curious about powervalves

[MILF_HUNTER]

im just curious as to what the benefits do 250r engine kits with powervalves provide. all info welcome

[Juggalo]

a powervalve restricts exhaust flow at lower rpm's thus giving more low end power, and then at a certain rpm's it opens up letting the motor breath to make all of its top end power. it will increase low end power and make the powerband much smoother.

[airheadedduner]

The PV is operated by a diaphram which moves with exhaust pressure. Low revs, less pressure=slide open less. High revs, more pressure=slide open more. At low RPMs a lot of 2 strokes loose a lot of the trasfer charge out of the exhaust port(because they tend to be ported more for mid-top power). With the slide lower in the port shortening exhaust duration there is less time the exhaust port is open so more of the fuel/air charge stays in the cylinder and is not lost. This allows the builder to port the motor much more radically and still retain a rideable motor. Further "porting" adjustment can be made by adjusting the preload on the spring in the PV making it open earlier or later. A very simple idea that should have been used in quads a very long time ago. PV is not a new thing. It has been around for quite a while. There is more to it but I am lazy and don't feel like going into it but that is the main part.

Also, the taller the exhaust port the less compression. Less compression means the motor will rev more freely.

[2004TRX450R]

Originally posted by airheadedduner
A very simple idea that should have been used in quads a very long time ago. PV is not a new thing. It has been around for quite a while.

Well about the time it was starting to be seen on the dirt bikes the hi po. quads were on their way out. I bet if the hi po ATVs were still in production into the early '90s that they would be sporting them.

[wilkin250r]

To explain, first you need to be familiar with two-stroke engine theory (http://www.exriders.com/techarticles/twostroke.html)

Along with the pipe is the porting of the engine, and we'll focus on the exhaust port. If you've read other posts on compression and octane, you know that compression equals power. Since the exhaust port is an actual hole in the side of the cylinder wall, you don't start building compression until the piston covers this hole.

If your exhaust port is very small, the piston will cover it early and it helps to build lots of compression. This helps low-end power, because you build lots of compression, but at high RPMs it becomes more difficult to push all that exhaust out of a small hole quickly. If the exhaust port is too small, eventually at high RPMs you won't be able to push all the exhaust out, which means less room for more fresh fuel/air mixture, and less power. Small exhaust port will give you good low-end, poor top-end.

If you have a very tall exhaust port, then it's very difficult for your engine to build compression, because you can't begin to build compression until the piston completely covers that port. This can hurt your low-end performance, because you'll just be pushing all your fresh fuel/air out the exhaust port. However, at high revs, your pipe will help build compression, and you have a nice free-flowing exhaust, so you build LOTS of top-end power.

A powervalve is the best of both worlds. It's a slide that controls the top of your exhaust port, and it is controlled by exhaust pressure. As engine speed increases, it will raise the top of your exhaust port. This means you get a small exhaust port at low speeds, which helps build low-end power. As engine RPMs increase, it will raise the top of your exhaust port (making it taller) giving you nice free-flowing exhaust for great top-end power.

[Toadz400]

Would paying for a Banshee powervalve engine be worth it?

[airheadedduner]

Originally posted by 2004TRX450R
Well about the time it was starting to be seen on the dirt bikes the hi po. quads were on their way out. I bet if the hi po ATVs were still in production into the early '90s that they would be sporting them.
True:o But what I mean buy that though is that road race bikes have been using them for decades.

[Juggalo]

half of the 2 stroke quads that came out used powervalves. 250R's,banshees, and blasters didn't have them. the LT250's and 500's had em and so did the kawasaki tecate. the motor the banshee is derived from (RZ350) used them i believe and so did the motor the blaster is taken from (DT200). don't ask me why they took them off for the quad versions but still half of the 2 stroke quads used em so its not like there was a total lack of powervalves. yes i know i'm leaving out polaris quads but i am talking about high performance quads here, plus i'm not sure if they used them or not.

[2004TRX450R]

No polaris doesn't use them or at least didn't last time I worked at a Polaris dealer but that was 3.5 years ago. You know how they are getting all hi perofrmance on us now! (yeah right!:blah: ) Anyway the guys that build the RZ350s like useing the Banshee cylenders because there is more room for porting them without the power valves there. The only thing a power valve does is help out bottom end. So if you want to build an all out drag racer with all out power then you don't need one. They come into play nicely when it comes to MX because it will have a decent hit off the bottom.