It might sound counterintuitive but all induction is forced. Yes, even naturally aspirated engines. Gale Banks explains that as the piston in the engine drops during the intake stroke, a near-perfect vacuum is created above the crown of the piston. The difference in pressure from that near vacuum to the air pressure in the intake manifold is what forces air into the cylinder. In a naturally aspirated engine that manifold pressure is simply the ambient air pressure (with a small margin depending

Naturally Aspirated Engines, How They Work.