I am new to this forum and tried to send this once before but I must not have done it correctly, so bear with me. I have a 2008 620i Gator to which I am adding an aftermarket tach/hour meter. Installation instructions ask me to select one of the following spark plug firing patterns: 1. fires twice per revolution; 2. fires once per revolution; 3. fires every other revolution. My engine has two spark plugs. Which pattern should I select?