Variable valves using DC motors. With the capacity to controle the degree of openness of valve, these are useful for controlling exhaust sound volume and tone. Automatic cut of power supply when valves are all open and closed helps to minimise the use of electricity. Advantages include easier installation than vacuum valves and an immediate use with connection to the power. Yet note that motor life can be shorted when working in a place of high temperature like near Down pipes or Catalytic convertor.