Manifold

This invention relates to an adjustable manifold which includes an external tube, an internal tube and a valve adjusting mechanism. The internal tube is arranged within the external tube so as to form a ring channel, and the valve adjusting mechanism is attached on the external tube. By this way, when the engine runs at a low revolving speed, the ring channel in the external tube is blocked by the valve adjusting mechanism, and the internal tube is passable for increasing fluid velocity and enhancing inertia effect and pulsation. When the engine runs at a high revolving speed, the ring channel in the external tube is unblocked by the valve adjusting mechanism, and the external tube and the internal tube are passable for enlarging the total sectional area of the manifold so as to lower flow resistance and increasing air admission efficiency. No matter the engine runs at a low revolving speed or a high revolving speed, the amount of air intake can be adjusted.