[texan]

To add to/clarify what DVS said, variable valve timing is any system which can vary either cam phasing (retarding or advancing the cam timing), lift or duration of the valve events while the engine is running. Most current systems on the market vary only cam phasing, such as BMW's Vanos system or Toyota's VVTi setup (which are basically eletronically controlled cam gears changing cam phasing throughout the RPM range). In contrast, VTEC actuates a 3rd lobe on the camshaft for each set of intake and/or exhaust valves, which actually changes the profile of the valve events. Both lift and duration can be varied, but cam phasing is not available.

In the newest and very best systems, such as Honda's i-VTEC or Toyota's VVTL-i, you get both cam phasing control and alternate cam lobes. The goal of all these systems is to of course maximize engine power and/or fuel economy (and possibly emmisions), the differences are as follows...

-cam phasing systems are not as ultimately powerful as actually having a seperate, optimized set of cam lobes for low and high RPM (because valve lift and duration are not changeable). They are however "infinitely variable", which means they work throughout the RPM range to smoothly increase power, fuel economy and minimize emmisions.

-variable lift/duration systems are usually two stage setups, meaning two cam lobe profiles for two large RPM ranges. This works pretty damn well as far as maximum power output is concerned, but generally has a noticeable step to the power curve as the big lobes engage. Anyone who has been in a Prelude VTEC, S2000 or Type R is well aware of this abrupt jump in power.

-combining both technologies is currently the furthest anyone has gone in valve actuation technology on production cars, and is the best of both worlds. Power output is maximized from a particular engine over an incredibly large RPM range, which is why Honda and Toyota have both moved to such setups in their newest, highest performance compact car motors. The abrupt jump in power can be minimzed if not completely removed from such systems.


Hope this helps explain things, peace.