The mean effective pressure is a quantity relating to the operation of a reciprocating engine and is a valuable measure of an engine's capacity to do work that is independent of engine displacement.
Mean effective pressure (MEP) It is the ratio of engine work done to the swept volume (Vs). The work output of the engine can be calculated from the summation of p*dV (product of pressure and change of volume at each crank angle). So, MEP = Σ(pdV) / Vs