BRZ/FRS AeroKits by WingsofWar

[WingsofWar]

Originally Posted by Dimman

Rear of the hood is a high-pressure zone because of the windshield. Look at old muscle cars with cowl-induction. They placed the intake at the back of the hood facing the windshield so the carb could pick up the more pressurized air.

Natural low pressure area is the curve of the hood/fenders where it starts to flatten out, or droop down in some cases. It can also be enhanced a bit with a gurney-type flat flap just in front of the venting.

exactly, the problem with this high pressure air at the edge of the hood is the slope of the windshield and on most cars suffer bad detachment at the roof. Many people and aftermarket designers don't take this into account when adding ventilation on the low pressure surface of a hood. Because the placement of the vent and style of vent can change the flow characteristics and either aid or negate performance.

Venting dry hot air from the engine bay angled at the high pressure zone of the windshield is bad juju.

Venting warm moist air from the radiator towards the high pressure zone of the windshield is more ideal.

Something that aids radiator efficiency is a well shrouded eletric fan which is typically effective enough to not need such a wide air dam. A clutch fan for the radiator as seen on most stock cars is effective, but isn't aerodynamically efficient because it pulls too much air into an already high pressure area, which unfortunately is also dry and hot. eg back half of the engine bay.

So with a well shrouded radiator fan that pulls at least 2500cfm of air, we can actually decrease the air dam for more pass through velocity towards a vented low pressure area at the bottom or ideally..the sides of the car.

If we want to vent the high pressure hot air that accumulates in the engine bay to aid cooling, and aid aerodynamics we need to shoot that air towards low pressure areas of the sides of the hood/fender towards the A pillar so it wraps around the car aiding the velocity of airflow towards the spoiler.


---sorry i just rambled---

I think the mistake we average guys make in tuning aerodynamics in cars is we always target the problematic areas of drag rather than looking to optimize the airflow from front to back. looking at the big picture is hard without a visual aid from CFD.

[Dimman]

I think mostly the point of the hood venting is downforce/lift reduction. The low pressure area is the source of lift. Venting the hood at low pressure areas allows the high pressure built up in the engine bay to 'neutralize' it. Then when we have less high pressure in the engine bay that also allows more flow through the rad/intercooler.

[WingsofWar]

Originally Posted by Dimman

I think mostly the point of the hood venting is downforce/lift reduction. The low pressure area is the source of lift. Venting the hood at low pressure areas allows the high pressure built up in the engine bay to 'neutralize' it. Then when we have less high pressure in the engine bay that also allows more flow through the rad/intercooler.

yep! a properly vented hood can;

Aid down force
Reduce top side Lift
Reduce bottom side lift
Aid heat exchange from Radiator
Aid engine bay temps to a minimum.
Reduce engine bay pressure

.. unfortunately just adding vents and flaps without considering its function like what most designers do **myself included** can have ill effects on performance.

For example, I believe that this hood by J's Racing is non-functional aero dynamics.



Here is why i believe this ,
-The shape of the open vent is square non-taper
-the location of the vent is smack right at the center of the hood.
-There is a lip at the opening that stops any sort of speed built up from the low pressure zone
-Now theres a awkward high pressure air zone at the front of the hood, in front of a vent that already shoots high pressure hot engine bay air into a high pressure zone at the edge of the hood.
-that floating high pressure area above the hood wont aid additional cold air to flow through which would probably resulting in no engine bay temp changes from stock.
-Also resulting in unwanted drag, in the form of non-productive downforce.

Im sure effects on performance are not extreme. But still if your going to buy a $1500 hood, it needs to be functional AND stylish.

i think im pretty correct in this analysis.

----

Also lets remember that as long as the boundary layer stays attached at low pressure zones, we don't see lift forces that negate performance. but we also have to remember that heat changes the characteristic of air.

**some footnotes to myself**

-Warm moist air is light and less dense resulting in low pressure air. Its the least stable form of air.
-Dry hot air is heavy, resulting in high pressure and can have the worst effect on drag.
-Cold Moist air is dense but still light, ideal kind of air for cooling and heat exchange.
-Cold Dry air is extremely heavy and dense, and results in high pressure. this is the kind of air you want to use for down force.

------------

I think im ready to try my hand at designing a hood that could be functional.

[Dimman]

I actually disagree with the J's hood being non-functional.

- The shape I don't really know about, so not sure. But louvers do the job, and an opening is an opening. So it may not be optimal, but it is at least functional.
- The location looks to be where the natural low-pressure area will be forming
- The lip is going to form only a slight high-pressure area in front, but also enhance the low pressure zone behind it. Right where the vent is.
- So because of the above, there is vented low-pressure that can 'suck out' the high-pressure bay air.
- Lift reduction probably is more positive than the drag increase is negative.

My .02

[serialk11r]

Originally Posted by WingsofWar

I think the mistake we average guys make in tuning aerodynamics in cars is we always target the problematic areas of drag rather than looking to optimize the airflow from front to back. looking at the big picture is hard without a visual aid from CFD.

Ehhh...have you seen ecomodder.com? The single biggest problem to aerodynamic design is probably the fact that a car cannot have a giant tail coming off the back. The front actually matters much less. Typically ecomodders don't really care about downforce so they focus on roof/trunk attachments to help direct air from the top down. Problem is, directing air downward naturally means having some lift. The downforce conscious way to do it would be instead of focusing on the top, to focus on the bottom (can't really do much about the sides of the car, they're more or less set in stone): a diffuser that extends out further behind the car.

About the top of the car, well completely altering the angle of the rear windshield isn't really possible without completely rebuilding the body, so this is pretty hard to work with. Typically people add wings and stuff but those cause some pretty bad drag.

Now on the front of the car, the base of the windshield is certainly a problem of sorts, but not nearly as big of a problem as the back. One thing you could do is to fabricate a piece that slightly fills in the gap right above where the windshield wipers are, this would direct airflow above them and improve things a little.

[Mr.Jay]

*waiting for FRS design*

[Giccin]

Response for the first post update:

Ooo... so my answers did get considered. :P

After you pointed out the details you've created I can see it a better now. :P

[SVTSHC]

Legit bro, you've got serious skill and an eye for aesthetics without going over the top. It's simple, clean, and wide enough to accommodate conversion spacers (even though personally I'll probably go with the full hub conversion and wider wheels). You'd definitely have some of "doll-hairs" I set aside for modification funds. BUUUUUUUUT, like you said it's Form vs Function and seeing as I tend to lean more toward the functional side I'd say I'd buy one of your kits but not this particular one (even though I seriously like the way it looks).

In other words, I like your style; were you to put together a function oriented kit my money would be yours.

[serialk11r]

Hey Dimman,
Your friend? 's roof uhhh "wingtip covers" would actually work on the rear window I think...the air traveling around the side of the car sees a sharp transition over the rear window which in most cars will create a rather large vortex coming off the 2 edges of the rear window. I'm thinking a rubber extension that is taped onto the rear window and attaches to the trunk would work. I'm not 100% sure but I think it would reduce the pressure difference across the edge of the window since the air meets a higher speed stream of air further from the rear window.

Ideally the car's ass should be a little higher and the rear window should "cave in" a little. See Pagani Zonda for reference.
EDIT: okay so apparently it's worth about 4% drag reduction if you extend C pillar correctly...that's not bad if it's easy to just tack on a triangle of ABS plastic (wouldn't look too bad if it had a matte finish too). Admittedly the trailing vortices do help airflow adhere to the rear window, but they come at a very high induced drag cost, so it's probably better to block off the rear windshield from the sides and then use vortex generators at the roof. Trailing vortices are probably also the reason why spoilers have those ugly sections coming out the sides sometimes.

[poormans_LFA]

Originally Posted by WingsofWar

I think im ready to try my hand at designing a hood that could be functional.

let's see it!!!

[Dimman]

Originally Posted by serialk11r

Hey Dimman,
Your friend? 's roof uhhh "wingtip covers" would actually work on the rear window I think...the air traveling around the side of the car sees a sharp transition over the rear window which in most cars will create a rather large vortex coming off the 2 edges of the rear window. I'm thinking a rubber extension that is taped onto the rear window and attaches to the trunk would work. I'm not 100% sure but I think it would reduce the pressure difference across the edge of the window since the air meets a higher speed stream of air further from the rear window.

Ideally the car's ass should be a little higher and the rear window should "cave in" a little. See Pagani Zonda for reference.
EDIT: okay so apparently it's worth about 4% drag reduction if you extend C pillar correctly...that's not bad if it's easy to just tack on a triangle of ABS plastic (wouldn't look too bad if it had a matte finish too). Admittedly the trailing vortices do help airflow adhere to the rear window, but they come at a very high induced drag cost, so it's probably better to block off the rear windshield from the sides and then use vortex generators at the roof. Trailing vortices are probably also the reason why spoilers have those ugly sections coming out the sides sometimes.

D'OH! Memory failure! Found the book that had the car I was talking about in it... And, er, ...nothing on the roof. Just the as-described fencing on the hood, with massive opening/venting.


I must be getting old...

[serialk11r]

On the hood hmmm that would be of limited use, because the hood angle generally is not very steep. Hey if you check out the NSX concept, you'll see my C pillar extension idea on the rear. A pillar vortices are weaker than the C pillar ones.

[Dimman]

Originally Posted by serialk11r

On the hood hmmm that would be of limited use, because the hood angle generally is not very steep. Hey if you check out the NSX concept, you'll see my C pillar extension idea on the rear. A pillar vortices are weaker than the C pillar ones.

Guy is (was?) apparently a lead engineer at the MIRA wind tunnel in England and the car was for him and his daughter to compete in time trials or hill climb. It was a Rover hatchback. They closed up the front grille significantly, had a deep airdam/splitter, deep side skirts, a pretty much open hood, those 'fences' along the entire length of the hood, nothing on the roof (argh...) and a rear spoiler at the back of the roof.

[serialk11r]

Ah hatchback changes things. I think fastback design has the greatest induced drag due to the C pillar angle.