Stock for stock isn't much of a competition, but like I said, a blaster can be made to compete.

 

Blasters are cheaper, the only advantage you may find. But then again, you get what you pay for.

 

i dont think i ever said vs. i am comparing the two just cuz i want to learn more about the R. thats all. im not an idiot. i know the R is faster and all. i have just gotten into the atv world in the past year and all i ever hear about are the new quads that come out. i just wanted to know more about some older ones and since i hear so much good stuff about the R i was interested. and thanks for your responses everyone. [img]i/expressions/face-icon-small-smile.gif[/img]

 

The good older quads were the: Suzuki LT250R (ecpessially the 87-92) the Honda 250r(all years) Tecate 4( rare ) And the LT500 A.K.A Quadzilla. The old 2-strokes were fast because they had a "powervalve" it is this little expansion chamber, that allowed gases to expand or something, i dont know, but it made more power [img]i/expressions/face-icon-small-smile.gif[/img] The older quads were Racing machines, Small, Light, Low to the ground, the blaster is more of a sport machine

 

b4 this year did u notice what kinda quads all the pros used?? mainly the 250R tecate 4 and the lt240r( not many used that) now they're using yfz450's and 450R's

 

The power valve is just that, a valve. It is located in the exhaust port and what it does is open and close the port, to create longer or shorter duration. By having a tall port you loose bottom end power, but a shorter port lacks top end power. So manufactures came up with a way to actually change the port duration "on the fly" which is when the power-valve was created. It's run off a motor called the servo motor, or on some older models, the crank. As RPM's increase the powervalve moves, or rotates to give the exhaust port more height. As we all know the exhaust port is open the same time as the transfers. A big problem with two strokes is that some of the mix was being pushed right out the exhaust due to lack of back pressure at low speeds. The power valve creates a smaller port at lower RPM's, which keeps less fuel/air from escaping the ehaust port, thus giving more power.

 

so elt me get this straight... the powervalve is to keep backpressure the same to reduse the amount of unburnt fuel? and also creating more power?

 

Kind of. As I said 2 strokes are always battleing the problem of no bottom end power. Do you understand the concept of the back pressure being created from the exhaust? On a 2 stroke engine the expansion chamber in the exhaust is what creates the "power band." The exhaust wave enters the pipe and hits the small end, often called the stinger end, and gets reflected back towards the port. This back pressure is what keeps the new intake charge from going right out through the exhaust. The problem is that it takes a certian amount of time for the wave to travell the distance of the pipe and back, and it also takes a certian amount of time for the piston to complete it's cycle. Because you cannot change the port height or length of the pipe when you are out driving, the only other variable you have to work with is the RPM's. When the RPM's are right, the exhaust wave will reach the exhaust port at just the right time as the piston is closing it off, which is where the term powerband or "on the pipe" comes from. Now how does this relate to powervalves? The power valve can now make the port smaller and taller, thus changing its duration. As I said a smaller port works great for bottom end power because it traps more fuel inside the cylinder. However power suffers on the top end. For example, A powervalve will start out closed and remain that way until lets say 5000RPM. Once it reaches that time it will open allowing the exhaust port to have a longer duration which improves top end power. It's getting the best of both worlds.

 

Ok thats kind of confusing.

Are you familiar with the term effective stroke?
The stroke of a engine is how long it is from top dead center (TDC) to bottom dead center (BDC). However the effictive stroke is the time from TDC to when the exhaust port first opens. At low rpm's, the engine needs a long effective stroke, which results in a high compression ratio. This is achieved by the powervalve closing which lowers the exhaust port giving it a longer effective stroke. At high rpm, the engine needs a shorter effective stroke, longer exhaust duration, greater port time area, and a lower compression ratio. The power valve is now open, giving it a smaller effective stroke, resulting in top end power?

got it?
it's hard to explain.

 

[img]i/expressions/face-icon-small-shocked.gif[/img] [img]i/expressions/face-icon-small-shocked.gif[/img] [img]i/expressions/face-icon-small-shocked.gif[/img] wow! i think im getting it. it is a little bit confusing but i understand the concept. this is all really ineresting info. so is there a reason why manufactures stopped adding power valves... or have they? did they not put a power valve on the blaster cuz it wasnt made to be a "race" machine? is there any way to add a power valve to a quad and does this only apply to 2 strokes? thanks for all the information. sounds like you know a bit about 2 stokes eh Shav? i want to ride a 250R sometime... trade my blaster maybe? lol!!!