How to use voting results to boost relevance for solr search?
Do you have the voting api results in your apache solr index?
If you do and they are stored under a name ss_votingapi, here is what you need to do:

/*
 * Implements hook_apachesolr_query_alter().
 */
function MYMODULE_apachesolr_query_alter($query) {
  $query-&gt;addParam('fl', 'ss_votingapi');
  $query-&gt;addParam('bf', "recip(rord(ss_votingapi),1, 1000, 1000)");
}


More here, here and here