318 MAX fuel economy builds?

As far as the heat chamber under the carb, Smokey Yunick did a rather elaborate design using a turbo back in the mid-80s. He was getting over 50 MPG with around 250 HP from a Fiero, Plymouth Horizon, and a European Fiat:

Smokey Yunick’s Hot Vapor Fiero; 51 mpg and 0-60 in less than 6 Seconds! See and hear it run in our exclusive VIDEO! - Information on collecting cars - Legendary Collector Cars