Exact Single Word Search not showing first

All of these posts have the word 210 in the title, that’s why they are getting the same score. The filtering function boosts them all.

Do you want “210” to find the “338-108-210-0” post in the first place? If not, go to Settings > Relevanssi > Indexing > Advanced indexing settings and set “Hyphens and dashes” to “remove” and rebuild the index. Now 210 won’t match to those partial part numbers at all. That should make the search more precise.

In that case do not adjust the hyphen indexing setting, but instead enable the “Exact match bonus” from the Relevanssi searching settings.

Ah, I see. The exact match bonus doesn’t actually help here, it has the same problem the original function you used. It just checks if the search term appears in the title, and it does appear in all of the searches (it’s really made for multi-word search phrases, where it works much better).

This should do the trick:

add_filter( 'relevanssi_results', 'rlv_perfect_match_boost' );
function rlv_perfect_match_boost( $results ) {
    $query = get_search_query();
    array_walk(
        $results,
        function( &$weight, $post_id ) use ( $query ) {
            if ( 0 === strcasecmp( $query, get_the_title( $post_id ) ) ) {
                $weight *= 100;
            }
        }
    );
    return $results;
}