Very interested In this topic. Love efficiency.
I agree that all the new stuff is 340-370ci for a reason. However, those displacements are matched to the vehicle weight and towing capacity. Much heavier than our a-bodies.
I think efficiency lies where having an engine that has less displacement so it drinks less on the highway to produce that 50-80 hp but can wind up high under a turbo to get 0-60 times. This is where direct injection, vvt, active intake controls, supercharging, etc come in. This is why so many performance cars that our in our weight range are 4 cylinder and turbo.
