| STI vs EVO IX aerodynamics, and the history of STi vs EVO aero design
I know these pics were sort of posted elsewhere, but I have archived them and am rehosting them to make sure they stay available. Anyway, the first really blurry pic is a test of the EVO 7 versus the STI v7. What this test shows is that the EVO has very poor downforce characteristics. Notice the first test, with the wing set to its minimum position, the car has literally 4 times the aero lift of the STI. Then, with the wing in its maximum position, it has similar rear lift, but now 6 times the front lift (thanks to cantilevering the rear downforce across the rear tire fulcrum point):
Prodrive did some testing of their own regarding the bugeye WRX with stock parts and with their aero parts, as well as with a 1" drop. The results include front lift, rear lift, and drag power required to maintain speed at 100mph. Notice that lowering the car (1" front and rear) reduced drag and reduced rear lift. If you lower the car more in the front than in the rear, you can expect exactly the opposite to occur. Front lift will be drastically reduced, but drag will be increased.
Code: Tested at 100mph
Vehicle Setup: Front Lift Rear Lift DragHP
WRX with no wing 59.2 70.0 60.5
WRX with stock wing 71.4 50.7 60.5
WRX with Prodrive wing and no lip 75.0 -10.0
WRX with stock wing & Prodrive lip 65.8 51.3 57.7
WRX with Prodrive wing & lip 20.8 21.8 58.7
WRX, prodrive lip/wing, 2cm lower 20.8 14.3 54.5
Now we have the 2006 iteration of the STI and EVO. The biggest changes for the STI include the rear underbody diffusor and the roof vane. These are significant improvements, whether you believe it or not and the test data shows. The v7 STi wing (and the whole rear end, actually) is identical to the 2006 except for the roof vane and diffusor, so the difference shown in the test is QUITE dramatic.
First, the STI vs EVO airflow off the roof into the wing. Note that the EVO's airflow is quite turbulent (seen in the smoke getting diffuse and swirly) which makes the rear wing far less effective, thanks to it being in a useless airflow:
Next, the STI vs EVO airflow over the hood and front part of roof. Not much difference shown here:
The STI vs EVO airflow over the 'hoodscoop' area. What we see is a huge disturbance in airflow right where the 'vent' on the EVO hood sits. This is probably exiting airflow from the radiator inlet blasting up out of the vent and disturbing the flow. On the Subaru, no such disturbance exists and in fact what we see instead is a nice clean scoop of air being directed into the intercooler:
Looking at this you might think "but the evo has the better front" but what you'd be neglecting is that the Scooby, with the massive increase in rear downforce causes the front end to experience 'lift' as well, leveraged across the rear tire fulcrum. Notice in that older test that I posted up above, when you have the EVO with its wing on the "LOW" setting, the front lift is apparently low, but when you raise the rear wing to the "HIGH" setting, the front lift is HUGELY increased. The fact is that the 06STI has a tiny bit more front lift in this test but massively more rear downforce... if you remove some of the rear downforce, you'd see a huge reduction in front lift... but the better option is of course to add the front lip which not only reduces the front lift, but improves the drag as well.
Finally, we have the Yaw/Roll/Pitch characteristics of the EVO and STI. I have no idea what most of this chart means so I can't say if the higher or lower numbers are better, but one part of the chart I can interpret is the Cd and Sf shown to the lower right of each car. The Drag Coefficient is slightly higher on the subaru but it has a smaller frontal area so the total drag is actually lower. Making downforce means creating drag, but the less drag you add the better your design is. I think it's clear that the Subaru is more efficienct at creating downforce than the Mitsubishi since it has more downforce and less total drag.
Also, from the Prodrive testing it seems obvious that the appropriate mod for the 06+STI is the V-Limited front lip, but this also begs the question, on the 07 Limited, which has an almost nonexistent rear downforce scheme (joke of a wing, no roof vane), but has the V-Limited lip in place, it must feel a little light in the rear at very high speeds.
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