In this paper, an idea to make the flow uniform inside the manifolds of four and six cylinder engines without changing their main geometries is presented. In general the uniform distribution of air flow among the engine cylinders is very important. Non-uniform flow in the intake manifold outlets causes inadequate feeding of the cylinders, inappropriate combustion and reduces engine volumetric efficiency. So this matter is vital for improving engine performance. A flow guide blade is added into the manifold and uniformity of the air flow is studied in different regime using transient numerical simulations. The amount of air exited from each outlet and the effect of this blade on flow uniformity is investigated. The results showed that the blade has made the flow more uniform in the manifold outlets and it was also found that in the four-cylinder and six-cylinder engines the flow uniformity is increased by 67% and 75%, respectively. In other words, as the number of engine cylinders increase, the effect of using this blade increases.