A measurement method to detect the knock-resistance and, especially, the knock-limited conditions of high-speed s. i. engines is described.Two instruments are used in conjunction: the first instrument measures a knock intensity index I. D. related to knock-typical pressure oscillations in the combustion chamber; the second instrument detects the knock-limited conditions in real time, by means of I. D.. This latter instrument, which can be properly used as a knock-limit detector in operating-parameter control systems for automotive engines, is analyzed in detail. Its operating principles, based on statistical criteria, are here described and have been extensively verified by means of experimental tests.The checking tests carried out at high speed, both on an automotive engine which was mass-produced and on an automotive engine prototype, are described. The knock resistance of the prototype, in its whole working range, is then analyzed.