 The unique feature that gave the 1989 and onward models their Exup name (for Exhaust Ultimate Power Valve) was Yamaha's four-stroke power valve system, a servo motor-driven exhaust valve.This allowed large bore exhaust header pipes (for excellent gas flow at high engine speeds) coupled with the valve restricting flow at lower revs, to speed the gas through.It gave pulling power from low revs, seamlessly, up to the red line at 11,500 rpm.