Off on a slight tangent can anyone tell me how variable valve timing works? :-/
Just as it says. It's a system that alters the point in the crankshaft rotation where the valves open and close.
The theory is that, if you have a fixed relationship between the crankshaft angle and where the valves open and close (as with most current engines) you can only tune the engine to work best over part of its' RPM range because the optimum valve timing (and lift) to achieve best power / economy varies over the rev range.
This is because it takes time to accelerate the gases that pass through the valves and they continue for a while once they are moving. Meanwhile, at higher RPM the engine cycle takes a shorter time so you generally want a proportionally longer duration of valve opening at higher RPM. In addition, resonance in the exhaust and inlet system can be used with a little overlap (both intake and exhaust valves open at once) to "pull" gases through the engine faster at high RPM.
If you go for a cam that gives you the greatest power at high RPM it will generally have a lot of lift, duration and overlap and will be lousy at low RPM (listen to a top fuel dragster idling sometime). At low RPM the engine is not capable of "breathing" as deeply, so you need less valve lift and duration to keep the speed of the gases high to ensure good "swirl" (mixing of air and fuel as they fill the cylinder), and overlap is generally a bad idea.
So, car manufacturers have always had a massive compromise in designing cams because, although customers want an engine that'll produce a high peak power and have a wide RPM range they also want something that has enough torque at 1500 RPM that they can pull away easily.
VVT is an attempt to get around this by changing the timing, sometimes just of the inlet cam and sometimes exhaust as well, and sometimes duration and lift as well as timing, in response to changing engine RPM. So, your engine has the cam from a Massey Fergusson at low RPM and when you hit the point at which that cam would run out of steam, it shifts a little bit further towards the top fuel dragster to give good high RPM performance.
That's the theory, anyway.
As always, Wikipedia is your friend
http://en.wikipedia.org/wiki/Variable_valve_timingKevin
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