Dual air intake

Back in the day cars had dual air intakes... a normal air intake (that sucked in engine compartment air) & a secondary "cold engine" intake that wrapped around the exhaust manifold.
I think that hot intake air would be good for higher fuel economy, but not for high performance. So how about a super high mileage fuel saver engine using intake air coming from behind the radiator.... it would (at speed) be heated and somewhat forced induction.
At idle the engine would suck air through a shorter intake just breathing engine compartment air. The air intake would be road speed/engine speed dependent, switching between modes or a combination of the two.