A bolt-on replacement intake manifold has an asymmetrical plenum with a first end including an inlet, a closed terminal end, a concave top surface and a convex bottom surface; a flange; and a plurality of runners extending from the bottom surface of the plenum and terminating at the flange. The plenum defines an interior space in flow communication with the runners. The bottom surface of the plenum is wider than the top surface. The plenum initially widens from the inlet to the first runner and then begins to narrow from the first runner toward the last runner adjacent to the closed terminal end. The runners are tapered, curved, and vary in length. The intake manifold causes air to exit each of the plurality of runners at substantially the same angle. The manifold balances airflow across each runner and increases swirl inside the cylinders enhancing fuel economy, power output, and torque.