Metal Gear Solid - rising will use a new video game engine because the Xbox 360 con not handle th Metal Gear Solid 5 game engine.Metal Gear Solid: Rising will be using a new engine that Kojima Productions Team needs to craft to compensate for the Xbox 360 and the PC version. The game can still look better than MGS4 in terms of graphics, but they’ll accomplish this feat by utilizing their tools and understanding of the hardware more. It also conveys the imagery that Rising won’t be your conventional MGS title as previous entries were. Games are always improving and it's expected that Rising will surpass MGS4. The real question is why couldn't Kojima just optimize the MGS4 engine and go from there?
Source: Examiner