Dual tower for 2 plugs per coil. Dual fire coil; sold in pairs so it can be used for single fire application in dual plugged heads (4 spark plugs total). Sold in pairs. 0.5 ohm primary resistance; designed for Twin Cam ignition systems that are designed to be used with Twin Cam style low resistance coils. 40.000 volt output. One of the smallest coils available. Great for customs where coils need to be out of sight. Overall dimensions 4.2" x 2.3" x 1.6" (10.7cm x 5.8cm x 4.1cm). Mounting hole distance 2.9" min. to 3.5" max. (7.3cm to 8.8cm). Note: It is recommended to use ignition modules & coils of the same manufacturer. Intermixing different brands may cause premature failure. Note: Standard Twin Cam ignition is a single fire system. Note: Difference between Single & Dual Fire: - Dual Fire - This is the way the earlier; pre-Twin Cam; engines work. A single coil is used and both spark plugs fire simultaneously. This is the simplest system. The ''wasted'' spark is sometimes said to be horsepower robbing. - Single Fire - This is the way the stock 00-up Twin Cam engines work. Two separate coils will fire the spark plugs independently and only the spark plug of the cylinder that is on compression stroke will be fired. It is said that these engines vibrate less and can improve power. The effects will become more pronounced with hotter cams and higher RPM.