Audi's opinion for years has been that Le Mans has far more relevance for road cars than F1, thus their involvement there. They've been running turbos since 2006 (when the R10 was introduced), and in 2013 were running a single-turbocharged variable-geometry-turbo diesel V6. There's far more liberty in the LMP1 class, especially when it comes to engine design. As of next year, the primary restriction on engines is fuel consumption.