There is no knock sensor/ignition retard in the M103 (at least not on my U.S. version).

Larry's right, it's false economy to use lower octane fuel. In fact, when I go to states where I can get 93 octane fuel, my mileage and performance increases markedly.

Here in Arizona, you should hear U.S. cars pinging in the heat of the summer despite their so-called ability to use 87 octane fuel.