"Basically a Super 12C." That is what Road & Track says about the 2015 McLaren 650S.
Apart from the 650S's face, the car is obviously 12C-derived. R&D says that had it not been for the new model designation, it would be seen as a mid-cycle refresh.
Underneath the body there is a Macca 3.8-liter turbo V8 which will produce 641 hp. The engineering team has made tweaks to the overall performance and handling too. The 650S has the same drag coefficient as the 12C, but the 650S has more downforce, and has had changes to the airbrake and a rear bumper that borrows from the GT3 race car.
Read the whole story here, but I've covered most of it for you.