Both Zetec and Duratec include 8v and 16v variants within the range of 4-cylinder engines.
The original "Zeta" Zetecs (Zetec, Zetec-E, Zetec-R) were superseded by the heavily revised "Sigma" Zetecs (Zeta-SE, Zeta-Rocam), developed in collaboration with Mazda and Yamaha.
Sigmas in some small model Fords (Fiesta, Focus, Ka) were renamed as Duratec 8v. To add more confusion some Duratec 8v are actually Endura-E engines.
Larger 4-cylinder Duratecs were originally heavily revised Zetecs but were replaced in 2003 with Mazda MZR engines.