Maybe I didn't explain it properly (I do that a lot) cause I do understand pretty well what I'm talking about.
The sum of my engines rotating parts doesn't allow it to go above 5000 RPM or so.
However, the sum of my intake system parts doesn't allow me to go above 3800 (say 4300 for shifts). So this is the first issue.
I want to change both of these. I want to build the engine to handle upwards of 5500 or 6000 RPM. That still doesn't make it worth my time to rev that high. That's where the turbo comes in.