The old 1.5 L voiturette formula was replaced with 3.0 L supercharged cars . Formula A permitted the old 4.5 L normally aspirated cars , but as the 3.0 L supercharged cars were more than a match for these ( and the pre-War German and Italian cars were no longer available ) , the old 1.5 L voiturette formula replaced 3.0 L supercharged cars in an attempt to equalise performance .