Initially, supercharging of aero-engines was introduced with a view to restoring the pressure in the induction manifold at altitude to that corresponding to sea level. During recent years this has been changed, and supercharging is now employed not just to maintain sea level pressure in the induction manifold, but to boost engines at altitude, in order to develop high B.M.E.P.s and thereby obtain high power outputs at high power to weight ratios.