Found a better picture of what the Air Design "cut a big hole" fender vent looks like. The cutting makes it a nonstarter for me. The raw black plastic looks a bit cheapo to me. Maybe painted?
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I like it painted too. Not that it looks bad black, but as someone mentioned a while back, it does have a F-150 vibe to it when it's black.
I'm no engineer but obviously air gets rammed through the grill and radiator, blown by the fan over the engine and exits the vents...cool air in hot air out..seems plausable right?
I am, but I haven't looked that closely at them or the engine compartment to see if they'd actually provide any function or not.. Your thought process is following logic though.
Shooting from the hip based solely on the above pictures, I doubt there'd be an appreciable difference. That said, if there is a path for air to evacuate the engine compartment through this vent you could potentially drop the temp in the engine bay briefly. I don't think it would be enough to effect a reading of the actual engine temp, but I don't see anything that the vent would hurt either. The main issue I see preventing it from having any real effect is the size. It's probably going to be too little air flow for as much radiant heat that is put off by the engine. While driving and the air is moving you may get a little change in the ambient temp of the engine compartment, but I suspect that as soon as you stop the heat will promptly build back up to normal. At the end of the day though, I don't think these will hurt anything.
Again, this is all purely speculation based on a photo.