The limitation of the single stage supercharger was that the maximum power dropped quickly as higher altitudes were reached; because air pressure and air density decreases with altitude the efficiency of a piston engine drops because of the reduction in the weight of air that can be drawn into the engine; for example the air density, at 30000 feet is 1/3 of that at sea level, thus only 1/3 of the amount of air can be drawn into the cylinder and only 1/3 of the fuel can be burnt.