Heynes was educated at Warwick School from 1914 to 1921 before joining the Humber Car Company in Coventry in 1922 where he worked in the drawing office before becoming head of the technical department in 1930. During this time he oversaw the introduction of significant models including the Humber Snipe and the Humber Pullman.
In 1935, after the Rootes Group takeover he left to join SS Cars Ltd also in Coventry. Initially he worked on the chassis but also was involved in increasing the output of the Standard Motor Company engines then being used.
After World War II, SS Cars was renamed Jaguar and Heynes persuaded the chairman William Lyons that the company should make its own range of engines. The result was the XK engine. As well as engine development Heynes also worked on many of the cars including the Mark V, the racing C-Type and D-Type, the Mark VII, E-Type, Jaguar XJ13 and Mark X.
His retirement from Jaguar took effect at the end of July 1969, following which it was reported that he intended to "devote his energy and enthusiasm to his farm". Shortly before his retirement he was honoured with a CBE (government award) for his services to exports. His responsibilities were taken over by R. J. ("Bob") Knight (Chief Vehicle Engineer) and W.T.F. ("Wally") Hassan (Chief Engineer, Power Units), both of whom were appointed to the company's board as part of their promotions.
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