Vtec

[h=Definition]1[/h]
VTEC is an acronym for Valve Timing and lift Electronic Control, and was first implemented by Honda in the late 1980s in the Japanese Domestic Market (JDM) Hondas.
VTEC allows an engine to have multiple cam profiles depending on engine RPM. That is, it provides a smooth idle at low RPM while having the ability to run the equivalent of a 'lumpy' cam at high RPMs.

[h=How it works]1[/h]
When the engine reaches high RPM (usually around 6,000+rpm) a small oil filled solenoid gets activated (VTEC solenoid) and this engages a larger cam which pushes down on the valves causing them to lift higher off their seats. This increase in valve lift allows more air to enter the combustion chamber and this combined with more fuel via the injectors allows the engine to make more power.
At low RPM's however this is not feasible because high valve lift results in a rough idle and for everyday driving this is not desireable. Hence VTEC allows a smooth idle at low RPMs while allowing F1 style high lift at higher RPMs.

[h=Improvements]1[/h]
Over time Honda have implemented a few variations of the VTEC system and have managed to achieve some great results. The most common VTEC system is the DOHC (Double Overhead Cam) system where each set of valves (Intake and Exhaust) has its own cam. DOHC VTEC configuration is optimised for high power output, and is capable of making upto 88kW per litre (as found in the current S2000).
The second impementation of VTEC is that of SOHC VTEC where both sets of intake and exhaust valves are run via a single cam. This mechanism is not for performance but for economy, while still making good power for practical everyday use. With SOHC VTEC, only the intake valves benefit from higher lift and not the exhaust valves.
The latest impementation of VTEC is that of i-VTEC where the system allows continuasly variable valve timing over the rev range which allows for upto 6kW more at peak power with improved torque and fuel efficiency.

[h=Where it is found]1[/h]
SOHC VTEC is primarily found in the early model Honda Civic Vti models (1993-1996) and the Accord Vti. These engines manage good fuel economy (6.5-7.0L/100km) while making a healthy 96kW of power from a 1.6L engine.

SOHC VTEC-E is found in a few of the early model Civic VEI's (1991-1993) and makes less power than the similar VTi but offers slightly better fuel efficiency.

DOHC VTEC is the ultimate performance monster and can be found in the Integra Vti-r (125kW), Civic Vti-r (118kW), Prelude Vti-r (147kW), Integra Type R (147kW), S2000 (176kW) and the NSX (206kW).