I know this is an old post but I made a tach adapter using a PIC 16F1824. It basiclly takes a single cylinder engine signal (1 pulse /720 degrees) and multiplies it by 4 to use a 4 cylinder tach. Works real well if you are interested, I can send you the code. It can be adapted to other situations as well. I also designed a real stable interface to the coil if you don't have a crank trigger.