For 1966, the Quattroporte was further refined on a number of points and the car received a slight restyling. The double round headlights, a feature previously reserved only for vehicles sold in the U.S.A., were now standard on all cars and replaced the single, rectangular units of the first series. The car's advanced De Dion type rear axle was for the second series replaced by a more traditional rigid rear axle layout with leaf springs that derived from the Mistral model. This modification was done after complaints from costumers of driving noise when driving on irregular road surfaces. In fact, tests showed that with this solution, the interior noise level was reduced by up to 20dB when driving at highway speeds. Furthermore, the car's interior was further refined with a new dashboard with fully integrated climate control system as standard equipment. Most Quattroporte I cars of the second series were fitted with the standard  4.2L V8 engine, but 64 cars received the larger 4.7L unit and two cars were even equipped with a 4.9L engine variant.