A method of measuring the fluctuating velocity of gases at any point in the exhaust system of an operating four stroke internal combustion engine has been described. The instrument developed permits periodic automatic crank angle selection in a preselected engine cycle, discharge and ionization in the region of measurement and finally detection at a known distance. The time interval between the ionization and detection was used to determine the velocity. The pulsating velocity was measured both for motoring and firing conditions at various crank angles.