Abstract: An insert for the receiver of an intake manifold for an internal combustion engine. The receiver receives air/fuel mixture from a carburetion system, usually from a plurality of carburetor barrels, and runners exit from the receiver to conduct the mixture to respective cylinders. The insert, which can be of a selectible or adjustable contour, serves to direct the flow of mixture to the various runners in accordance with the demands of the respective cylinders. Correct selection of insert contour and size enables an individual manifold configuration to be adapted to the unique airflow pattern and volumetric requirements of an individual engine, rather than to an average engine, and also to specific performance ranges, rather than to a wider average performance range. The insert can also be movably mounted to provide for various load conditions. The invention comprehends the combination of the insert and a manifold.