Originally Posted by wstar

The goal is increasing the airflow through the radiator by giving the hot air coming out of the radiator somewhere to go. With no hood vents you tend to just build up pressure in the engine bay and that restricts radiator flow. If you install hood vents in the most wrong way possible, you'll end up pushing cold air *into* the hood through those vents, which will counterintuitively make the car run hotter because you're making the pressure situation worse (because radiator airflow is vastly more important than just stuffing cold air randomly into the bay). A rearward-facing vent that's too close to the windshield can be bad that way, because there's usually a flow of air that rolls under at the base of the windshield and moves forward along the upper part of the hood. Each car is a little different though, and it's hard to say without careful analysis.