A few months ago, since both of my aging MN-2 nicad packs were a bit too old, and needing a very small amount of higher speed work, I 'boosted' one of my F3/MD-4 by adding on another set of 4 lithium AA's, bringing the total up to 12 AA's giving 18+ volts. Motor and body ran fine, at over 6FPS. My MN-2 Nicads got up to 20v+ when they come off the charger, in the fun old days I'd drain the pack shooting all week then give one full big charge Friday afternoon so for Friday Night Football (Highschool coverage for the newspaper) I'd get a full blast of max FPS, usually for one or two rolls. Smokin!