Personally I question the frequent need of these things. I see a LOT of very shiny Land Rovers and Toyotas with these giant monstrosities on them, and all it does, IMHO, is block the driver's vision and make the trucks look silly.
Anyway, one thing I've seen from the desert people is to run the intake into the cab... if you were playing U-Boat Commander you'd still want to run the intake UP above the water level, but the air inside is cleaner and easier to filter.
I suppose at high speeds the air would get sucked *away* from a snorkel pointed to the rear of the vehicle, which is why they get painted forward.
Ideally, I imagine, you'd have it shaped like a question mark, i.e. folded over all the way DOWN at the top, so as not to suck up anything other than air.
Again, however, my experience is that the vast majority of snorkels, especially the commercially available kits, are aimed at the mall crawler crowd. (Doing it yourself with parts from Home Depot is not only rednecked and un-stylish, but technically superior

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-- A