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The air needed for combustion in the engine flows through the variable intake manifold to the intake valve. In naturally aspirated engines, the length of the intake manifold is subject to variable demands at different engine speeds. This ensures the highest possible continual air inflow rate into the combustion chamber. At low revs you need a high level of torque delivered as fast as possible - by means of a long intake manifold - while maximum power output at high revs demands short intake manifolds. The variable intake manifold features a flap system governed by the engine control unit that meets both those demands. It channels the air via the long intake manifold at low revs and through the short section at high revs. Turbo engines do not need a variable intake manifold because the boost pressure of the turbocharger determines the volume of air entering the combustion chamber. Intake manifolds in turbo engines are therefore very short for reasons of space and cost.