if the M-B engineers have specified spark plug cables with a certain amount of electrical resistance, they have in mind a certain firing voltage across the plug for the hotness of the spark desired. If the plug is a resistor type, then some of the desired sparking voltage is dropped in the plug and not available for the magnitude of sparking desired. If the spark is less than desired, combustion is not as strong or complete and the power from the detonation in the cylinder is diminished. Yhink of it this way. If the fireman responding to your house fire takes the nozzel off of the hose and inserts a length of garden hose between the fire truck's hose and the nozzle, he won't be nearly as effective fighing your fire!