Manifold Pressure

The air/fuel mixture entering the engine reaches the inlet valve at the pressure in the inlet manifold (the Manifold Pressure). Unless the throttle is wide open, this pressure will be less than atmospheric pressure as a result of the pressure drops due to flow restricting sections of the inlet system. In addition, the inlet valve also restricts flow into the cylinder and causes a further pressure drop. These pressure drops increase the work that has to be done to pull the air/fuel mixture into the cylinder, work that is done at the expense of the work output of the engine. On the engine (p,v) diagram the net work output is the difference between the cycle work and the work done in "pumping" the inlet and exhaust gases each cycle. The top shaded area on the diagram minus the bottom shaded area represents the net work output of the engine.