Probably the best liquid cooled engine ever developed was the Rolls Royce Merlin. Since 1935 the British had the engine under continuious development. designed to be hand made, however, the enginedid not lend itself to mass-production techniques. On June 24, 1940 Packard was asked by the British goverment to redefine the engine and make it practical for American mass-production. The process required a complete set of new drawings which Packard prepared under the direction of Jesse Vincent. The Rolls Royce drawings from which Packard worked lacked details and specifications and were not in the third angle projection as is the American practice. The Rolls Royce drawings also omitted tolerances which Packard had to develop from an actual engine. It became a matter of taking an actual engine apart and going backwards to develop the needed drawings and specifications. Packard, under the direction of William H. Graves, chief metallurgist for the company, also had to develop the foundry specifications for the Packard-made engine.

1942 - Packard engines at war. The war effort was one of Packard's finest hours and included the development of a marine engine used to power PT Boats and the Merlin aircraft engine used in the P-40. Prior to the development of the Packard Merlin engine the P- 40 used the inferior GM developed Allison engine.

1944 - Packard's war record was unequalled. More than 50,000 Merlin aircraft engines and more than 4,000 marine engines had been made. The engines are known for their reliability and quality. The feat is unmatched by any other manufacturer.

The engine room of an Elco PT Boat contained three Packard engines