FYI: The manifold vacuum has nothing to do with the opening of the secondary air valve or vacuum secondary in a Holley carbs. What causes the secondary to open is the air traveling threw the venturi in a carb. Manifold vacuum drops down to almost zero when the throttle is at WOT. Secondary open on demand when the motor needs it.
Last edited by lg1969; 10-11-2012 at 03:08 PM.