On the other side of the head, there are three fuel injectors, sandwiched between the fuel rail and the cylinder head. The injectors are controlled by the computer using a duty cycle signal (percentage on-time vs. percentage off-time). When the computer wants to add more fuel, it increases the injector on-time, (also called pulse width). When the computer wants to reduce the amount of fuel, it reduces the on-time. The fuel is also controlled by the fuel pressure regulator. The fuel pressure regulator sits on top of the fuel rail. The vacuum line coming off the regulator goes to the intake manifold (not shown in this picture). When the engine is under load, the vacuum will drop. A diaphragm in the fuel pressure regulator will respond to the vacuum, and increase the fuel pressure.