Changes due to magnetic field actions are very small at the molecular level by permanent magnets such as Neodymium (NdFeB).Therefore, it is not rare that different lacks of precaution result in a frequently limited global changes in the fuel and in an inefficient performance of devices using magnetic action with or without previous treatments by hydraulics changes; such as sections enlargements and/or fuel flow splitting to optimize the combustion.In addition, those changes can be attenuated or totally neutralized by variations in fuel environment conditions and/or by a defective installation of the device that give rise to unsatisfactory results or render the device completely ineffective.That is why it is necessary to precisely design magnetic devices to build them under a continuous rigorous technological control and protect them as shielding them against random electromagnetic field actions and to install them taking all the precautions to prevent environmental influences as the earth magnetic field in some instances, tending to minimize their efficiencies.There are few formal studies in the open literature on the engineering design of magnetic devices.In this study, intended to improve the efficient performance of magnetic minimizers using permanent magnets, with or without hydraulic fuel flow changes, some criteria based on theory and experience on engineering design, building and installation, based in the comparison of the most representatives magnetic devices with and without fuel hydraulic flow changes with a chosen prototype of efficiency, proved in an ADC, are presented.