Pre ignition is not ignition ocurring before spark, it is basically the fuel burning too fast. If the timing and octane are adequate, the fuel will burn slowly providing even pressure as the piston goes down.
If octane is too low or ignition is too advanced, pre ignition occurs at the time of spark. Basically this means that there is a very sudden pressure "spike" as the burning occurs. This is why you hear it. If I could easily draw a curve it would have pressure on the vertical axis and time on the horizontal. With preignition at the time of spark there would be a very high spike that showed tons of pressure at one instant. The curve for proper ignition with adequate octane fuel would have a lower maximum pressure than the other curve, but would spread out over time.
Proper ignition provides burning over a relatively long period of time. Since the piston is going down increasing volume of the cylinder over time, the combustion maintains similar pressure during the entire descent of the piston.
Any of the MB engines sent to the US in the '89 time period have relatively high compression ratios requiring high octane fuel. You can use low octane fuel and the knock sensor will detect knock and the ignition module will retard timing enough to eliminate the knock. With the timing retarded you are decreasing the efficiency of the engine thus requiring more fuel to be used. For this reason, using a fuel with less octane than premium is usually false economy. The fuel is less expensive, but it will get you fewer miles down the road.
Hope this helps,