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The idea of having two intake valves is simple. The only thing that draws air into the cylinder (unless you have forced induction) is the vacuum created by the piston moving down. The freer the air flows into the cylinder with less restriction, the more air you get. more air = more oxygen. More oxygen= more fuel you can burn. More burning fuel = more power. Exhaust flow is less important (tho it IS important) simply because as the piston moves up, its going to force the spentair out.