YPVS - Yamaha Power Valve System

The two-stroke power valve system is an improvement to a conventional two-stroke engine that gives a high power output over a wider RPM range.

Yamaha engineers realized that by altering the height of the exhaust port they could effectively change the engine power delivery thereby having optimal power and torque across the entire rev range, so it was how the YPVS was born. The valve is of a cylindrical design running across the top of the exhaust port, it is turned by a servo motor controlled from a control box taking information from the CDI (and other locations). The valve is a slightly ovular shape. This changes the height and size of the exhaust port at different engine speeds, maximizing the available power at all rev ranges. Opening up firstly at 3000 rpm for low end power, gradually in between 3000 - 6000, fully opening at 6000 rpm for maximum power, on most 125cc. It was fitted to all of the later models of the RZ/RD two-stroke road bikes (125, 250 350 and 500 cc), the TZR range. It was also added to the DT(125lc 2/3) range after 1984 (but was locked closed to comply with UK learner regulations until the R in 1988-04 which had a fully functional YPVS valve. The YZ series of motocross bikes has a mechanical power valve which is activated at RPM speed. The YPVS is only found on the liquid-cooled bikes not air cooled versions. Yamaha have also used a guillotine version in some of their later models such as the TZR250R SPR model.

Yamaha was actually the first company to produce consistent results with their YPVS in their race bikes. The 1977 OW35K was the first race bike to incorporate the power valve system and it won the Finnish GP in 1977. The Kadenacy effect was harnessed and controlled to a point that gave Yamaha great advantage over all the other manufacturers throughout the late '70s and into the mid '80s. The first street bikes with YPVS were the RZ/RD350 YPVS (LC2-onwards), and RZ/RD500 GP Replica in 1983-84.

Port timing (intake, exhaust and scavenging) is a very important factor that determines the characteristics of a 2-stroke engine. In general, fast exhaust timing means that the engine will perform well in the high-speed, high-power range, while slow timing means the engine will have good low-end torque. However, a road racer or motocrosser which must perform well especially in the high power range will also encounter many other situations, such as coming out of a corner, when it will need high torque and good mid-range performance.

The YPVS answers both of these needs.

The YPVS employs a spiral shaped valve located on the top of the cylinder’s exhaust port. This valve is operated by means of a cable from a computerized mechanical control unit which detects the ignition frequency and determines the required turning angle of the valve. In this way the valve moves upward and downward out of the exhaust port. This means the exhaust timing is constantly adjusted in accordance with the rpm level, thus giving more effective timing over the entire speed range.

"A simple diagram of the VTAC system - similar to the YPVS"

Similar systems in other manufacturer bikes are;

• Suzuki - AETC - Automatic Exhaust Timing Control

• Honda - ATAC - Automatic Torque Amplification Chamber

• Honda - V-TACS - Variable Torque Amplification Chamber System

• Honda - RC-Valve - Revolution Control Valve

• Kawasaki - KIPS - Kawasaki Integrated Powervalve System

Sources: Wikipedia, Yamaha Parts