Question for Powervalve Owners/Tuners

[rustyATV]

I've been refurbishing a Duncan Powervalve cylinder and found something that made me curious.

Look at the attached image. Notice the guillotine neither matches the contour of the piston, nor does it really seal up close against it.

It does, however, perfectly match the roof of the exhaust port when it is fully retracted.

So what's more important for a powervalve cylinder; sealing against the piston or matching the inside of the port?

[wilkin250r]

I can't say I've ever TRIED to get a powervalve to match against the piston, so I don't know what the difference in power would be.

Personally, I would think the PORT is the most important.

The powervalve is there just to provide a restriction at low RPMs. It's not going to act exactly like a lower exhaust port, just similar. That restriction helps build pressure to boost your low end and midrange.

But at peak horsepower and high RPMs, you want that exhaust port to act exactly like a full , open exhaust port. That's were you have maximum airflow, you don't want ANY unecessary restrictions or turbulence. This is where you want maximum performance. You want that powervalve to match the exhaust port exactly.

My thoughts on the matter.

[C-LEIGH RACING]

Its nice to have the edge of the valve blade as close as you can get it up to the piston, but, you dont want the blade to touch the piston at all.

I've seen engine damage from the blade getting to close & the rings or either the piston touching it, no, you dont want the blade to touch.

Two strokes & power output, is mostly all about cylinder port timming. With the blade in the roof of the exhaust & extended down, it has changed the port timming of the exhaust to a lower rpm rate shifting the output.

That is the sweet part about a PV cylinder.
You can have a full range of power from one port job, say like bottom end to midrange while the valve blade is extended down into the port & then with it fully open, depending on how much you have cut the top of the exhaust port roof, it could be a top end cylinder.

Problem with a PV cylinder, again depending on how much the exhaust port is cut, with the valve blade extended, its off setting the port timming enough to cause a mis match of the pipe & porting reducing power output during that time.
What would realy be a fantastic deal, would be if the pipe build could change at the same rate as the pv valve to in a since match up with the porting.
Having all this on one engine would be to complecated & + you would be getting away from the simpleness of a two stroke which is more than likely the very reason most would want a two stroke.
Neil

[rustyATV]

I guess I'm just a little concerned about how effectively the PV is working when there is such an asymmetry to it .

[C-LEIGH RACING]

I guess in a sence, you could say its fooling the port timming in the cylinder to some extent, but not completely.
Works about the same as cutting a pistons timming edge, changing the opening time for that one port.
Neil

[rustyATV]

What I'm really looking for is not an explanation of how it works, just a reassurance that with the shape of the guillotine being so lopsided, I'm not losing any of the effect the powervalve has on the engine.

See the picture: Fully extended the guillotine is very close to the piston on the right side, but about 1/4in away on the left. Granted, this is looking down almost from the top of the cylinder, but I would think that it should be equal, or near to it, all the way across.

[C-LEIGH RACING]

Ok, I see what your saying now.
Didnt think much about the pic of the blade, cause I was thinking it was just the angle the pic was taken causing it to look like that.

Hummm, now I'm wondering if you straighten the edge up, make it straight in the port, will it still be long enough to support a bottom end port timming.
It makes a difference in how long the blade is as well. Shorting the blade changes the timming just like cutting the roof edge.

When you had it out, it look like somebody had been hacking on it.
Normally they are machined smooth & straight, not at an angle like that.

You want to see odd shaped power valves, should see some of the Suzuki & Kawasaki valves. Two set of plungers comming in on each side at the roof of the exhaust port.
Neil

[rustyATV]

I actually still have it totally apart. I'm debating trying to find a new guillotine, and seeing if I can cut it myself, to make it contour better, and keep the original as a back up in case it all goes wrong.


Anyone know where to get these things? (Other than Duncan)

[C-LEIGH RACING]

What brand of cylinder is that your working on. If its the Duncan, you may have to check with them because the PV on theirs according to that pic in the other thread dont look like a complete Rotax valve. Looks like the lower part is special made, which probably would include the blade, but I could be wrong.
I dont remember the lower part of the Rotax valve being shinnie like that, reason I'm saying that.
Neil

[All250R]

I think it was Loren that told me once that their PV setup, or some portion of it at least is their own design. I want to say he was explaining that the blade specifically was an item they designed themselves. I haven't seen it first hand though to say if or what though.

Can you call them to see if they sent you a bad part? It looks awful crooked. You wouldn't cut a port to look like that. The valve shouldn't misshape the port that severely on purpose. I would think the sound wave would enter the cylinder irregularly and that would effect some portion of the output curve, on paper at least. Gas would flow asymmetrically too at port opening at least and possibly have a flow effect that impedes some fluid activity. Generally speaking you want the gases to exit (and enter) the ex port as easily as possible. In the meantime, just a 101 question, you're sure you can't fix it through installing it differently?

[rustyATV]

This is a Duncan cylinder

This is the guillotine that came with the jug. It actually (mostly) matches the "throat" of the port when the guillotine is fully retracted, but I think the exhaust port was cut a bit off center.

I'll have to call their tech line and see what they say. I don't suppose Loren would let a jug out of his place that didn't perform like it should, would he?

BTW, the guillotine has "TOP" cast into one side, so I'm pretty sure I've got it in the right way.

[All250R]

You could try to make a port map and see if that tells you something about the exhaust port. Once you pull the map out it's a lot easier to see what the port's symmetry is like.

Once I had Loren make me a head in a pinch to go riding and he was supposed to cut it a certain squish clearance... blah blah... when I get it home I wind up having to lap a pretty large amount of material off and split the head gasket to get it down to a normal level. For that trip I had to commit to pure C12. I dunno about Duncan. I haven't worked with them a lot, so could be I just got a fluke, but things happen even in good shops too. The problem comes when you call with a problem and they tell you to eat it.

Try a port map. If you're still not satisfied or don't have another answer I'd call down there and see what they say.
Jason

[C-LEIGH RACING]

Rusty,
Looked at that pic real good, I see what your saying now.

The piston ??, is it on the rod or you just got it stuck up in the cylinder.
If its on the rod & cylinders sitting down on the cases, lookng at the exhaust arrow, its as if all the ports are shifted, rotated to the left.
You can see in the pic, the blade is deeper back up in the port on the left, than the other side.
I would say, you probably cant correct that, at least not with out rotating those ports back to center, which cant be done.

Only thing you can do, is check to see how close the extended edge of the blade itself is to the right side of the cylinder bore. Stick a straight edge down in the bore & see how close.
Around 0.040 should be plenty enough clearance to keep it away from the rings or piston.


Alls,
You remember some time back, when Rick was showing a bunch of cylinder on MacDizzy & one of the Duncans, he said something about the ports not being right, this may be what he was talking about.
Neil

[Witz]

Originally posted by rustyATV
This is a Duncan cylinder

This is the guillotine that came with the jug. It actually (mostly) matches the "throat" of the port when the guillotine is fully retracted, but I think the exhaust port was cut a bit off center.

I'll have to call their tech line and see what they say. I don't suppose Loren would let a jug out of his place that didn't perform like it should, would he?

BTW, the guillotine has "TOP" cast into one side, so I'm pretty sure I've got it in the right way.

If the guillotine has "TOP" cast on one side then it is more than likely a Rotax guillotine. Along with a couple 250R pv motors, I run Ski Doo sleds in the winter and the Rotax manufactured guillotines all have "TOP" cast on one side of them in raised lettering. If that was a guillotine that Duncan had machined, then I would guess it would either have no marking or something engraved/machined on it rather than cast into it.

I'm not sure what part number to start with for a replacement Rotax part, but Rotax PV assemblies usually go very cheap on ebay (like $50 or less for the whole valve assembly).