Vacuum helps but it's not needed if the boosters are the right type (annular or triple-stack and/or at least sharpened lower edges), short-side radius in the intake port has the right contour and the port velocity is decent for the intended application (displacement and RPM range). I haven't seen one in person yet but direct-injection engines don't have a SSR because its sole purpose is to shear the wet flow in port-injected/carb'd engines. That's actually the most critical thing, much more than vacuum unless we're talking strictly street engines at part-throttle.
