https://ecomodder.com/forum/member-a...speed-aero.jpgThat sharp inlet of the coroplast is introducing a vena contracta entry loss, knocking the flow efficiency from 98%, down to 74%.
You need to provide a bell-mouth entry radius, all the way around to get rid of it.
If you'll look closely at Spindletop's fascia, you'll notice that the 1963,Walter Korff, NASCAR inlet, is radiused like a velocity stack on a Ford GT 40's 427.
You'll need something similar.