OK, I've owned, ridden, raced and been around 2-stroke quads and dirtbikes my whole life. But, I have never did any engine work (which is a good thing since I'm far from being a mechanic). My idea of engine work is "yeah, now it runs like I want it to, how much do I owe you"!!!

Now the one concept I have never understood is the difference between piston port, reed valve and rotory valve intakes. Of course a piston port doesn't have either of the other two mechanical parts. I understand that much. But, what is the "mechanical" difference in these three intake systems? What "performance" differences do they make?

I started to brew up some interest in this after reading about the reed cage kit HPD is testing for the Trailblazer. After thinking about it, the whole idea of the conversion just didn't make sense. It was then that I knew I was dumber than I thought?*&@#$!!!

Give the gift of knowledge to a dummy this Christmas season!!!.........THANKS

 

I can help you on a couple. A port allows the air/fuel mix in, and exhaust out.
A reed valve is simply a one-way valve in the intake side of the engine.

A 2 cycle engine 'breathes' like this. On the piston upstroke, fuel in the cylinder is being compressed. There is now a vacuum UNDER the piston, inside the crankcase. This draws air through the reed valve, into the crankcase, under the piston. The oil mixed in with the fuel air mix is now lubricating the innards of your engine. After the spark plug fires, the piston is forced down, compressing the mix in the crankcase. This causes the reed valves to close, trapping the mix in the case, compressing it.
When the piston is far enough down, it exposes an exhaust port cut into the cylinder, letting exhaust gases (under pressure from combustion) out. The piston goes down farther, opening the intake port cut into the cylinder. Remember the fuel/air mix being compressed under the piston? Now that is allowed into the cylinder, pushing out most of the remaining exhaust gases out. The piston rises, sealing off both ports & does it all over again.

All the exhaust is never quite pushed out, and usually, a small portion of the unburned air/fuel mix is allowed out the exhaust, which is why a 2 cycle is not as efficient as a 4 cycle. However, it fires every turn of the crank instead of every other turn, so performance is greater.

By varying the reed valve tension, opening, port size and location, reed spacing, etc. a knowledgable engine builder can 'tune' the engine's power to suit a rider's particular style. That is why you want to let whoever is working on it your use. A motor set up for motocross would be a terror on a slow trail, and a trail machine would be in everyone else's way on a track.

Don't know enough about rotary valves to go there. I will let those smarter than myself explain that to you.

 

Your explanation makes sense, Farmr123.

I always imagined reed (or rotary; I reckon they perform the same function) valves helped insure one-way flow of mixture into the crankcase; without valves of some sort, although 2-strokes will run without valves of any kind, there must be some kind of pressure wave or partial flow reversal when the piston transitions from intake to transfer mode (covering up intake port and opening transfer port); while not magic, I expect reed or rotary valves help the flow dynamics along in the desired direction.

Diogenes

 

Is that why a 350 Banshee does not have the power of a 350 RZ street bike even though they share the same motor?

 

performance wise reeds may be a little better but rotax makes killer rotary valve motors a reed valve is not really mechanical reed pedals are plates of fiber material like a flap that open and close from (suction,vaccum)caused by piston movement a rotary valve is a disk with a slot cut out that spins (with rpm's) between the intake and carb. on every intake stroke the slot passes by and allows the air/fuel mixture to enter the cylinder

 

A good place to start is at "HowStuffWorks" have a look at the link below
http://www.howstuffworks.com/two-stroke2.htm
and follow the other links on that page.
Enjoy

Andrew
97 Sport 400

 

Thanks for all the replies. It seems I did understand more than I thought. I'm just confused by this....Piston port engines like the 250 Polaris lack reed or rotary valves....So, I now assume that the piston is the "valve" in a piston port engine. I assume the negative pressure created in the crankcase during the up-stroke creates suction, filling the crankcase with new fuel/air mixture as the piston rises above an "intake port" in the cylinder wall. The fuel/air mixture comes into the cylinder below the piston, filling the crankcase. Then, during the down-stroke, the piston covers this port, then becoming the closed "valve", building pressure in the crankcase as it continues down, forcing the mix to rise up against the side of the piston until the piston drops below the "topside intake port". Then the mix rushes in on the topside of the piston. As the fresh mix comes in, the exhaust leaves through the "exhaust" port, creating negative pressure and the up-stoke, starting the process again.

THAT'S MY ASSUMPTION AT LEAST!!!