If the free flow of the exhaust from the cylinder head faces any obstructions, twists, bends or turns to guide the pressure, the flow is reduced the pressure however remains the same. This creates back pressure. Backpressure is caused in two strokes and four-stroke automotive, however, it is quite undesirable for two stroke automotive. Back Pressure has a negative effect on the engine efficiency that can result in a decrease of power and speed output, and this is often compensated by increased fuel consumption. Much fuel is wasted due to this and unburned mixture can also increase pollution reducing engine life and efficiency considerably. This can be avoided by increasing the pressure at the exhaust port than the pressure in the cylinder.