I have read numerous articles stating that spoiler cause and decreases drag. I was told that only correctly designed spoiler can help reduce drag, and the factory ones actually causes more drag. I also heard that it depends on the geometry of your car. Can anyone tell me whether a 2008 with a factory OEM pedestal spoiler will decrease or increase drag?
Any added surface in an airflow adds drag. However, good spoiler designs can reduce the overall drag of your vehicle. This is achieved by minimizing the wake turbulence. So its a favorable trade off, slightly increased skin drag but much reduced overall drag.
Usually factories have testing facilities that help them make very efficient spoilers. Your source might have been referring to decorative spoilers which are put for aesthetic purposes.
PS: Many answerers have mentioned about downforce. That would be inaccurate here, because a downforce generating surface is a "wing" and not a spoiler.
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