I spent most of the afternoon studying this direct injection idea and talked with two engine builders. So far the best answer has already been studied as (click on)
What Ever Happened To Smokey's Hot-Vapor Engine? - Hot Rod Magazine
I learned that Brodix has an injection head for SBC that sprays fuel behind the intake valve but at ambient temperature so that is not supercritical injection. Overall the basic idea falls under Carnot's efficiency law for ANY heat engine:
Eff = ((T-hot) - (T-cold))/(T-hot)
Most normally aspirated gasoline engines run at about 25% efficiency because of the heat and fuel wasted in the exhaust. The old trick of putting a spark plug in the end of the exhaust pipe with a wire to the coil resulting in a long flame from the exhaust tells us that a great deal of fuel is not burned. The way the supercritical fuel injection as well as Yunick's "Hot Vapor" method work is by increasing the temperature of the input fuel mixture. So far I appreciate Jerry's suggestion to look for an LT1 from 1992-1997 as a better SBC 350 with emission controls but I found most of the Impalas of that time used 3.08 rear gears so that probably helped mpg more than the computer controlled injection??? Now I am going to study Yunick's tricks but he used a turbocharger as well as hot air and a very lean mixture. Overall it would seem that these methods border on burned valves and probably require forged pistons and even a diesel type heavier block to handle the higher compression ratio. Maybe I should try to find one of those old Buick diesel V8 engines for starters but were those blocks any sturdier to handle what was about 13:1 C.R.? Anyway I learned a lot today but other than mabe putting an aluminum carb spacer to heat the mixture a bit (without vapor lock in July?) I don't yet see any easy ways to improve my 17 mpg. Even so Yunick's Fiero project is very interesting. Are sodium-filled exhaust valves available for SBC?
Don Shillady
Retired Scientist/teen rodder