How to send a custom Solr Query using Search API Solr
From version RC4 you can use makeServletRequest() to generate custom solr queries. See some examples on GitHub.

Here are some code examples:

  // Ping example
  // class_exists('Apache_Solr_Service') ?: spl_autoload_register('_search_api_solr_autoload'); // make sure that SolrPhpClient class is loaded (use only for &lt;=rc3)
  $server = search_api_server_load('my_solr_server'); // CHANGE THIS
  $solr = new SearchApiSolrService($server);
  $result = $solr-&gt;ping();
  drupal_set_message(print_r($result, TRUE));

  // Simple search query (not fully tested)
  // class_exists('Apache_Solr_Service') ?: spl_autoload_register('_search_api_solr_autoload'); // make sure that SolrPhpClient class is loaded (use only for &lt;=rc3)
  $server = search_api_server_load('my_solr_server'); // CHANGE THIS
  $index = search_api_index_load('my_index'); // CHANGE THIS
  $query = new SearchApiQuery($index);
  $query-&gt;keys('foo'); 
  $query-&gt;fields(array('title')); 
  $solr = new SearchApiSolrService($server);
  $result = $solr-&gt;search($query);
  drupal_set_message(print_r($result, TRUE));

  // Custom path example for rc4 and above (not fully tested)
  $server = search_api_server_load('my_solr_server');
  $uri = SearchApiSolrConnection::LUKE_SERVLET . "?show=schema"; // or any other custom URL
  $connection = $server-&gt;getSolrConnection(); // or: new SearchApiSolrService($server)
  $response = $connection-&gt;makeServletRequest($uri);
  drupal_set_message(print_r($response, TRUE));

  // Example for assembling the right URL
  $server = search_api_server_load('my_solr_server');
  $url = 'http://' . $server-&gt;options['host'] . ':' . $server-&gt;options['port'] . $server-&gt;options['path'];


Above code tested in hook_init with Drupal 7 and Search API Solr [rc3].



Acquia

If you're using Acquia, you can use Acquia-Search-Proxy which allows you to send custom queries against Acquia Search (Apachesolr + Search API Solr). See also my fork of Acquia-Search-Proxy which include some extra improvements.

Example custom queries via CLI:

PATH_INFO="/admin/ping" php search-proxy.php
PATH_INFO="/admin/luke" QUERY_STRING="show=schema" php search-proxy.php
PATH_INFO="/select" QUERY_STRING="q=foo" php search-proxy.php
PATH_INFO="/admin/file" QUERY_STRING="contentType=text/xml;charset=utf-8&amp;file=schema.xml" php search-proxy.php