Widen open transfers?

[David Young]

200 t has a windowed piston. So not much duct work needed.

This was mid porting. I raise the rears but didn't get a picture

 

[smokey7]

Open transfers don't need to be widened all the way down? I am wondering if widening the outlet only would drop velocity too much? I still prefer a closed port jug but I like all the piston support I can get. Good info so far guys please show more pictures and what difference it made

 

[Mastermind]

Back in the day......lol

The thinking was that most flow occurs in the first few degrees of port opening, so the wider transfer port would take advantage of that. Over the years, I've begun to see that wider is not always better on transfers because of the loss of effective scavenging. Can widening help? I would say yes. Is it needed? I don't think so. Port placement is far more important than width when we are discussing transfer ports.

 

[paragonbuilder]

Back in the day......lol

The thinking was that most flow occurs in the first few degrees of port opening, so the wider transfer port would take advantage of that. Over the years, I've begun to see that wider is not always better on transfers because of the loss of effective scavenging. Can widening help? I would say yes. Is it needed? I don't think so. Port placement is far more important than width when we are discussing transfer ports.

So than if wider isn't needed but heights are more important, could it be possible that narrower would be better in some situations? Where is the balance?


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[Mastermind]

Scott and I have had a lot of discussion about transfer width. He tells me that he thinks in a two port engine, they are already too wide.

He knows this stuff a lot better than I do, and I think he is spot on.

 

[Keith Gandy]

Scott and I have had a lot of discussion about transfer width. He tells me that he thinks in a two port engine, they are already too wide.

He knows this stuff a lot better than I do, and I think he is spot on.

What determines if u wanna widen them on a quad?

 

[Mastermind]

I can't see any reason to widen them on a quad port engine. What advantage would you gain if you widened the opening wider than the passage?

Add another? Maybe.

 

[Keith Gandy]

I can't see any reason to widen them on a quad port engine. What advantage would you gain if you widened the opening wider than the passage?

Add another? Maybe.

Im still pondering the intake side quad port. Make most of the timing change in the 1st 25% of the window. Like a dip and then back up quick to the factory timing. Crazy thought I know but was thinkin it may purge that intake side 1st with a quick blowdown and then the majory of the transfer would happen after that 1st 25% opened

 

[Zombiechopper]

Scott and I have had a lot of discussion about transfer width. He tells me that he thinks in a two port engine, they are already too wide.

He knows this stuff a lot better than I do, and I think he is spot on.

Well then. I'm gad I asked the question before firing up the grinder. Thanks Mang.

 

[Mastermind]

Im still pondering the intake side quad port. Make most of the timing change in the 1st 25% of the window. Like a dip and then back up quick to the factory timing. Crazy thought I know but was thinkin it may purge that intake side 1st with a quick blowdown and then the majory of the transfer would happen after that 1st 25% opened

Don't sound crazy to me brother. I say try it out.

My biggest problem right now is being able to do good solid testing. Never enough wood, or enough time. Then I injure myself to boot. For now, I'm counting on you guys to share with me what you are learning.

 

[Zombiechopper]

this reminded of that echo top handle that has stuffers in the transfers. Maybe I should spill some epoxy in a 200t.....

 

[Keith Gandy]

Don't sound crazy to me brother. I say try it out.

My biggest problem right now is being able to do good solid testing. Never enough wood, or enough time. Then I injure myself to boot. For now, I'm counting on you guys to share with me what you are learning.

Well in my thinkin the width, 25% , more or less in relation to the ° it opened would be the balancing act? In other words more width or quicker opening on the port height ?

 

[Terry Syd]

The thinking was that most flow occurs in the first few degrees of port opening, so the wider transfer port would take advantage of that. Over the years, I've begun to see that wider is not always better on transfers because of the loss of effective scavenging. Can widening help? I would say yes. Is it needed? I don't think so. Port placement is far more important than width when we are discussing transfer ports.

On the exhaust most flow occurs in the first few degrees, but on the transfers max flow occurs just before BDC. I don't understand your comment about "because of the loss of effective scavenging". Perhaps you mean if you only widen the port windows and thus have a restriction in the tunnel that drops velocity at BDC. I'd agree in that case, otherwise increasing time/area whether by time or area will still raise the RPM of peak delivery. I widened the transfers on my Husky 450 12.5% and the engine really liked the extra area, however I had to spend a LOT of time to completely rebuild the tunnels in order to get the ports to flow.

As far as me modding the 039/390 jug, I would definitely rework the tunnel and use the slab sided piston to make sure I could feed the port opening. Otherwise, I could see a problem at BDC where the intake side transfer would be flowing faster than the exhaust side transfer. I doubt that having one transfer port flowing faster than the other is desirable as I have never seen any factory or race engine that is set up that way.

I'd agree on a two port engine that widening becomes a problem, but on a quad port there is plenty of room. Heck, high performance two-strokes have transfers that take up all the available room. On a saw there is plenty of room left. Widening just 10% will increase the time/area 10% and lift the powerband about the same amount.

Another thing I like about widening the transfers is that the effective stroke (from transfer closing to TDC) is not reduced. When you raise the transfers it shortens that stroke.

I think the biggest problem with widening the ports is that many times it requires a lot of extra work to get the tunnels right. Just opening up the port window is not an effective way to widen the port.

 

[Mastermind]

Perhaps you mean if you only widen the port windows and thus have a restriction in the tunnel that drops velocity at BDC

That is exactly what I mean Terry. Not many folks are gonna take the time to widen the entire tract.

 

[Keith Gandy]

That is exactly what I mean Terry. Not many folks are gonna take the time to widen the entire tract.

Flat all the way or sloped?

 

[BGE541]

 

[BGE541]

First shot with the RA on a Chinese 350 jug. Runs well so far!

 

[Mastermind]

Also.....what I said was...

"The thinking was that most flow occurs in the first few degrees of port opening, so the wider transfer port would take advantage of that."

Not my thinking.......but at the time that was what everyone was saying. I was less than "experienced" so I took it as the "truth".

If you notice......I got away from that line of thinking on transfers.....

 

[Terry Syd]

Flat all the way or sloped?

The tunnel should be more open towards the crankcase and gradually get smaller as it approaches the port window. On my 450 I had to grind out the back of the transfer covers and build them up with GB Weld and fiberglass. I spent a lot of time with a micrometer to get the shape I wanted.