#NYIAS That wing on the 2011 Subaru Impreza WRX STI? It's tough to love. But there's some aerodynamic function behind it. It drops the STI sedan's coefficient of drag to 0.35 versus 0.37 on the hatchback. It does this by creating a more continuous steam of airflow, as air passes over the roof and straight over the large wing. In contrast, air hits the hatchback's roof-mounted wing and then drops off more abruptly. Also helping is the STI sedan's 6-inch-longer body.
All this, says Martyn Harding, product manager for the Impreza and Forester line, is enough to increase top speed.
The STI hatchback tops out at 250 km/h (155 mph). But the STI sedan keeps on going to 255 km/h (158 mph). A negligible difference, maybe, but it's something.
Here's something else: Subaru will offer a Limited version of the 2011 STI sedan, but not the hatch. That means if you're soft enough to require leather upholstery and a sunroof, you're getting the sedan.
One other bit of trivia: Subaru of America wasn't able to source the correct front spoiler for the 2011 Subaru WRX STI sedan and hatch in its booth at the New York auto show. Eagle-eyed fanboys will note they're wearing the same spoiler as the normal WRXs on stage. The production versions, which go on sale in August, will have a different spoiler.
There's video of the STI sedan unveiling after the jump, plus more photos. You'll notice somebody's head is obstructing the brilliantly captured footage -- that head belongs to none other than Hiroshi Mori, the chief engineer for the STI.
amorak says:
10:27 AM, 04/ 1/10
"2011 Subaru WRX STI Sedan: Love the Wing, It Increases Top Speed" Actually, no it doesn't. Further to your article, and per Subaru PR - "the wing negates lift at high speed". That means it puts downforce on the car at high speeds. Downforce, Erin, is WEIGHT on the car. This wing causes the car to have a LOWER high speed than the same car without the wing. The wing provides more control at the limmit and in high speed handling, at the EXPENSE OF a higher top speed.
Erin, my dear, the reason for the higher top speed is that the sedan is LIGHTER than the hatch (also, per Subaru PR).
amorak says:
10:28 AM, 04/ 1/10
Further to my original post, and DIRECTLY FROM a previous Edmunds post:
"Said wing weighs 33 pounds, which is exactly how much heavier the 2011 WRX STI sedan is compared to the five-door hatchback. But the wing assures the four-door STI has zero rear lift, just like the five-door. Also, if you're looking for rivia, the sedan has a 0.35 coefficient of drag compared to the hatchback's 0.37. "
the lower CD of the sedan also contributes to the higher top speed.
stovt001 says:
10:34 AM, 04/ 1/10
Since I grew up seeing WRX sedans with huge wings, this new STI sedan with wing brings back some good memories.
eriches says:
10:58 AM, 04/ 1/10
@ amorak: Nope. The word from Subaru is that the huge wing on the sedan creates a more continuous path for airflow, smoothing it. On the hatch, the air flows over the roof wing but then drops off, so it's not as slippery. So, yes, indeed, the sedan does end up with a higher top speed than the hatch.
Weight difference is negligible, but the sedan is a little heavier. Also rear wing on sedan ensures zero rear lift. Doesn't create more downforce than on hatch.
Thanks for giving me the benefit of the doubt. --Erin Riches
carlisimo says:
11:55 AM, 04/ 1/10
Amorak, I can’t quite agree with your first post. Weight plays into acceleration (mass, actually), but not top speed on a level surface. Top speed is a function of power, internal friction, and drag… this was on a fluid mechanics final so I remember it well. Weight only affects top speed on a slope.
This wing reduces drag, so regardless of its downforce or lift it gives the car a higher top speed. At low speeds, before drag has much of an effect, it reduces acceleration due to its weight.
Downforce-producing aerodynamic objects often create drag, and the masters of downforce – F1 cars – have a ton of drag, and that’s where the misconception comes from. Furthermore, downforce doesn’t affect acceleration because it’s massless. (Acceleration is force divided by mass.) That’s why racing cars love downforce. You get an advantage in grip without any of the disadvantages of carrying extra mass.
subytrojan says:
01:14 PM, 04/ 1/10
Well said, Erin and carlismo!
hybris says:
02:53 PM, 04/ 1/10
Finally I can get a sedan STI instead of a freakin hatchback!
/rant
stijockey says:
03:40 PM, 04/ 2/10
the subaru PR shots show the hatchback with a sunroof as well as the sedan...so I'm guessing that the limited trim will be available across the range. The also make no mention of the limited trim being a sedan only option on the subaru global site...finally I can replace my beloved STI hatchback with another hatchback with heated seat!
cr_driver says:
10:08 AM, 04/ 3/10
It`s always funny when someone gets OWNED!
Cause they think they are totally correct, and only them.
And its always good to learn from a "fluid mechanicals final" !!!
philsfan3 says:
11:25 PM, 04/10/10
I would like to say that amorak is naive for forgetting that WEIGHT does not affect top speed only acceleration. Example you can go the same top speed with a car load of people or just yourself. I'm only recently into sti's but I'm a physics major and weight cancels in equations (commonly known fact).
xsk8rat says:
11:58 AM, 04/15/10
The overall effect of any aerodynamic device is more complicated than this discussion of first order effects will uncover. The proof will be in the performance numbers demonstrated by third party reviews.
Air flow over road cars has a characteristic where the point of detachment moves up and down the back of the car over time. This irregular flow causes vortices which increase the drag of the air. At high speeds the effect on the overall drag can be big. The wings on cars like the STi help fix the point where the airflow detaches from the back of the car. Proper placement of this detachement eliminates the tendency for vortices to form and decreases this contribution to overall drag. The overall effect of the wing can result in a higher top speed and improved accelleration (at higher speeds).
The rear downforce can be increased due to the effect of the wing. By itself, increasing downforce will not affect the acceleration. However, if the wheels are limited by traction, then any gain in downforce can work to improve the car's accelleration.
The wing has a real mass which will affect acceleration at all speeds. Additionally, it is located quite far from the (presumed) center of rotation and this will affect the rate of change of direction (quickness of turning). However, the relatively small weight of the wing (relative to the car) probably has a negligible effect in this regard.
If the effect of the wings weight were appreciable, drivers would see better performance if they lost some weight and keep the wing.
Ciao,
Mark
xsk8rat says:
12:05 PM, 04/15/10
My bad,
The effect of "negating lift" is not the same as adding "down force". I could discuss the difference, but i have to get back to work... maybe later.
Ciao,
Mark
evo9allday says:
07:49 AM, 08/14/10
you all should shut up and just love this wing...really if this wing is your problem on performance just switch it for a carbon fiber wing or just simply take it off. it doent matter any ways evo 9 will still smoke this car..
id take this sedan sti over the hatchback ...
detail_devil says:
10:32 AM, 08/16/10
"I would like to say that amorak is naive for forgetting that WEIGHT does not affect top speed only acceleration. Example you can go the same top speed with a car load of people or just yourself. I'm only recently into sti's but I'm a physics major and weight cancels in equations (commonly known fact). "
Correct, but if you want to get really technical then your example needs an extra provision. The same top speed is achieved if you are by yourself or a car load of people with all else being equal. I realize we all know you meant on the same streatch of road, same conditions (wind, atmosphere, etc...). However, in order to achieve the same speed by yourself as with another 3 or 4 passengers, then you would technically need to increase tire preasures to the point of matching the same tire profile (would affect Cd) and resistance (or friction) as with just you in the car. This is why you should always set tire preasures based on what is on the door jam of the car (pressures calculated based on mass of car, often has second set of higher pressures for fully loaded vehicle) and NOT what is on the sidewall of the tire (simply a max pressure capability of the tire). Also, you would need to upgrade the dampers/springs (most likely in the rear) to allow the car to have the same overall "angle of attack" or wedge/profile (generally a car is setup with a slight increasing rake from front to rear, 1 degree I believe). This rake does generally promote more high speed stability (at least for sporty cars); so with a car full of people you might actually get a little lower Cd and possibly a higher top speed, along with more of a floaty feel. I think the higher rear is also there for sedans/wagons so the car does not sag with rear passengers and a trunk full of luggage.
I knew what you meant, I just thought I would mention the above because it might allow you to win a bet with a physic's egg head after a few drinks! Oh, and to demonstrate that what I write is not as nit picky as you might think, I remember when the 2005 997 C2 came out, and I believe the CD was 0.28 or 0.29 with the stock 18" wheels with rear 265 width tires. If you opted for the 19" C2S lobster wheels, then you got an inch wider wheel in the rear with a 295 width tire. This little change actually increased the CD by 0.01 to either 0.29 or 0.30! I thought it was interesting anyway.
r1xsvspd says:
05:49 PM, 01/17/11
The wing installed on the 2011 STI has a flat top surface and curved bottom surface. It is in effect an upside down airplane wing. The STI wing produces downforce but at the expense of air friction drag. Both the downforce and the aerodynamic drag increase with the square of the speed. This aerodynamic fact by itself is not conclusive as to whether the top speed of the car is faster with or without the wing.
As noted above, the wing has an affect on the way the streamlines of the separated air come back together behind the car. Just as the shape of the front of the car has a significant affect on aerodynamic drag, the rear of the car has a significant, but lesser, affect on drag.
Unless anyone has access to Subaru's wind tunnel test data, the definitive answer can only be found by running the car with and without the wing at top speed. Yes, I too studied Bernoulli's principle in fluid mechanics.
Now, who has contact information for an aftermarket company that produces an F-duct for the STI?