when did kentucky fried chicken start getting called kfc
In March 1991 the KFC name was officially adopted
History of KFC
In March 1991 the KFC name was officially adopted, although the chain was already widely known by that initialism. 
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History of KFC [SEP] International growth and franchisee disputes under John Cranor III
KFC found itself competing against the growing El Pollo Loco restaurant chain, as well as with Burger King, which had just introduced the BK Broiler, a grilled chicken burger. Delays in product development, cramped kitchens, and the ongoing franchisee contract dispute prevented the chain from rolling out a grilled product of its own. In March 1991 the KFC name was officially adopted, although the chain was already widely known by that initialism. The change was advised by the Schechter Group brand consultancy agency. Research demonstrated that 80 percent of customers already associated the "KFC" initials with Kentucky Fried Chicken. A spokesman for the chain said that it represented its diversified menu, which was moving away from solely fried products.