Yamaha’s Power Valve System is used mainly on high-performance two-stroke engines. A two-stroke engine produces optimum low-end torque with short exhaust port timing (low exhaust port height) and maximum high rpm horsepower with a high exhaust port. YPVS eliminates this conflict by incorporating variable exhaust port heights that change as rpms increase.
The YZ125 utilizes a slide or guillotine-type YPVS system derived from the very successful TZ250 roadracer. This system features a two-piece slide-type/guillotine exhaust valve that not only varies the exhaust port height according to engine rpm, but also better matches the curvature of the cylinder and improves the sealing at the exhaust port for increased power output. The YZ250 uses a similar slide or guillotine-type system however, the YZ250 has a total of 3 slides. One for the main exhaust port and two smaller auxiliary slides that control the two sub-exhaust ports. The major benefit is a wide spread of power from low rpms right through the powerband to redline.
