I've tried a few different combos of AND, OR,the wildcard characters, and the radio buttons - and sometimes can get things narrowed down to less than 10 threads to dig through - but there's usually more.
For instance - looking for "fuel pump" and selecting the "search for all terms" radio button will bring you back 422 topics (955 posts) that contain the words "fuel" and "pump" but they don't necessarily have the exact phrase "fuel pump" in them. Using the "search for any..." without the AND brings back thousands of hits.
As much as we point out using the Search function before posting a question, it would be nice if phpBB made some type of 'exact phrase' search available.
That way, someone looking for "Hooker Header" will find only posts/topics with that EXACT phrase in it - and can avoid digging through the threads about Olympic diving or what we did on Friday night.
