Only index open threads?

[Joe]

How can I only index open threads for the vB search?

I'm going to auto-close old threads, the plan is to keep my search index small and quick, and lessen my server load in the process.

Anyone have a quick solution? Right now i use Full Text search, but with 3 million posts, MySQL has a hard time keeping up. I'd be willing to go back to the vbulletin search engine if i can get the index to less then 1MM indexed posts.