http://www.caranddriver.com/reviews/car/11q1/2012_mclaren_mp4-12c-first_drive_review/tech_trickledown_page_2
"Early in this project, McLaren engineering director Neil Hannemann nominated an engine for the MP4-12C that was originally developed in Nissan’s racing department. Nissan and Infiniti had raced this spinoff from the Infiniti Q45 V-8 at Le Mans (1997 and 1999) and in two Indy Racing League series (1997–2005)."
"Other than a 93-mm (3.66 inches) bore dimension, little of Nissan’s VRH engine exists in today’s M838T V-8. Developed in collaboration with Ricardo, this is a compact 90-degree twin-turbo V-8 with aluminum construction, Nikasil bore surfaces, dual overhead cams, and variable intake- and exhaust-valve timing. The modest 3.8-liter displacement is McLaren’s nod to the environment."
I doubt that little remains in difference between the two engines except the fact that one is turbo. It wouldnt look to good for mclaren if everyone thought they were using a nissan motor..