If the lettering on the valve cover is raised, it is likely a 94-97 1.8 engine. If the lettering is recessed, and the Intake manifold is one piece, it is a 90-93 1.6. If the lettering is recessed and the intake manifold looks like you can take the top half off, it is a 99+ 1.8. If the lettering is raised, but the valve cover is red crinkle painted and there is a turbo hanging off the side, it is a 2004-05 Mazdaspeed 1.8 motor.