It is well known that a appropriately designed Exhaust manifold is important for the optimal performance of an IC engine. Though, the intake system is dominant on the cylinder filling process, the exhaust manifold has ability to influence the gas exchange process in various important aspects, like the piston work done during the exhaust stroke, the short-circuit of fresh charge from the intake into the exhaust. We know that if the back pressure of Exhaust gases is higher, it reduces efficiency of the engine. So aim should be to reduce the back pressure of the engine. Also amount of pressure drop is equal to back pressure. So ultimate goal is to reduce its pressure drop by changing its geometry, reducing friction in the manifold. In this paper only Geometry optimization is done. This paper presents 3-D simulation of Exhaust manifold of 4 cylinder, 4 Stroke, turbocharged diesel engine(4G11 Greaves Engine) of generator by using ANSYS-CFX code and results are discussed. A Steady state CFD simulations have been accomplished and experimental results are compared for validation. The CFD results gives an idea for changing its geometry.