Direct injected engines still have throttle butterfly valves, or something complex like VANOS to simulate one with valvetrains. Air and fuel must mix at a corre

Direct injected engines still have throttle butterfly valves, or something complex like VANOS to simulate one with valvetrains. Air and fuel must mix at a correct ratio, if you're requiring little power, you need little air and little fuel. Lots of air and low fuel gets you detonation.