[Ecky]

Here's a real world example:

I'm currently in the process of replacing my 1.0L Honda engine with a 2.4L. With the existing 1.0, peak efficiency is around 2000rpm. At 55mph in 5th gear it turns around 2000rpm at 75% load. At this speed I can get 100mpg on a flat road. It's almost perfectly optimized for those conditions.

However, if I wanted to drive at 75mph instead, it's way outside of peak efficiency - spinning too fast, and there isn't enough reserve power for me to gear it taller. Likewise, if I'm climbing a hill (and I live in a mountainous state) I have to downshift, which also hurts efficiency. Normally the hybrid system adds torque with virtually no penalty for the periods where I'm climbing hills, but without it the ideal engine size for my car in the environment it's used is probably a little bigger than 1.0L - maybe 1.2-1.5L. Finding the correct balance of all these factors is a moving target.

The 2.4L engine I'm installing is mostly for fun, but I'm still doing my best to make it efficient. I'm gearing it extremely tall to hopefully get load up, though this will be at the expense of often operating *below* its peak efficient RPM. I suspect it will do worse than the 1.0L engine on flat ground, but might do nearly as well or even a little better if combined with engine-off coasting when driving through mountains and/or at higher speeds.

Efficiency improvements to the 2.4L include significantly reducing rotating mass, helping it to warm up more quickly, deleting accessories where possible, deletion of balance shafts to cut down on parasitic drag, and playing with a lean burn tune to get load up during level ground cruising. I don't think I can out-engineer Honda when it comes to camshafts, stroke, cam angles and that sort of thing. I expect an efficiency loss overall, but I'm trying to minimize it while gaining 400% more horsepower for some laughs.