Discussion Starter

[Hundred Acre Wood]

Too much diesel tech, lol. For proper scavenging in a two-stroke, you don't want mixing of the spent gases and the fresh charge. You want the fresh charge to blow in under the escaping exhaust and loop up & around toward the combustion chamber. The swirl you are talking about can be helpful up near the combustion chamber after the ports are closed. This is why it is important to have a tight squish band clearance to push the charge into the chamber closer to the spark plug.

There's also a tradeoff to be had. Turbulent air is better for dispersing the charge, but it's worse for moving the spent charge out.

So you have to balance turbulent air for maximizing combustion with laminar flow for maximizing exhaust. That's a tricky optimization.

 

[huskihl]

so then the upper opens on the intake side at the same time on both sides does it create a specific flow pattern like a whirlpool or like a mushroom cloud effect up the side and towards the chamber?

Usually in a dual port motor, one side opens 2–3° earlier than the other

 

[srcarr52]

So why has everything gone to the exhaust side of the upper opening first? It seems like scavenging would work best if the intake side filled first and then pushed the exhaust from the intake side of the piston crown up towards the chamber and then down to the exhaust. I guess the exhaust side opening first has enough force to shoot all the way across the crown and hit the cylinder wall, then the intake side assists travel up the cylinder wall towards the chamber? Seems like opening intake side first would work just as well. Maybe it’s about putting more momentum in the exhaust side to create a stronger boundary between exhaust and fresh charge. If the exhaust side opens with less “pressure” (or momentum?) it may short circuit out the exhaust more easily.

Opening the intake side first allows some exhaust to be mixed in once the exhaust side opens. My thinking is trying to get the exhaust and intake side transfer charges to meet the charges from the opposite side like a unified front at your peak power RPM.

 

[pbillyi69]

everything is happening so fast in these tiny engines. how do you guys that do the port work really know how much any one change you make to any port affects the overall performance? when doing a cylinder for the first time how many things do you adjust or change at once?

 

[Powerstroke Cowboy]

Opening the intake side first allows some exhaust to be mixed in once the exhaust side opens. My thinking is trying to get the exhaust and intake side transfer charges to meet the charges from the opposite side like a unified front at your peak power RPM.

Honest question. How does/can that work when the exhaust side is part way open before the upper transfers start to open?

 

[huskihl]

everything is happening so fast in these tiny engines. how do you guys that do the port work really know how much any one change you make to any port affects the overall performance? when doing a cylinder for the first time how many things do you adjust or change at once?

I go back into new models 3-4 times looking to see what does what. And after I’ve done a few like that I start experimenting with little changes every time I port the same model saw. I know what I want and don’t veer from it on about 15 models. The rest are still an expensive experiment

 

[huskihl]

Honest question. How does/can that work when the exhaust side is part way open before the upper transfers start to open?

That’s the game. Adjusting exhaust and transfer heights to get them to purge enough spent exhaust but close the exhaust off early enough to not let too much fresh charge out behind it

 

[pbillyi69]

i really do admire you guys who take the time to put the effort and work into a saw and the are willing to share what you have learned and sell your services. thank you to all of you pro porters.

 

[srcarr52]

Honest question. How does/can that work when the exhaust side is part way open before the upper transfers start to open?

To clarify, the exhaust side of the transfers.

 

[pbillyi69]

i have read alot of the porting info on here and there is a lot of info on numbers and intake and exhaust size and shapes. but what about the transfer ports how much bigger do you make them if at all and once you start grinding in them how do you look in there to see if you are achieving what you are after. do any of you have mirrors or cameras that you use to look in there? some of the cylinders are so small it seems like a very difficult task

 

[huskihl]

i have read alot of the porting info on here and there is a lot of info on numbers and intake and exhaust size and shapes. but what about the transfer ports how much bigger do you make them if at all and once you start grinding in them how do you look in there to see if you are achieving what you are after. do any of you have mirrors or cameras that you use to look in there? some of the cylinders are so small it seems like a very difficult task

Transfer size depends on what the purpose is. Larger transfers flow more if there’s enough to push the charge through. Smaller tighter transfers limit rpm and promote torque and forgiveness. More case pressure pushes on the charge harder to feed tighter transfers. All depends on whatcha got.

Daughter gave me her old dental mirrors for seeing inside.

 

[Ketchup]

i have read alot of the porting info on here and there is a lot of info on numbers and intake and exhaust size and shapes. but what about the transfer ports how much bigger do you make them if at all and once you start grinding in them how do you look in there to see if you are achieving what you are after. do any of you have mirrors or cameras that you use to look in there? some of the cylinders are so small it seems like a very difficult task

I look down the bore, down the tunnels, through the exhaust and intake ports. Then I take photos and look at the photo. Then I look down the bore, down the tunnels, through the intake and exhaust ports. Then I take photos and zoom in on those. Then I look down the bore, down the tunnels, through the intake and exhaust…etc.

I haven’t seen a lot of quality jugs that need the transfers hogged out or the uppers enlarged. Mostly I focus on shape, angle and timing. I usually try not to make them bigger.

If you do go bigger the increase needs to be consistent down the tunnels to the lowers. Enlarged uppers with stock tunnels haven’t worked for me. The lowers can be enlarged or blended within reason to achieve better flow.

 

[pbillyi69]

thank you for the lat two post that explains a lot

 

[Powerstroke Cowboy]

To clarify, the exhaust side of the transfers.

Gotcha. That makes more sense. I did wonder if that's what you ment. After I asked the question.

 

[5155]

Friend of mine worked with setup and repair of natural gas meters of all sizes for over 30 years.
He often referred to a tool at the shop named "the flow bench".
He turned out some wicked SB chevy heads, Also ruined a few.

 

[drf256]

So why has everything gone to the exhaust side of the upper opening first? It seems like scavenging would work best if the intake side filled first and then pushed the exhaust from the intake side of the piston crown up towards the chamber and then down to the exhaust. I guess the exhaust side opening first has enough force to shoot all the way across the crown and hit the cylinder wall, then the intake side assists travel up the cylinder wall towards the chamber? Seems like opening intake side first would work just as well. Maybe it’s about putting more momentum in the exhaust side to create a stronger boundary between exhaust and fresh charge. If the exhaust side opens with less “pressure” (or momentum?) it may short circuit out the exhaust more easily.

This is what I think. Also sweeps more spent gasses off of a greater area of the piston. Gotta remember that the piston is still falling when the transfers are opening.

 

[drf256]

everything is happening so fast in these tiny engines. how do you guys that do the port work really know how much any one change you make to any port affects the overall performance? when doing a cylinder for the first time how many things do you adjust or change at once?

There’s some general combined shared knowledge of do’s and do nots for saw motors. The rest is experimentation.

Generally one change at a time is the only way you’re gonna know what’s going to work.

Necessity is the mother of invention. A few of my own best recipes happened when I had to fix something in a saw and had no choice but to use odd numbers.

If we knew it all, none of this would be any fun.

 

[Red97]

What if.

The newer saws switched to exhaust side transfers first/earlier because a bit of raw fuel is less of a strike than burnt emissions? I would be very curious to know how far back the "emission" were a large factor in design?

I'm sure currently the new designs have to meet requirements that are not even rule yet?

Just looking at it from a different angle.

I tend to open transfers at the same time. May play with the angle some.

Saws sure can be fun...

 

[HYPERSAWS]

Only if someone could print up a few clear cylinders to watch what is going on in there in super slow motion

 

[Thesandman]

Ok I'm wondering about trans port lengths. This design being very short and able to move a short column of air very fast compared to a bottom of cylinder feed. Has there been a design that consistently yields mobedd,r?