Originally posted by eye_con:
the basics of it is simple, at WOT you have a vacuum in your intake or a lower pressure than the outside pressure the fan just helps increase the pressure inside the intake to what the outside pressure is. that being said it doesn't really give you boost it just eliminates the vacuum.



The fan does give a boost, and will create a positive pressure if the remaining restrictions in your intake are mostly downstream of the fan. On a Zetec with a custom intake, the biggest restriction is going to be from the throttle body, manifold, and valves, so upstream of that point you'll probably have positive boost. On my setup I believe the pressure within the intake pipe was about 0.4 PSI above the surrounding atmosphere.