Properly designed exhaust systems help minimize the amount of raw fuel loss in the exhaust process, but a carbureted two-stroke engine will always waste some fuel (modern direct injected engines avoid this). Many producers of two-stroke performance bikes fit them with the exhaust power valve systems. These valves act to vary the height (and width) of the exhaust port thereby broadening power delivery over a wider rev range. Exhaust ports with fixed dimensions only produce usable power in a narrow rev range, which also affects fuel consumption and emissions. In a race bike, this is not a problem as the engine will be operating at high RPM almost all the time. However, in a road/commuter bike, the limited power range is a problem. To provide more low RPM power, as well as enable the engine to produce a lot of high RPM power, a power valve system is used. All power valve systems vary the duration of the exhaust port open time, which gives the engine usable low end power combined with excellent top end power. Manufacturers have also included sub exhaust chambers that extend the 'tuned length' of the expansion chamber. Power valve actuation can be by mechanical (RPM dependent) or electric (servo motor) means increasingly with electronic control. Electronic control offers a greater degree of accuracy as well as being able to vary the opening of the valve and be tuned to conditions. Suzuki AETC and Super AETC AETC and Super AETC Suzuki engines, Automatic Exhaust Timing Control: The two-blade version was fitted to the VJ21 RGV250, and the three-blade version, to the VJ22 RGV250 and Suzuki RG150. With the AETC system, the power-valve systems are normally partially closed at low RPM; when closed, it enables the engine to make more power. Up to a certain point, however, power drops off as the engine is unable to expel enough gases out of the exhaust. When the power-valve is opened, it allows more gases to flow out of the exhaust port. This system is recognizable by a small box above the exhaust outlet; the power-valves are situated in this box. Depending on the valve, they may be made of two (older version) or three (newer version) separate blades. YPVS-Yamaha Power Valve System YPVS Yamaha engines, Yamaha Power Valve System: Yamaha engineers realized that by altering the height of the exhaust port they could effectively change the engine power delivery