engine question

[Mopsdrops]

Hi,

I've got some questions:

If you are tuning a car into high bhp's by installing a turbo setup, can you still do a N/A tune up afterwards like changing the pistons, valves, valve springs, retainers, camshafts, etc... ? Cause like in GT4 you would have to chose: N/A or Turbo, so I don't know if you could mix the two in real life

2nd, what is it all about those compression rates like 11,1:1 ? Can someone explain a bit more about this, like why they calculate this etc...

Thank You