All the different motorcycle companies had different versions of power valves. Some you just looked at and shook your head at with all the complications they had. We bored cylinders a lot and you could see the technology as it came out and the other ones that were way too complicated. But they all did what it was designed for. Give you somewhat low end but open raise the exhaust port up for top end. Some even had rotating ports by the sides of the exhaust along with the top of the exhaust port that would raise and lower.