I look at this this way. About 15 yrs ago I rebuilt a Chevy 265 cu in V8. It was basically the same cu in afterwards with only four changes but these changes made a world of difference.
What I changed was the stock camshaft to a mild racing cam, duration and lift profile changes. Along with variable duration lifters. Then I free flowed the exhaust by changing to a turbo muffler.
I gained greater torque at takeoff and better slightly better fuel economy. Then I went one step further and switch from the stock 350 transmission to a 350C with locking torque convertor. That when went from the factory 16 mpg combined to nearly 23 mpg combined.
The end results just shows how just a few minor/major changes can really affect an engine power curve and hp even the cu in never changes.
