For my hypothetical question lets use a stock LG4 as an example. If one were to replace the engine internals and with light weight components without changing anything like the cam profile, compression ratio, etc. Just replaced the rotating assembly , valve train, the flexplate, pulleys(same diameter) with light weight. The max HP and torque would remain the same or not? I realize the engine would wind up faster, thereby making the car faster but it should still peak the same, right? I know I'm missing something here, because if its faster then its got more HP.

Next question is anybodys quess so guess away. If one were to replace the rotating assembly and drive train with lightweight components what kind of fuel savings could be expected? 5% 10%? More?

 

this is a purely theoretical question, so lets keep it that way:

your would have less loss, when moving lighter weight components, for a net gain in overall power.

revving up faster, due to it being lighter, would increase your horsepower, and keep your torque the same (on paper.)

Kinda conflicting thoughts, but that's all I can add...

 

At a steady RPM, only those things that are stopping and starting would increase power. Flexplate/flywheel, pulleys, would not (although pulleys to slow down alternator/water pump still would).