i-vtec and vtec are same technology but different implementation..
vtec is purely engine rpm dependent..will always kick in at a certain RPM...
i-vtec intelligently monitors rpm,load,vehicle speed, acceleration,throttle position,gear and various other factors before dis-engaging vtec..
in short, with the R18, u may never dis-engage vtec if u accelrate smooth and gentle even upto the tacho redline...and this improves fuel economy and controls emissions!