Here are the airflow efficiencies I found in a library book for smooth duct openings with different shapes. (Ignore all the dots, I had to add them to get the solid lines to position correctly.)
I thought you people making your own cold air intakes and ductwork might find this useful.

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Plain opening with sharp edge, efficiency = 72%

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Flanged opening, efficiency = 82%

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Funnel-like tapered opening, efficiency = 90%- 98%
Angle of taper 20° to 65°

Curved "Bellmouth" shaped opening, efficiency = 98%

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Can type opening, efficiency = 82%

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Can with funnel-like tapered bottom, efficiency = 90% to 96%
Angle of taper 20° to 65°

Can with curved bellmouth shaped bottom, efficiency = 97%


The sharp edged plain opening is the least efficient because most of the air is sucked in from right next to the duct. As it makes the sharp turn into the duct, its inertia makes it overshoot a little bit and the flow constricts to as little as 70% of the duct diameter a short distance inside the end of the duct.

 

Isn't this all negated however once you slap on an open element filter? I can see how this is a good tech tip for those w/ dual snork intakes but for someone whos running a single inlet, this doesn't matter. I'm gonna do a bell opening for my ram air project

*Tubing from fog light box on 1990 Iroc running up to gutted air boxesand sealed off*