To adjust the output flow of an axial piston pump, what must you change?

Adjusting the output flow of an axial piston pump is primarily achieved by changing the angle of the swash plate. The swash plate is a critical component in the pump design that influences the pistons' stroke length. By altering its angle, you effectively change the displacement of the pistons during each rotation of the drive shaft.

When the angle is increased, the pistons are pushed further out, increasing their stroke length and thereby enhancing the volume of fluid being displaced with each cycle, leading to increased output flow. Conversely, decreasing the angle reduces the stroke length, resulting in lower flow output. This versatility allows for precise control over the system's hydraulic flow based on operational requirements.

The other options do not directly affect the flow output in the same way the swash plate does. Adjusting the maximum pressure relief valve setting or changing valve porting does impact system performance, but it does not modify the actual flow volume produced by the pump. Shortening the pistons would not be a practical adjustment since it’s a design feature rather than an operational setting.