"Hot V" Turbo Boxer Engine

Porsche and Subaru both make turbocharged four-cylinder boxer engines with the turbocharger at the bottom of the motor. This makes exhaust packaging somewhat difficult, as all the exhaust tubing has to route around the oil pan at the center of the engine. It also means that the turbo needs a separate sump to move oil and coolant through it.

Mercedes has a twin-turbo V8 with the turbochargers in the valley of the engine, where an intake manifold would normally go, which they call a "hot V" arrangement. The exhaust and intake are flipped, with the intake manifolds on the outside of the engine and the exhaust in the V. This allows for more compact packaging of the engine and better efficiency, as all the hot parts are kept at the top of the engine (hot air rises). The design doesn't require twin-turbos -- a smaller and less powerful engine would get by just fine with a single turbo.

If the cylinder heads were flipped on a boxer engine with the exhaust ports on top, it would be easier to arrange the turbo and exhaust headers to be compact and equal-length, because the oil pan wouldn't be in the way. This would help to spool up the turbo faster and increase thermal efficiency. A regular front-mount intercooler (for a front-mounted engine) would route compressed air into the intake manifold on the bottom of the engine. Heat management would be easier, with fewer parts getting cooked by hot exhaust pipes. Natural convective air flow would keep cool air moving up through the engine bay and out the hood vents, even at a stoplight. Oil and coolant would drain out of the turbo by gravity.

This would require some redesigns to the engine block, cylinder heads, and the intake and exhaust systems. And of course you would need heat shields and vents in the hood. But the gains in power and efficiency might be worth it.