On my '73 K5 I've used both. Don't have the original Q-jet so I'm not sure if it came stock with both or which was used. Edelbrock calls for "timed" vacuum on emmisions vehicles and full or "manifold" vacuum on non-emissions. The '73 came with emissions (not CA) but all has since been lost. Would I see a performance or mileage difference with one or the other? Right now I'm using timed and running an HEI and Edelbrock Performer square bore. Thanks! PS-I did a search of the forum using the above title but nothing came up. If we get some replies to this one maybe it'll provide a hit for others looking for an answer to this question.