EXUP (EXhaust Ultimate Power valve) is a device fitted to selected Yamaha motorcycles (FZR,YZF,R series) that constantly adjusts the internal diameter of the exhaust system to suit engine revs. This ensures good low to mid-range performance for a linear power output all the way to the rev limiter. This is achieved by using an internal valve inside the exhaust at the point where the four pipes from the cylinders meet.