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

So what degree are you setting the diffuser at? I assume 7/10? Also is there an advantage to be had by sweeping the diffuser gradually to the desired angle vs. a straight panel? I haven't come across such info in my book, at least not yet.

Also, I've wondered about the affect (good or bad) of maybe having a naca duct pointed at the diff? My diffuser will not meet the bumper (don't think you plan on it either?) but rather hang down however far is needed to achieve the desired angle. I'll also have a bottom for the bumper that meets the unibody so air can't travel inside the bumper.. with that said, I wonder if there would be a downside to having air flow on the top side of the diffuser via the duct?
 

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Discussion Starter #922
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

So what degree are you setting the diffuser at? I assume 7/10? Also is there an advantage to be had by sweeping the diffuser gradually to the desired angle vs. a straight panel? I haven't come across such info in my book, at least not yet.

Also, I've wondered about the affect (good or bad) of maybe having a naca duct pointed at the diff? My diffuser will not meet the bumper (don't think you plan on it either?) but rather hang down however far is needed to achieve the desired angle. I'll also have a bottom for the bumper that meets the unibody so air can't travel inside the bumper.. with that said, I wonder if there would be a downside to having air flow on the top side of the diffuser via the duct?
Yes, aiming for around 10 degrees. I'm pretty sure I was a little over that last time which still seemed to work (on the one side anyway).

Yes my book also uses simplified angle transitions but does not discuss the possibility of rolling into the angle slowly. It does however talk about that being a critical area to be sealed up and free from anything that would generate turbulence... In my mind, making the transition a little slower would aide in minimizing airflow separation. I certainly don't think its hurting anything. But hey, it might aide in running a steeper angle:notsure:

As far as the NACA duct goes, I've thought about that a little myself. The idea of introducing some extra airflow across the rear diff seems like a good idea but I think it might be detrimental to the airflow under the car. I'm worried that rear edge of the NACA duct would introduce some turbulent airflow under the car which would be very bad so close to the diffuser. But, on the other hand, if it doesn't introduce turbulence (their design is to minimize turbulence and drag to begin with I believe) then it could actually lower the air pressure and increase the air velocity just behind it by scavenging some of the airflow up to the rear diff. You would be introducing a small amount of drag for that air to escape again but it would probably be negligible.

To me, the chance of generating that small amount of turbulence right at the throat of the diffuser is too high to be worth the small amount of cooling it would produce for the diff.
 

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

Well the lowest pressure occurs right at the throat of the diffuser then increase as it travels along. I'm thinking since it's naturally the lowest point of pressure, it may not create too much turbulence but rather scavenge air as it can? Makes sense in my head but I could be way off.. if I did it, I would want to use this style duct that doesn't have a back side lip. Just cut out the shape and install other side.



Here's an Audi R8 with a couple ducts in the same general area:



I wonder..
 

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Discussion Starter #924
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

Even if it is low pressure back there, any turbulence is going to cause airflow separation and basically ruin the airflow in that part of the diffuser. Does your book discuss airflow in and around NACA Ducts? I'd be curious to know just how they affect airflow around them. Looking around online it appears the good ones actually have a airfoil shaped edge for the back site of the duct.

Don't forget pressure is what drives flow, so if you achieve a pressure under the car that is less than whats inside the tunnel with the diff, then the duct won't be effective at all.

There would still be a back side to that duct somewhere if it is to actually duct air upwards. That edge is the important one for what we are discussing as well.

That R8 also has that duct in about the center of the car rather than all they way back by the rear diff. Although it does appear to have the engine exposed right at the back which can't be too good either. Do we know how effective the rear diffuser on the R8 is? Some super/hyper car diffusers are more for looks than function. I saw some relatively recent Ferarri with a "rear diffuser" at like a 45 degree angle :rofl:
 

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

I have no idea how effective the aero is on that car.. I just assumed super cars have to be functional?! I'm shocked to hear they have some stuff for aesthetics honestly. Just thinking out loud, I wonder if boxing the duct in with strakes to keep the air Channeled in its own area would keep any unwanted turbulence centralized?

Maybe even have some sort of strakes on the top side to do the same?
 

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Discussion Starter #930 (Edited)
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

What was originally a weekend for trimming and fitting the first large underbody panel quickly turned into a hood vents upgrade weekend. The difference in free-area for air movement between these and my old setup is HUGE.

Lining up the templates


Taped/covered everything


much bigger than before:tup:


Both sides cut out


Started trimming the backside


First louver test fit


Cleaning up the cuts a bit


2nd test fit, this time with louvers bent


ziptied in place until I drop them off for paint


Out in the sun after being rinsed off :D
 

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Discussion Starter #931
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

So what degree are you setting the diffuser at? I assume 7/10? Also is there an advantage to be had by sweeping the diffuser gradually to the desired angle vs. a straight panel? I haven't come across such info in my book, at least not yet.
Kind of a slow day at work so I've been doing some internetting. Mostly on F1technical about diffuser shapes.

Mostly all I wanted to know was how the leading edge shape of the diffuser effects the downforce/drag but that is very hard to find information on (so far).

There are three main "types" of diffuser shapes being discussed, flat rake, concave curved rake, and convex curved rake. There is also some mention of a hybrid concave/convex diffuser but that’s probably not real relevant here. My current diffuser design has a convex entry and flat rake exit.

Concave (which I didn't know was a thing) has generally lower drag and lower downforce but in a given set of regulations can be the best option. From what I've read though if you have freedom in your design (as I do) then concave is not likely to be the best choice. Interestingly it seems as if the concave design actually relies on airflow separation to create a bubble of air in the diffuser to energize the airflow.

Convex seems to be a better performer in general, by producing more downforce but also more drag. I’m sure the increase in drag (as compared to concave) is associated with the continuously attached airflow.

Most of the "comparison" information I have read so far only compares concave to convex and not to the flat rake that a lot of "cars" seem to be using (possibly due to simplicity or to motorsport regulations). In fact motorsport regulations are really confusing the whole subject because people are doing the best with what they are allowed to do and that can look very different from one car to another.

The flat rake diffuser with a sharp transition a the throat of the diffuser has a higher expansion coefficient and moves the point of lowest pressure forward closer to the throat than the convex diffuser.

The convex diffuser has a smoother/gentler shape for airflow and has a lower expansion coefficient as a result. I believe the “effective length” of the diffuser is slightly reduced as well, which could be a determining factor for a motorsports team working within design regulations.

One thing that does seem clear though, is that the gentler shape of the convex diffuser is likely to be less prone to airflow separation. To me this indicates the possibility for raising the angle of attack a degree or two, without negative results as compared to a similar flat rake diffuser. But this still includes a lot of variables such as how the airflow behind the car is acting and how the angle of attack corresponds to the wake. This also does not speak to the tradeoff of diffuser length versus angle of attack for the flat rake versus convex diffuser either.

Another topic discussed is the ability to use exhaust gasses to try and seal the airflow inside the diffuser from the turbulent airflow off of the rear wheels. This, while being a useful design element will not be practical for anything less than open wheel cars or bonkers time attack cars.

There was also some discussion about having a gap between the diffuser and the rear bumper for airflow from the engine bay/transmission tunnel to be relieved at the rear, and that you should NOT seal that up. The discussion catalyst was actually someone who had taken the diffuser and sealed it up to the bottom of the rear bumper and wound up having heat retention issues because there was no way to relive that heat. The gap over the diffuser solves this, although heat retention with a full on racecar/endurance racer is still potentially an issue and may necessitate trans/diff coolers.

I have also seen several flat bottoms that include a NACA intake for the rear diff as well, but they are usually at least a foot or two upstream of the diffuser throat. I’m starting to think that they may not be detrimental after all, but I’m still not ready to try it myself.

Another common design element I am seeing is the vertical “gurney flap” on top of the rear edge of the diffuser. My understanding is that this helps to further energize the diffuser and the small(er) amount of air above it and reduce the size low pressure zone behind the car

On a related but not super applicable note, there are a lot of venturi shaped flow meters for pipes/ducts. The only thing I came up with about straight vs curved edges is that the straight edges produce sharp pressure changes and that by using curves instead the flow meter is more accurate. To me that says the curved edges produce a more stable airflow profile. But that says nothing about the actual overall pressure change or how that relates to creating downforce :rolleyes:

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So after all that reading, my takeaway is that without testing or working within a given set of regulations there are no fixed “answers” regarding diffuser design that applies to all situations, and that airflow separation (except in the concave design) is always a bad thing. With that in mind, I feel good about the entrance of my diffuser having a curve to it for a smooth airflow transition. :tup: I will keep looking around to see if I can find anything else.
 

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

Good information, thanks! I didn't realize concave diffusers were a thing either.. That one's a bit of a head scratcher. One thing that I took away from what you just said is that the top of the diffuser should not be sealed to the bumper (which is what i plan) in order to relieve heat / engine bay pressure. With that said, if air is indeed traveling along the top in such a manner, i see no reason not to add the naca ( albeit maybe further up stream than the throat) to channel flow to the diff.
 

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Discussion Starter #933
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

Back to pictures. Got the louvers painted and decided to get the hood repaired as well. I did a pretty good job of cutting carefully but the heat of the dremel made some of the clear bra bubble up and some paint around the edges flake off... But now its all fixed, and lookin good!

picking up the hood at the body shop:


(never mind the dust on the hood, its not stuff IN the paint...)


hood back on:



Riveted on the louvers (added one coat of touch-up paint on the rivets and will need to add more):


 

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

Looking very good there.
 

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Discussion Starter #936
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

Looks great man! Did you ditch the weds, or are they track wheels?
The weds are track wheels. I drive the SSRs on the street with all seasons. :tup:

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Fun fact for you, Ive seen some interesting data over the past week on how effective the TMIC is with varying pressures across it.

Taking the hood off, very poor function as no airflow is "ducted" through it. Temps rose quickly, stayed high, and barely came down with movement.

Hood back on with giant holes (no vents) increased under-hood pressure, although still wound up being more effective than no hood due to the hood scoop. Temps started to come back down a little quicker but still not as good as a plain OEM hood.

Hood back on WITH louvers in place, lowered under-hood pressure; MUCH more effective TMIC. Temps come down after a stoplight much quicker. In fact i sprayed the IC on the way back from lunch once, while at ~50mph. The IAT dropped from 122 to 113 (it was still coming down from stoplight traffic) in a matter of about 5 seconds (I was seeing one or two degrees per 10-20 seconds previously). That's a much bigger response than I have ever seen from the IC sprayer before... I know there's no repeated testing here so there could be some anomalous conditions, but that's still a pretty stark difference even from my old (not very effective) hood louvers.
 

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

The weds are track wheels. I drive the SSRs on the street with all seasons. :tup:

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Fun fact for you, Ive seen some interesting data over the past week on how effective the TMIC is with varying pressures across it.

Taking the hood off, very poor function as no airflow is "ducted" through it. Temps rose quickly, stayed high, and barely came down with movement.

Hood back on with giant holes (no vents) increased under-hood pressure, although still wound up being more effective than no hood due to the hood scoop. Temps started to come back down a little quicker but still not as good as a plain OEM hood.

Hood back on WITH louvers in place, lowered under-hood pressure; MUCH more effective TMIC. Temps come down after a stoplight much quicker. In fact i sprayed the IC on the way back from lunch once, while at ~50mph. The IAT dropped from 122 to 113 (it was still coming down from stoplight traffic) in a matter of about 5 seconds (I was seeing one or two degrees per 10-20 seconds previously). That's a much bigger response than I have ever seen from the IC sprayer before... I know there's no repeated testing here so there could be some anomalous conditions, but that's still a pretty stark difference even from my old (not very effective) hood louvers.
That's cool! Makes me want to get a temp sensor put in post-intercooler.
 

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Discussion Starter #938
Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

I did it before I had the car retuned so they could tune using the actual realtime IAT that enters the throttle body :tup: Its also nice for being able to see what is actually happening instead of interpolating based on assumptions too.


I have more data on IC performance. The higher the temperature, the faster the temps fall when you get moving again (basic thermodynamics so I'm not really sure why I'm so intrigued...haha). I sat a stoplight yesterday waiting to turn left onto the freeway. IAT got up to 155ish. By time I got to the end of the on-ramp (accelerating pretty good but far from WOT) I was down to 130ish IAT, and within another 15-20 seconds I was down to about 95 degrees IAT. Thats about a 60 degree drop in 30 or so seconds. (IAT stabilized around 95 or so since OAT was about 85)

I have read before that at higher speeds a TMIC will become less effective because the pressure buildup under the hood starts to limit airflow through the TMIC. Clearly good hood vents lower the under-hood pressure because my TMIC effectiveness at freeway speeds has increased :)
 
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