These MSD shift lights have a built-in rpm sensing circuit that will turn the light on when the engine reaches the desired rpm. The rpm is chosen with MSD plug-in modules. They feature a bright cluster of LEDs, making them easily visible, even in broad daylight. The lights also turn on for a second or two when the ignition switch is first turned on, to inform you that they're functioning properly. The Shift Lights will work on 2, 4, 6, or 8-cylinder engines. They plug directly into the tach output on MSD 6, SCI, 7, 8, and 10 Series ignitions, or can be connected to the negative coil terminal when used with points or inductive-type ignitions.