Actually, we still don’t, because like so many modern engines, the direct-injected turbo four under the Mustang’s hood is clad in more plastic than the average Comic-Con cosplayer. Nonetheless, the image, via a dealership employee's post on Reddit, gives us our best idea yet of what’s going on in the engine bay.
What we can see is the engine block is right up against the firewall, explaining part of the car’s rather excellent 52/48 front/rear weight distribution. We can also see just the hint of the turbo and exhaust manifold sticking out below the plastic atop the head.
Another hint gleaned from the position of the turbo (very close to the block) is that the Mustang, like the Focus ST, and the brand’s three-cylinder engines, almost certainly uses an exhaust manifold integrated into the cylinder head. That makes for good efficiency, but it may stymie buyers looking to boost power with aftermarket goodies—or at least put a crimp in the engine’s potential. Thus far, it hasn’t rained on the Focus ST’s parade too hard, however, so there’s always hope.
Other than that, there’s not a whole lot to be seen under the hood of the 2015 Mustang EcoBoost: acres of plastic, a coolant overflow tank, a brake fluid reservoir, some hoses, the hint of the intake hose and throttle body, and the edge of the battery box.
Another factoid, anecdotal as it is, from the Reddit thread: the car photographed was said to be averaging 31 mpg on the highway with the air conditioning on.