One thing to keep in mind is that the rotor and the crankshaft spin at different speeds. When your tach says you are revving to 9,000 rpm, the rotors are only spinning at 3,000 rpm. The engine fires at the same rate that a 4 cylinder piston engine fires. So if you want to buy a tach for an RX-7, you go out and buy one for a 4 cylinder car.