To max out back; try weighted pull-ups as mikelong23 said, but the best way I feel is heavy bent-over rows and deadlifts. back is kind of hard to max since there are many angles and it is easy to cheat a little here and there. I personally suggect a 20 rep set of deads to give back girth....
and watch it now with 170lbs comment, there is no need to slander on weight basis alone. i'm only 187 now, then again I'm 5'7-8 at 7-9%