According to Mike of Mulsanne's Corner, from the analysis of the exhaust hole of the Porsche, next LMP1 will be fitted with a 4-L engine.
http://www.mulsannescorner.com/newsjuly13.html
I would be glad to see a V6 turbocharged engine, instead of a 4-L, wich is closer to Porsche philosophy.
It seems FIA and ACO has the same plan of the late '60s: engines easily convertible from endurance to formula 1 and viceversa.
These engines will be close enough that Renault is thinking to the conversion of their F1 V6 to an endurance V6, under Alpine flag.