Distributorless Tach Driver Not For Use On Odd-Fire Engines MSD IgnitionThe compact Tach Driver measures only 1. 5 Inch x 3. 5 Inch x 2 Inch 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 tachometers use as a trigger signal. It can be used on 4, 6 or 8 cylinder