The first P6 used a 2.0 L () engine designed specifically for the P6. Although it was announced towards the end of 1963, the car had been in "pilot production" since the beginning of the year, therefore deliveries were able to begin immediately. Original output was in the order of . At the time the engine was unusual in having an overhead camshaft layout. The cylinder head had a perfectly flat surface, and the combustion chambers were cast into the piston crowns (sometimes known as a Heron head). With the rapid construction of motorways around the world, many of which (including those in the United Kingdom in this period) had no upper speed limits, the P6's engine was developed to be efficient, smooth and reliable when driven at high speeds for long periods. It had 'square' dimensions where the cylinder bore and stroke were the same (85.7 mm or 3.4 in) to minimise piston speeds and bearing loads, which was very unusual amongst British saloon cars: these tended to have under-square (long stroke) engines, a lingering legacy of the pre-1947 horsepower tax system and the driving conditions encountered on British roads in the pre-motorway era.