[oldschool]

LMAO, now we can completely confuse the folks here Coyote, when I was doin my vapor carb building in the 80's we had an exhaust heated exchanger mounted directly on top of the stock carb so we could use both the carb and the vapor unit. This was a 74 Chrysler Newport with a 400 cu in engine. In stock form it got 12 mpg, runing hot air(265 degree air) in the stock carb it went to 22 mpg.

It could be that high compression effects the intake air temps viability to increase mileage.My truck is running a 10.75 :1 compression ratio and like you avoiding knock has been an issue. In the winter however I run a stock air cleaner with the heat stove off the exahust manifold and disabled the temp sensor inside the aircleaner, running the snout butterfly directly off manifold vacumn so when I am cruising I have hot air and when I am accelerating cold air; while I don't drive the truck enough to get any accurate mileage figures , I do know the hotair stove helps the trucks drivceability.