The first big breakthrough was the launch in October 1948 of their new record-breaking engine design in their XK120 sportscar to replace the prewar SS Jaguar 100. It was powered by a new twin overhead camshaft (DOHC) 3.5-litre hemi-head six-cylinder engine designed by William Heynes, Walter Hassan and Claude Baily. The XK100 4-cylinder 2-Litre version had broken records in Belgium travelling at 177 mph. This XK 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".