repatched for custom dynmaic field definition : https://drupal.org/node/1846860#comment-7805861

search_api_solr.api.php

@@ -100,6 +100,46 @@ function hook_search_api_solr_multi_query_alter(array &$call_args, SearchApiMult
   }
 }
 
+/**
+ * Define how Search API Solr should index different data types.
+ *
+ * It is important to make sure that any types you define are also declared to
+ * Search API using hook_search_api_data_type_info().
+ *
+ * @return array
+ *   An array containing data type definitions, keyed by their type identifier
+ *   and containing the following keys:
+ *   - prefix: The prefix used by the dynamic field type.
+ *   - always multiValued: (optional) Whether the single/multiple prefix should
+ *     be skipped for this data type. Defaults to FALSE.
+ *
+ * @see hook_search_api_solr_dynamic_field_info_alter()
+ * @see search_api_solr_get_dynamic_field_info()
+ * @see hook_search_api_data_type_info().
+ */
+function hook_search_api_solr_dynamic_field_info() {
+  return array(
+    'example_type' => array(
+      'prefix' => 'ex',
+      // Could be omitted, as FALSE is the default.
+      'always multiValued' => FALSE,
+    ),
+  );
+}
+
+/**
+ * Alter the data type indexing info.
+ *
+ * @param array $infos
+ *   The item type info array, keyed by type identifier.
+ *
+ * @see hook_search_api_solr_dynamic_field_info()
+ */
+function hook_search_api_solr_dynamic_field_info_alter(array &$infos) {
+  // Change the prefix used for example_type.
+  $info['example_type']['prefix'] = 'ex2';
+}
+
 /**
  * @} End of "addtogroup hooks".
  */