check performance you could always tape or hot glue small lengths of string/yarn under the car then affix a hero cam under there. That will probably give you a good idea of how the air is flowing over the panels.
 
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