The air required 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 in order to ensure the highest possible air inflow rate into the combustion chamber at all times. Whilst at low revs a high level of torque needs to be delivered as rapidly as possible – best attained by means of a long intake manifold – 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, channelling the air via the long intake manifold at low revs and through the short section at high revs. Turbo engines do not require a variable intake manifold because the volume of air entering the combustion chamber is determined by the boost pressure of the turbocharger. Intake manifolds in turbo engines are therefore very short for reasons of space and cost.