James Cowl inlet size is 4.750" ID, outlet hole on mine trimmed flush with firewall is 14" along the top, 3.5" tall and has a radius edge to a 12.5" bottom lip. I have Vetterman 4 pipe (1.75" OD) So here is how I calculate the areas:

Inlets=35.44 sqin
effective Outlet=35 sqin

what does that mean? I have no idea, but looks like a 1 to1 ratio. I like the speed my plane has and don't want to make a change that will hurt those numbers, but could use a little better summer/climb cooling.