Attached is an article I scanned in from a May 1974 Car and Driver. They measured the drag and downforce of different spoilers and air dams on a 240Z, and even provide performance and fuel economy improvement numbers. Very good stuff for those who are wondering whether they should add aerodynamic aids to their Zs. For example, a non-ducted Spook is a pretty good add on, as others have noted from their own experience.