Re: So maybe I'll just make a DIY Heat Extractor hood
You can't have it both ways. As you said, size is important - you want the things to be big enough to move enough air that they actually make a difference. But, the bigger they are, the less that venturi effect is going to work - you have some pressure difference between the relatively stagnant air underhood and the fast moving airstream on top of the hood, and the larger the area of the vent is, the smaller the differential pressure. You'll see on a lot of race cars that there's some sort of wicker on the leading edge of the hood vents to help generate a greater D/P.
Originally Posted by bneal1022
And, just to clarify, placement does matter as far as where the radiator is. Going back to the Volvo S80 diagram I posted before, you can see that the light blue (lowest pressure on top of the hood) is towards the front. To maximize the D/P and make the vent more effective, I would want to put it in that light blue zone. But if I do that the vent is probably in front of or maybe right on top of the radiator, which means it's not doing much to help air flow. That's the mistake I made on my Mustang's hood - the hood vents I had added are too far forward, so I don't think they're as effective as they could've been a couple inches farther back.
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