No, you will still get some benefit without the fender louvers or tire reliefs, just not as much. Air will still make it out, but you are correct that it will be more difficult to get out.AJ, am I safe to assume that a car without the fenders vented sees no improvement from the ducting on the splitter that is leading to the wheel well. If the air doesn't have any place to go, it will stagnate. No flow equals no downforce.
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