VTEC is a system which optimizes the power output and fuel consumption of a motor. Some VTEC models were built towards fuel economy , while others were for performance.
VTEC is engaged at all times. It enhances the engines low torque torque by enabling a cam for low rpm torque duty. Then crosses over to a high rpm cam profile which takes advantage of the high rpm and optimizes the power there. This in turn creates two motors in one , Honda has used this engineering along with lower powered 4cyl motors to create fuel efficient motors that produce power levels of V6 motors , with a more peppier , rev happy and refined feel.
It takes about .1-.01 seconds for VTEC to engage to the higher rpm cam mode. A big misconception with VTEC is that it kicks in at only one point. This is wrong , it is on all the time. Its setup to give a little surge when going into it for a number of reasons, this is why most tend to believe it only comes on at higher rpm or have VTEC off and on. It is always active at low and high rpm. The main reason it is so effective.
To take into consideration a 5.0 8cyl 93 mustang vs a 2.2l 4cyl prelude will beat the mustang at low and high speeds. This is due to the optimized system and low vehicle weight. This is the basis for all of honda's vehicles. Which provides a cleaner environment and has lower costs to operate. More info can be found on VTEC systems @ superhonda.com
even though they are essentialy the same, each one dose have it's flaws for example:
vtec has very little torque.
VVL parts are rare and hard to come by.
VTC only works on the intake cam.
nissan guy: *coughs* no torque *coughs*
ford guy: you know he's right
Honda and Nissan guy: shut up, you car doesn't even run.
VTEC keeps the hamsters from getting too tired.
Wait, you mean a bottle of Mountain Dew displaces more than my engine?