roots blower instead of butterfly valve
 

Let the intake vacuum do work on the roots blower.

My brother's car is 2L, 60mph is 2600 rpm.

That would be about 73 cfm at wide open throttle, actual engine intake is 17 inHg cruising.


my math is a bit fuzzy but it looks like maybe 2 hp theoretical is available.

Yeah 4cyl gasoline engine cars typically lose about 2hp to isobaric losses.
Easiest way to fix that is get a diesel engine.

Yes, you will probably gain efficiency with a roots blower anyway. On a 2L engine I recommend an SC12 from Toyota, it's clutched like an A/C compressor, so you can even toggle it on and off at will.

This has been invented many times.

My version first heated the air by extracting heat from the exhaust system, used a turbine to produce power, and finally cooled the air again before it went into the engine.

Will someone please explain... any single part of this? Thanks! :D

It takes POWER to suck air through/past a partially open orifice (throttle).