here is some traditional imbert gasifier math from gasser guru Dutch John

calculating gas required to run an engine...

G = V x n x 0.5 x 0.48 x 0.72 [l/s]

60

G is the needed quantity of gas in liters per second

V is the engine displacement in liters

n is the rpm

0.5 is the four stroke factor

0.48 is the mixture composition (1: 1.1)

0.72 is the filling degree of the engine (assumption)

60 for the conversion to seconds

DJ's volvo example

Example: G = 2.32 x 2,750 x 0.5 x 0.48 x 0.72 = 18.4 [liter cold gas per second]

60

calculating the restriction

d = square root from (4/pi x G/Vi)

d is the restriction diameter

pi is 3.142

G is quantity of cold gas per second

Vi is the superficial velocity being: 2.5 [m/s] = 25 [dm/s] for an Imbert

Example: d = square root from ((4/3,142) x (18,4/25)) = 0.97 [dm] = 97 [mm]

from this you can calculate the rest of the dimentions

all of that should get you in the ball park.

I build what i consider a modified imbert so I usually calculate restriction sizes and use that as my starting point.