Upvoting and intelligent links to info sites would be my top two suggestions. Allowing users to showcase how they tuned their cars would make it something like

Upvoting and intelligent links to info sites would be my top two suggestions. Allowing users to showcase how they tuned their cars would make it something like a facebook for cars. Featured cars could be a simple slightly random spotlighting of a few cars that are in the top ~30% upvote ranks. Right now it looks like it is geared towards the "Ohh shiny! Lemme get my camera!" crowd than the "Here is the work of art I poured my blood swear and tears into!" types. Appealing to the former nets more users, while providing a forum for the later gives you more dedicated users.