The compact Tach Driver measures only 1.5" x 3.5" x 2" and is potted with a polyurethane compound for vibration and water protection so it can easily be mounted under the hood. It wires inline on the coils. 12 volt supply wire where it senses the current going through this wire and converts this information into a 12 volt output signal that most aftermarket tachs use as a trigger signal. It can be used on 4, 6 or 8-cyl engines.