The high-pressure hydraulic circuit of the Multi-jet Common Rail (C.R.) system has thoroughly been investigated in the last few years by researchers of the automotive field. However, shortage of knowledge is still present about the high-pressure pump performance. Hydraulic-mechanical efficiency of the pump is only known as mean value and no published data are available on the Radial-jet compression volumetric efficiency. Due to the fact that part of the pumped fuel is partially expelled by the pressure-control valve and because of the presence of the oil flowing in the cooling and lubrification circuit, the determination of the compression volumetric efficiency seems to be a hard task. In the present paper a detailed description of the Radial-jet performance has been provided. The dependence of the flow rate sucked by the high-pressure pump, on speed and load has been studied and the characteristic curve of the cooling-lubricant circuit has been determined. A special procedure was designed and applied for the experimental evaluation of the fuel leakages from the pumping chambers, so as to allow the calculus of the volumetric efficiency. The actual head-capacity pump curves at different revolution speeds were plotted and compared with the electroinjector flow-requirements so as to allow the evaluation of the efficiency of the pressure-control strategy. Furthermore the pump mechanic-hydraulic efficiency dependence on head and speed was also experimentally assessed.