With three versions of the Gold Wing boxer motor spanning a dozen years, by 1987 further development of the flat-four engine was regarded as being constrained by the law of diminishing returns. Piston displacement had been increased twice to generate more torque, but this also made each power stroke more intense. During the same time period, gear ratios had been raised to decrease engine RPM (boosting fuel economy and reducing vibration as well as noise levels) which in turn made pulses through the drivetrain seem rougher to the rider because firing intervals were farther apart. The obvious way to deliver power more smoothly (as Irimajiri-san had demonstrated with the M1 engine) was to step up from four cylinders to six.