This paper presents the results of an experimental analysis on a multi-cylinder diesel engine, in which in-cylinder pressure and accelerometer transducers are used with the purpose of developing and setting up a methodology able to monitor and optimize the combustion behavior by means of non-intrusive measurements. Previously published results have demonstrated the direct relationship existing between in-cylinder pressure and engine block vibration signals, as well as the sensitivity of the engine surface vibration to variation of injection parameters when the accelerometer is placed in sensitive location of the engine block. Moreover, the accelerometer trace has revealed to be able to locate in the crank-angle domain important phenomena characterizing the combustion process (the start of pre-mixed combustion, the crank angle value corresponding to the beginning of diffusive combustion and to the in-cylinder pressure maximum value). The objective of the present work is to quantify the strict relationship between the in-cylinder pressure signal and the accelerometer trace, in order to obtain information concerning both the pressure maximum value and the pressure rise rate from the accelerometer trace. In this paper, the previously developed steps of the research program are briefly presented. The processing technique is described and representative results are presented and discussed, in which different values of speed and load have been imposed on the engine.