with my N/a set up i just raised my vtec engagement point to 5900 rpms and the difference was night and day as far as power and smoothness, but a VAFC should be one of the last things u should put on, cause i got that type of a gain only after i had done all of my cams, header, intake, and air intak and so forth and so on..

It basically alter the vtec engagment point and u can adjust the fuel ratios if needed, however the VAFC is very limited its it ablity to tune with.