if (in_array($wp_query->query_vars['meta_key'], array("pa_pph","pa_tonnage","pa_capacity","pa_horsepower","pa_zones","pa_screw_diameter"))) {
        $wp_query->query_vars['orderby'] = "meta_value_num";


    // this is what breaks the sorting... BUT I get all the posts that I need.
    // if I remove this, then I only get posts with the meta data
        $wp_query->query_vars['meta_query'][] = array('key'=>$wp_query->query_vars['meta_key'],'compare'=>'NOT EXISTS');
        $wp_query->query_vars['meta_query']['relation'] = "OR";


    $wp_query->get_posts();
}