I didn't think it was possible to have valve timing with no overlap; when my VW was on the street, the accepted wisdom was that it was almost essential to have some overlap in the works. With the air-cooled engines, the overlap actually helps cool the exhaust valve by letting some of the incoming charge flow over it, and also helps pull more charge into the cylinder. Kind of like having to open the door and an opposing window in an apartment to get a draft going.
