Originally posted by chknhwk:
So then, is the VCT a cheap imitation of, or an improvement on, the DOHC design?
DOHC means that the engine uses two camshafts per head. On an inline engine, like the Zetec, this means we have two camshafts. One on the intake side, one on the exhaust side. The main reason that a double overhead camshaft design is used is to all for more intake and exhaust valves. More valves = more openings for gases to flow = free-er flow of intake and exhaust gases = more power.
Variable camshaft timing is a way of improving performance. The opening and the closing of the intake and exhaust valves needs to be adjusted as engine speed increases. What works good at low speeds may not necessarily work good at higher speeds (referring to engine RPMs). The exhaust valve is opened a little sooner, and closed a little later. There's alot more out there on how the VTEC in Honda engines work, not so much on the Fords. Same basic principle, just that the Zetec VCT only works on the exhaust side, and only to help out emissions. This isn't anything new, the idea of adjusting when the valves open has been around since the early 1900's.
Here is an interesting link that talks about how variable valve timing and how it affects performance and emissions.
