Yea, ya have to use something to rid the 0.6 coeff of entry "the discharge coeff", yet at some length the form becomes a nozzle or venturi that really helps the velocity rise for flow.
I use an intake port form ground from 3/4" aircraft plywood. I've seen some folks go to 2" longk clay forms, to get the good numbers. Pedestrian Disclaimer is i go for lowest noise, not numbers.
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