Auto-Assist 4WD works in forward and reverse, automatically controlling the drive loads on the wheels during turns. Auto-Assist 4WD lets you turn with greater efficiency and ease with less wear and tear on the turf. Full-time 4WD can also be engaged. Powerful Kubota diesel engines mean faster, more efficient job performance. These F Series engines have power to spare; their high torque rise ensures faster, smoother recovery under sudden loading, letting you power through tall or wet grass without skipping a beat. They're also environmentally and economically friendly thanks to their compliance with the EPA's Tier 4 emissions regulations, enhanced combustion efficiency, low vibration and noise levels, and reduced fuel costs. Stable, consistent engine speeds for superior results. The F3990 diesel engine features an electronic governor that stabilizes the engine speed at 2,500 rpm with the PTO switched on for enhanced cutting and discharge performance, and increases engine speed to 2,700 rpm with the PTO switched off for faster traveling speed.