variable valve timing? Please explain

Very simplified:
To work efficiently at high revs an engine needs a different volume of fuel/air mixture than at low revs. Therefore the engine varies the opening times of the valves to alter this as the revs rise

To add a bit more... this variable timing affects the amount of time* the fuel/air mix valve and the exhaust valve at the top of the cylinder are open for. This changes the way the gases flow around the in cylinder optimising it for high/low rev operation.

The benefit is an engine which is more responsive, economic and efficient over a wider band of the rev range

Edit: *it can also affect the amount they open and the way they open (their opening shape/profile)
 
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vetak includes variable lift too. google an image of a vtec camshaft, similar principal to VVT, but very different execution
 
VTEC is a version of Variable Valve Timing.

It's a Honda technology which has 2 cam shafts above the engine and switches between the 2 when it deems suitable.

Which is why (I think) VTEC have/had a small but noticable clunk/lurch when the engine switches between the 2
 
not all vetaks have twin cam, some are single.

however, it does switch between two cam profiles

http://us1.webpublications.com.au/static/images/articles/i1109/110991_15lo.jpg
110991_15lo.jpg


mosts VVT systems are variable, vetak isnt, its one or the other
 
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