Here is the before and after of a H23 NON-VTEC head with a (competiton port and polish). Which means its polished 100% inside the intake, exhaust, and combustion chamber. The Chamber is ported to match the head gasket. Ofcourse, its had the valve seats reground, valve job, and resurfaced head. You want the surface as smooth as possible so air can travel across its surface faster (Velocity) the speed air can travel. (Volume) of air is increased by porting.