If your talking B16 engines a 1600 16v DOHC compared to a 1600 16v VTEC, the difference is about 30bhp. How this is acieved at low RPM the valves only open around 60% of there total lift caperbility this inproves engine torque as there is very little valve overlap ( when Inlet and Exhaust valves are open at the same time). With conventional fixed camshaft systems they have a compromise of the valve lift, the profile of the cam is aimed at economy so when cruising its great on fuel but at all other ranges its a compromise.The VTEC system has a good low range cam profile that gives good fuel economy below around 5700rpm with most performance VTEC engines there is a extra high lift cam lobe that will extend the valve lift and increase valve overlap. At high rpm this overlap is good because as the Exhaust gases are leaving the cylinder the gases create negative force inside the cylinder pulling in the new air fuel mixture with more efficiency. This technique is known a scavanging and increases Volume Metric Efficiency. Simply put more efficiency more power.There are a number of different VTEC systems, VTEC-E is designed for economy and basically operates on 12 Valves below 2500rpm after this rpm the VTEC control solenoid energises and oil pressure is directed through the camshaft to the VTEC rocker arms locking the two rocker arms together allowing 16Valve operation. Performance VTEC's normally run DOHC and have Variable valve control on the Inlet and Exhaust camshafts 16Valve engines 4 Valves per cylinder. The camshafts have three cam lobes for the two valves Inlet and two for the Exhaust valves, the two outer cam lobes are operating below 5700rpm as mentioned and the third operates when the rockes are locked together giving high lift. Then there is i VTEC, as a rule the i VTEC system is identified by a cam variator fitted to the Inlet camshaft (VTC controller). The VTC controller is to control the camshaft phase angle at low range rpm allowing improvments in torque as there is now even greater control of the valve overlap, again this system is oil pressure controlled via the ECM.Hope this helps and isn't to much of a head mess for you.