OS .91, great engine!! Pumps, all my four strokes are now pumpers except one, an older OS .91. Pumps allow you to place your fuel tank just about anyplace you like in the air frame, I like mine just a tad forward of the CG for ballance reasons. They also keep a positive fuel pressure at all times to the engine so you aren't running lean and rich during stunts or higher or lower G forces. I like pumps!!!! I just don't care for the prices of the engines now days!![:@][:@]