roll front windows down 2 inches

[aspera]

Racecar Engineering had a great article a while back where they wind tunnel tested a BTCC car. IIRC, one of the curious things that they found was that the front downforce increased and drag decreased when they rolled the front windows down about 2 inches. I think it has something to do with air filling the low pressure areas behind the A-posts. I assume the air came from the high pressure zone in the cowl.

On my car, I have no carpet and all of the drain plugs removed. I get a nice breeze when I lower the windows an inch or two.

I've also spaced the rear of my hood (for cooling reasons) and noticed a change in pressure when I have the HVAC on 'vent'. Most of the air seems to go over the wipers and not pressurize the cowl area.

About a year ago I added window vents to my WRX. (The ones for a WRX are really slick.) They make slightly more wind noise when the windows are rolled up, but really suck out the air when the windows are cracked a bit. I can get by without using the A/C quite as much, and it really gets the hot air out of a parked car quick.

As you guys know, the WRX has a huge non-aero hoodscoop. Rally versions have roof scoops instead of A/C. That's a trade-off, but I've seen an even better solution on one of the RallyAmerica cars. The guy used ducting hose (think drier hose) to vent from the high pressure cowl directly on the driver.

[SVOboy]

I've found similar results on my CRX. I can feel the breeze all the way down to my feet just from a small 2" crack

[wyatt]

I tried both front windows down about an 1.5 inches last night. I didn't see any MPG gains, but no losses either. May be a good candidate for some A-B-A style testing...

[NeilBlanchard]

Hi,

I'm pretty sure that this effect will vary for different vehicles? The slope of the windshield, the angle of the 'A' pillars, and the aerodynamics of the front of the vehicle will affect it. If the operable window opens just at the top, or if it opens also along the leading edge of the window, and how close it is the 'A' pillar, and the general flow of air in this area; will all possibly affect this tweak.

As some have already said, even if the FE doesn't improve -- if it helps with the air flow and doesn hurt the FE (much), then this will be a good technique for ecodriving, for sure.

I know that in my xA, if I open just the front windows, it has a different effect than if I also open the rear windows; or just the rear windows. If I open just one rear window too much -- I get this massive low-frequency "beating" that pummels the eardrums! Conversely, I can open just the front windows all the way up to say 45mph, and get almost no additional noise at all.

[aspera]

Originally Posted by NeilBlanchard

As some have already said, even if the FE doesn't improve -- if it helps with the air flow and doesn hurt the FE (much), then this will be a good technique for ecodriving, for sure.

Great. You said it better than I could.

I think one of the odd things in the Racecar Engineering article was how this air effected the ends of the rear wing on the subject car. I guess the air that gets sucked out the front edge of your windows by the A-posts ends up curling around the C or D-posts and influencing the airflow on the rear window. So the results would very much depend on each individual car.

There might be some kind of swirling vortex at the A-posts that you're helping to feed (with interior air) that helps hustle the exterior air around the outside edge of the A-posts. I think that's how it works. Sort of like a Gurney flap.

You guys should check that magazine out at the bookstore. They've got some great wind tunnel test results on little stuff like splitters and dive planes.

[cfg83]

SVOboy -

Originally Posted by SVOboy

I've found similar results on my CRX. I can feel the breeze all the way down to my feet just from a small 2" crack

I just confirmed this today on the freeway with both front windows open 2".

CarloSW2

[whokilledthejams]

There is a kit available for the Impreza/WRX to add that roof vent, if you're brave enough to cut a hole in your roof. I believe it was a dealer option in Japan. It's pretty slick, though.

[aspera]

There are several neat things about Imprezas. One neat thing is that you can buy race prepped versions from the factory. The Subaru Impreza WRX STI Spec C Type RA-R is a nice one. It's even better if you know what all the letters mean. The important one here is 'C' for 'Competition'. A *normal* competition package means A/C delete. Cars without A/C get hot, so that's why they get a roofscoop. The factory cars get a roofscoop that folds flat when not used. The big fixed roofscoops mount over the inside scoop and are usually open at the rear. Some race roofscoops I've seen are full carbon fiber and have a false floor built into the outer scoop to keep out rain and drain tubes to get rid of the water.

I'd love to see more race homologated production cars like the WRX, but I won't hold my breathe. If there was a production-based race series for high fuel mileage, we'd see some real wild cars. For example, Toyota is slated to make a RWD small car using a Subaru flat 4. It's supposed to be a spiritual successor to the AE86 Corrolla hatchback (the Hachiroku). Subaru offers a turbo-Diesel flat 4 in Europe. Combine those ideas and you get a slick little 2 door Diesel coupe.

[Will]

Thanks for this thread. It's a good thing to remember when I start back to the mpg quest next spring.

[Sean T.]

I wonder what effect this would have on a car with front ventvisors? I would imagine there is a pocket of air beneath the visor, but it definitely makes it less stuffy in the car when I crack the front windows.