A basic rundown, 18cc domes are for higher compression. 21cc is for lower compression. The cc number is basically for how much volume is in the dome. More compression will help give you a better low end torque, but will lose some top end. More timing will help alot down low, but will also help some through the entire rpm range. Yes, you need more octane for higher compression and advancing timing. Octane ratings are basically numbers for rating when a fuel will detonate. Adding timing and compression both make it easier for fuel to detonate, so you need more octane to prevent that. In a nutshell.