because think about the physics of the situation. The Zetec has negative pressure from the exhaust gas forcing out of the cylinders. When the intake opens, that negative pressure creates a VACUUM that sucks the air right into the engine.
In the case of the M45, the supercharger is actually blocking off the intake manifold so the only air the engine gets is the air coming through the rotors.
Now the rotors themselves aren't creating a vacuum as powerful as the result of an explosion in the engine cylinders. They working by displacing the air that is around them, the rotors just spin around and around. As a result of the small rotors spinning, only a small vacuum is created.
What this is showing is that the M45 simply cannot create a large enough vacuum to suck air all the way up through a restricted CAI tube. Especially a vacuum as large as the engine itself can generate. Just another parasitic feature of forced induction.
Now because work is defined as force times distance, the supercharger then has to do more work to pull the air all the way through the tube which then inhibits the efficiency of the supercharger.
Visually,
Suck air through a 3" straw and suck air through a 10" straw.
Lengthening the straw adds resistance
What I plan to do visually
Suck air through a 3" straw
Suck air through a 10" straw with a larger diameter.
Lengthening the straw adds resistance, increasing diameter decreases resistance.
Using fluid dynamics equations that I'm not going to flood the thread with lol,
or maybe I willl.....
R = (8hL)/(pi)r^4
R resistance
h=fluid viscosity (air in this case)
L=tube length
p=air pressure
r=pipe radius
You can calculate the exact length and diameter needed to have the same resistance as the SRI so that the CAI doesn't restrict airflow anymore than the original SRI by setting the resistances equal to each other.
(8h(L1))/(pi(r1)^4)=(8h(L2))/(pi(r2)^4)
8, h and pi are constant so they go away so it becomes..
(L1)/(r1)^4=(L2)/[(r2)^4]
by isolating r2...
r2=(L2*[(r1^4)/(L1)])^(1/4)
Now say our SRI has a length of 2.5 feet (30 inches) and an inner diameter of 2.5" (radius=d/2=1.75")
And lets say our CAI is 4.5 feet (54 inches) long
r2=(54*[(1.25^4)/(30)])^(1/4)
r2=1.45"
So for a 4.5ft CAI to have the same resistance as a 2.5ft SRI with a 2.5" diameter, you'd need a 2.9" pipe.... SO THERE OSCAR JACKSON

lol..
Yeah my major is Aerospace Engineering, so what lol....
/end of my longest post ever
EDIT I ACCIDENTALLY SAID 2.5/2 = 1.75. I HAVE CORRECTED THIS, YOU DONT NEED A 4" PIPE, YOU ONLY NEED A 3" PIPE!!!!!!!