...as one of the folks posted above, the OEM pop-up spoiler is actually a "Gurney Flap".
The way it works is simple, when it pops up, it creates a high pressure area on the trunk lid. The faster you drive, the larger the high pressure area becomes. As you drive faster and faster and faster, the high pressure area fills forward from the spoiler, up the trunk lid, all the way to the roll bar if you go fast enough. So, the faster you drive, the more downforce you create.
Those aftermarket spoilers that let air pass THROUGH them kill this downforce.
Read about the concept here:
https://en.wikipedia.org/wiki/Gurney_flap
Save your money.
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