The big breakthrough was the launch in 1948 of the XK120 sports car, powered with the new XK twin overhead camshaft (DOHC) 3.5 litre hemi-head six-cylinder engine designed by William Heynes, Walter Hassan and Claude Baily. This engine had been designed at night during the war when they would be on fire watch in the factory. After several attempts a final design was achieved.
That is until owner William Lyons said “make it quieter”. The car had originally been intended as a short production model of about 200 vehicles as a test bed for the new engine until its intended home, the new Mark VII saloon, was ready. The XK120’s exceptional reception was followed in 1954 by the introduction of the derivative XK140, and a much revised XK150.