Quote:
Originally Posted by SentinelAeon
Is there a formula to calculate how much is lost ?

Had to dig up some of that old aerodynamics theory, but I think you could try with:
Delta P = f * Rho * (L / D) * (v^2 / 2); f=64/Re
Delta P = Pressure loss
Rho = Density air
L = Length pipe
D = Diameter pipe
v = velocity air
Re = Reynolds number
The flow Q1 = Q2; v1 * A1 = v2 * A2
Which means that the flow speed will increase in the smaller tube and thus increase the drag
Disclaimer: All in metric, laminar, isotropic flow!
It's not possible to do it without knowing some basic data about the fan, but as you can see from the equations the higher the flow speed the higher the losses.