 The power valve revolutionized the performance of two stroke engines, and one might even say it saved the two stroke technology back in the days. Let's find out how it works and how adjusting the power valve affects the engine's performance. The power valve was firstly developed by Yamaha in the late 70s. At the time, most manufacturers were still stuck to two stroke engines with a limited power band, even with the best expansion chambers money could buy. Manufacturers could produce two stroke engines that provided a huge amount of top-end power, best suited for motocross, but lacked bottom-end power. Or they could produce two stroke engines with a fairly good amount of mid and bottom-end power and torque, but couldn't maintain power at higher RPMs. In a two stroke engine, it is a piston that determines the timings for both the exhaust gasses and the fresh charge. And since gasses have a travel speed limit, this creates a problem. When the exhaust port is small, at low to mid-range RPMs, the exhaust gasses exit the combustion chamber, leaving it all for the fresh charge, which provides the most amount of low-end power and torque. However, when the RPMs start picking up, the small exhaust port is not big enough to let the fresh charge get back in on time, reducing top-end power. When the exhaust port is bigger, at low RPMs, the exhaust gasses and the fresh charge have more than enough time to go back inside when pushed by the expansion chamber's pressure wave. While entering together, the exhaust gasses contaminate and heat up the fresh charge, reducing bottom-end power. At higher RPMs, the exhaust gasses no longer have time to get pushed into the combustion chamber, and the fresh charge is left alone in the engine, providing the most amount of top-end power. Yamaha engineers figured out that if they could vary the size of the exhaust port according to the RPM, they would hit the jackpot. So they came up with the YPVS, the Yamaha Power Valve System, the first system of its kind. The principle of a power valve is really simple. The power valve of a two stroke engine is in essence a simple flap placed on the exhaust port that rotates and can either reduce the size of the exhaust port or allow for maximum port size. You can look at it as something holding the door a bit longer for the fresh charge to get back inside the combustion chamber before the piston closes the exhaust port at higher RPMs. But how does the power valve know at which RPM to open? On one of the sides of the engine, you have a long lever that is attached to the power valve. The vertical movement of this lever is what determines the rotation of the power valve. At the bottom of the crankcase, this lever is attached to a linkage system which in turn is connected to the governor. No, the governor is not a small leprechaun inside the engine that makes the valve open or close. You don't have to feed it, don't worry. The governor has a gear coupled to the engine, so it will rotate at the same RPM as the engine. Between the gear and the crankcase, there's a spring compressing a housing with four small metal spheres inside. When the engine's RPM increases, the governor's RPM increases as well. At a certain RPM, the centrifugal force of the spheres will be great enough to counter the preload of the spring exerted against the sphere's housing. The spheres will move further away from the center, displacing the housing. The lever attached to the housing is also displaced, which in turn converts the displacement of the housing into a vertical displacement of the power valve's rod, making the power valve rotate. Who would have thought that there's so much going on on your engine? This clever system fixes the problem of having an exhaust port too small at higher RPMs and an excessively large one at lower RPMs. The power valve can then start off by blocking part of the exhaust port, and as the RPMs increase, it gradually opens, allowing for maximum power on the entire rev range. At this point you might have figured out that changing the preload or stiffness of the spring compressing the governor has a considerable effect on the power valve's behavior. Let's take a look at what happens when you adjust the power valve. When you apply more preload or change to a stiffer spring, the engine's RPMs will have to be higher to make the spheres push the housing against the spring. This means that the power valve will remain closed for longer, reducing maximum power on mid to higher RPMs. This will make the power curve more progressive. When you apply less preload or change to a softer spring, the engine's RPMs won't need to be that high to make the spheres push the housing against the spring. This means that the power valve will open more easily, providing a more linear and higher power output. To be clear, the power valve isn't like a turbo. It won't make the engine produce more power than the engine is able to produce by itself. An engine with a power valve has a specific maximum power output and power curve. Usually manufacturers apply some preload in order for the bike to meet the manufacturer's power output needs and to provide some adjustability to suit the rider's needs. Hopefully you're now comfortable to adjust the power valve as needed. Nothing is truly complicated. Knowledge is power. Thank you for watching and don't forget to subscribe.