The vBulletin search engine is something of a pain-in-the-backside, and doesn't support complex searches. For a more powerful search of Autopia, try using
Google's advanced search, and use "autopia.org" for the domain name. Or, you can use the CGI shorthand and type "site:autopia.org/forum
MySearchCriteria" in the regular Google dialog to limit searching to the Autopia forums.
Tort