An engine anti-knock control arrangement controls both the ignition timing system and an evaporative cooling system so that when the engine is operating in a predetermined engine speed and load zone and engine knock is detected, the cooling system is operated to reset the temperature to which the coolant (and therefore the engine) should be controlled, to a predetermined minimum value and the ignition timing incrementally retarded until the knocking phenomenon terminates. While the temperature is dropping and knocking is absent, the ignition timing is incrementally advanced until the knock threshold is reached and thus brings the timing as close as practical to the MBT value for the instant set of operating conditions. Cyclic repetition of the ignition timing advancement and retardation holds the timing at MBT while the engine is operating in the predetermined zone.