I want to boost a 98 civic. I would like 15 to 20 psi and I know stock can only take about 8psi. What all internal changes would I need to make to the engine for this? I am relatively new to this so I don’t know if that is even realistic. Eventually I would like 350-400 hp.

 

you're not getting 400 hp out of the d-series any kind of easy or on a reasonable budget. Once you do you'll just start breaking everything else... transmission, axles, mounts... not stopping you, it's been done, you can do it, but I no longer see the point in this.