Porting

[ApePilot]

From the same article (Blair) --
"The particle flow entering the atmosphere (reversed flow) is more pronouncedly strong for the weakest
bellmouth, i.e., the plain-ended pipe, and vice-versa for the elliptical
profile bellmouth. Indeed, it is so strong at the plain pipe end that
it has formed a toroidal vortex (smoke ring!) at the pipe end (spitback)."

This may give credence to my previous statement about eliminating the spitback through the carb. Because of the major radiusing I did to the lower transfers, the possible spitback from the transfers (w/ sharp edges) back into the case was eliminated or mitigated and therefore the case pressures were reduced before the intake phase, thus eliminating the carb spitback from the higher case pressures. I may be all wet on this topic, but maybe just damp. Something changed for the better after improving the transfer flow.

 

[Bjorn T]

Hello, I have been thinking alot about porting and how a 2-stroke is running. I have come up to this.

crankcase compression only reaches like 10 psi. If you think about the engine running at 50-250 revolutions per second or 3000-15000 rpm, think about how long the transfers are open for each stroke. If you have an air hose pressurized to 10 psi, and could open the gate for 1/50th of a second, and that is a full stroke of the engine at idle, how much air is coming out of that hose?
The only way these engines can pump efficiently is from the energy produced by the burning mix.
At first the expanding gases push the piston down, then as the exhaust opens and the gases hit the muffler can and expand, it created a pulling effect on the rest of the cylinder, vacuum. And a lot of it. So the transfers want to open just as the pressure is changing to a vacuum for the most efficiency.

After pondering this for a bit I see this as the only way to get enough transfer to run the engine. The crankcase compression combined with a vacuum on the other side means the transfer port sees a much greater differential than just 10 psi to atmosphere

 

[Lightning Performance]

From the same article (Blair) --
"The particle flow entering the atmosphere (reversed flow) is more pronouncedly strong for the weakest
bellmouth, i.e., the plain-ended pipe, and vice-versa for the elliptical
profile bellmouth. Indeed, it is so strong at the plain pipe end that
it has formed a toroidal vortex (smoke ring!) at the pipe end (spitback)."

This may give credence to my previous statement about eliminating the spitback through the carb. Because of the major radiusing I did to the lower transfers, the possible spitback from the transfers (w/ sharp edges) back into the case was eliminated or mitigated and therefore the case pressures were reduced before the intake phase, thus eliminating the carb spitback from the higher case pressures. I may be all wet on this topic, but maybe just damp. Something changed for the better after improving the transfer flow.

Wow you really went for it.

The short version is it ran out of air. Take off the air filter.

You get bonus points for three post. When you understand all the stuff you said put it in layman terms for the class to read lol.

 

[Lightning Performance]

Hello, I have been thinking alot about porting and how a 2-stroke is running. I have come up to this.

crankcase compression only reaches like 10 psi. If you think about the engine running at 50-250 revolutions per second or 3000-15000 rpm, think about how long the transfers are open for each stroke. If you have an air hose pressurized to 10 psi, and could open the gate for 1/50th of a second, and that is a full stroke of the engine at idle, how much air is coming out of that hose?
The only way these engines can pump efficiently is from the energy produced by the burning mix.
At first the expanding gases push the piston down, then as the exhaust opens and the gases hit the muffler can and expand, it created a pulling effect on the rest of the cylinder, vacuum. And a lot of it. So the transfers want to open just as the pressure is changing to a vacuum for the most efficiency.

After pondering this for a bit I see this as the only way to get enough transfer to run the engine. The crankcase compression combined with a vacuum on the other side means the transfer port sees a much greater differential than just 10 psi to atmosphere

Your right it sees a big pressure hit right back down the transfer ports on stock saws.

 

[Lightning Performance]

It's already been said, but stock saws are limited by sound and emissions regulations. Power isn't their only consideration, they also have to have a quiet saw and one that runs cleanly. It seems pretty reasonable that, if you don't care about sound or emissions, you can build a saw that's optimized for power.

I'm not a guy that's a professional porter, just merely an informed "redneck," although I mostly drink hipster IPA beers or whiskey instead of gin. Just because I'm not a Stihl engineer, doesn't mean that I'm just some idiot with a dremel...I'm actually an idiot with flex shaft grinders and a lathe.

Even some idiot hillbilly like me has an understanding of how compression, port timing, and even port shapes affect the powerband of the saw. Coupled with exhaust mods, I can build a saw that is a definite improvement over stock. All of my saws are work saws...they are used for falling trees or are climbing saws. I'm usually not reinventing the wheel with them, just prioritizing power over noise and EPA compliance.

I'll add that that I don't hold saw engineers on some pedestal...like we already established, they are designed as a compromise for factors that a lot of the end users don't care about. Besides that, saw technology is in the dark ages compared to other types of 2 stroke motors. Just look at dirtbikes...they've had powervalves and reed valves for the better part of 40 years now. Going even further, most of them are now case-reed inducted with boost ports located where the intake port would be on a cylinder fed jug. Just looking at how much the transfers can flow compared to a traditional 2 stroke motor is eye opening. Suddenly, saw motors don't look so high tech. I understand that they have to build the saw in a small, lightweight package, but it's not the dark art you think it is.

Where can we buy these dark arts and idiot grinders?
I need two please

 

[Lightning Performance]

Since you are taking that track…….why do folks that mill a lot have them ported for milling……actually milling………not just talking.

Ported milling saws suck. They damn thing keeps killing my chain cutters every few tanks.

 

[ApePilot]

Wow you really went for it.

The short version is it ran out of air. Take off the air filter.

You get bonus points for three post. When you understand all the stuff you said put it in layman terms for the class to read lol.

Yeah, but why did it 'run out of air?' Same kind of thing would happen to an older McCulloch but at lower rpms -- but we're a bit beyond that nowdays. Taking off the air filter might help a little because of the extra drag it creates, but a bigger air filter would be better (don't worry about fitting it inside the nice, shiny cowlings). I've gone the sans filter route and I'm not doing it again.
Saying it 'ran out of air' is completely dismissing the possibility of promoting better flow up into the combustion chamber. Somehow Randy (mastermind) got there (or real close to 'there') with at least his 261 builds.
I can get far better flow, but it's difficult without a tuned expansion chamber.
As for layman terms? I think all this is laymen terms. I don't know how to put it any more simply. Pretty dang succinct, don't you agree?

BjornT hasn't thought about the thermodynamics yet. As hot gasses expand from being expelled from the exhaust port, what do they do? You got it. The gasses cool a bit.
What happens when they cool? They contract in volume.
What happens when a moving gas contracts? It creates more dynamic pressure.
The typical exhaust-can on a chainsaw is not contributing to a vacuum. It's creating more back pressure than if it was not there. The whole point of the can is to attenuate the exhaust noise by cooling the gasses, creating a more constant pressure release, compared to releasing the raw pressure waves straight from the exhaust port.
Now if you subtract the can and properly design and install a tuned expansion chamber, then yes, you will create a low-pressure wave, then a high-pressure wave; this cycle begins at the time of exhaust-port opening and concludes (for the most part) before the exhaust port completely closes, when the rpm's are within the designed resonance of the pipe. Otherwise, it's not helping. Much.

 

[Lightning Performance]

Yeah, but why did it 'run out of air?'

Because the carb is too small on them if you port one. Ive ported the carbs and they are still too small.
Pretty simple.

Change all the filters you want. If you take off the filter and it goes faster great but you can't just add on anything to a climbing saw top or rear handle. 201T same thing. It has a bigger filter but not much bigger. These saws are about as close as you can get to being over the edge moving more air through one. They choke once you open the mufflers and raise the transfer ports a bit. Same thing with many other models built on an emissions standard or limited fuel supply to hit a targeted run time on one load of fuel.

 

[link]

Hello, I have been thinking alot about porting and how a 2-stroke is running. I have come up to this.

crankcase compression only reaches like 10 psi. If you think about the engine running at 50-250 revolutions per second or 3000-15000 rpm, think about how long the transfers are open for each stroke. If you have an air hose pressurized to 10 psi, and could open the gate for 1/50th of a second, and that is a full stroke of the engine at idle, how much air is coming out of that hose?
The only way these engines can pump efficiently is from the energy produced by the burning mix.
At first the expanding gases push the piston down, then as the exhaust opens and the gases hit the muffler can and expand, it created a pulling effect on the rest of the cylinder, vacuum. And a lot of it. So the transfers want to open just as the pressure is changing to a vacuum for the most efficiency.

After pondering this for a bit I see this as the only way to get enough transfer to run the engine. The crankcase compression combined with a vacuum on the other side means the transfer port sees a much greater differential than just 10 psi to atmosphere

That's a pretty thought through reply to this subject, I'm looking forwards for @huskihl reply on this, thank you sir.

 

[huskihl]

That's a pretty thought through reply to this subject, I'm looking forwards for @huskihl reply on this, thank you sir.

I don’t have an opinion on it.

Bjorn and I usually agree about porting theory. I can’t prove him wrong or right and don’t really care to one way or the other, so…?

 

[link]

OK.

 

[ApePilot]

Lightning wrote: ... They choke once you open the mufflers and raise the transfer ports a bit. ...

I'm thinking he had a few beers too many before writing this one. Who raises the transfers in relation to the exhaust? No one that I know. Maybe they do in error? One of the big reasons that a 2T 'runs our of air,' as Lightning would say in layman terms, is because as the rpms increase, the drawdown time period shortens too much , thus the combustion pressures don't have enough 'time' to subside and evacuate through the exhaust port, and the pressure remains too high for the crankcase charge to be scavenged efficiently. Thus there is no vacuum generated by the exhaust can... it's just creates too much excess pressure. If an exhaust can did create a vacuum as does a tuned expansion pipe, then we wouldn't have need of tuned expansion pipes on other 2T engines.

The typical torque curve hits a peak at typical chainsaw working rpm, but then takes a dive, and I believe it's primarily as Lightning would suggest.... it runs out of air.... because as the rpm increases past max torque rpm, the drawdown time decreases too much and the combustion chamber pressures during drawdown remain too high for complete and efficient scavenging and therefore combustion forces don't increase at a similar rate as the increasing rpms after max torque rpm.
If we can decrease the scavanging drag as much as possible, we can extend the upward or constant trend of the torque curve a few more hundred rpms and possibly reduce the torque-dive after our extended max torque rpm. If we can LOWER the upper transfers a bit in relation to the exhaust roof, and perhaps create new 'fingers' or more transfer/boost ports (as do the 2T bikes and sleds), then we've given the combustion gasses more time to evacuate the combustion pressures to subside before we allow scavenging to start. By creating more scavenging pathways (boost ports, etc.), we help decrease the scavenging drag (lower transfer velocities) at higher rpms. That's likely why a dual transfer port engine 'runs out of air' at lower rpms compared to a quad-port saw.
At least this is what I've been trying to wrap my little pee-brain around.

 

[huskihl]

Lightning wrote: ... They choke once you open the mufflers and raise the transfer ports a bit. ...

I'm thinking he had a few beers too many before writing this one. Who raises the transfers in relation to the exhaust? No one that I know. Maybe they do in error? One of the big reasons that a 2T 'runs our of air,' as Lightning would say in layman terms, is because as the rpms increase, the drawdown time period shortens too much , thus the combustion pressures don't have enough 'time' to subside and evacuate through the exhaust port, and the pressure remains too high for the crankcase charge to be scavenged efficiently. Thus there is no vacuum generated by the exhaust can... it's just creates too much excess pressure. If an exhaust can did create a vacuum as does a tuned expansion pipe, then we wouldn't have need of tuned expansion pipes on other 2T engines.

The typical torque curve hits a peak at typical chainsaw working rpm, but then takes a dive, and I believe it's primarily as Lightning would suggest.... it runs out of air.... because as the rpm increases past max torque rpm, the drawdown time decreases too much and the combustion chamber pressures during drawdown remain too high for complete and efficient scavenging and therefore combustion forces don't increase at a similar rate as the increasing rpms after max torque rpm.
If we can decrease the scavanging drag as much as possible, we can extend the upward or constant trend of the torque curve a few more hundred rpms and possibly reduce the torque-dive after our extended max torque rpm. If we can LOWER the upper transfers a bit in relation to the exhaust roof, and perhaps create new 'fingers' or more transfer/boost ports (as do the 2T bikes and sleds), then we've given the combustion gasses more time to evacuate the combustion pressures to subside before we allow scavenging to start. By creating more scavenging pathways (boost ports, etc.), we help decrease the scavenging drag (lower transfer velocities) at higher rpms. That's likely why a dual transfer port engine 'runs out of air' at lower rpms compared to a quad-port saw.
At least this is what I've been trying to wrap my little pee-brain around.

All depends on what the intended purpose of the saw is. Raising the transfers will raise the upper rpm torque (increasing rpm in the cut) until it doesn’t

 

[ApePilot]

All depends on what the intended purpose of the saw is. Raising the transfers will raise the upper rpm torque (increasing rpm in the cut) until it doesn’t

So when raising the transfers, are you also raising the exhaust roof? Everything I've read and done suggests that the greater the blowdown period, the better the high rpm torque should be. Obviously the goal is to make the transfers as efficient as possible, but if the residual combustion pressure is still too high while the spent gas is exiting, no transfer flow will occur no matter how high and enlarged the transfers are made until the residual combustion pressure has been relieved to below that of the primary compression. And... and.... if the transfer roofs are too high, the residual combustion pressure will not only exit through the exhaust port, but also exit the combustion chamber through the transfers, creating a reverse flow that must be somehow reversed again for the new charge to flow in the correct direction. Attempting to reverse a contrary flow creates a lot more resistance from flow momentum, but also with more surface drag, all within the transfer tunnels. In my head, increased rpm torque requires a proper amount of blowdown time, unless you have a tuned-pipe effect that does create a vacuum that not only acts on the spent gases, but also helps pull up a fresh charge from the crankcase. A stock can does not have the proper geometry to function as a Helmholtz resonator and produce any viable vacuum,.... or does it? I'm pretty sure that the volumes of typical cans on saws are just not big enough to function as resonators. The main objective of the engineers is to make enough power while reducing radiated pressure (sound) waves to appease guberment safety and environmental agencies.
Am I incorrect? Please give us something more substantial than 'until it doesn't.'
Just trying to learn more.

 

[Bjorn T]

Hello. I Can explain more about my Theory. But it Will take some time. My english is not good enough, so I Will need some help to get it right.

 

[huskihl]

All depends on what the saw wants. After lathe work, I don’t know of a saw that runs better with more than factory blowdown

 

[Bjorn T]

Hello, What I was meaning in my first post. Is that the burning mix ceratet a pulling effect in the cylinder vacuum, when the gases is leaving The cylinder out in the exhaust port. An then to the muffler.
A original muffler is restrectiv and Will not make a strong pull, But it wil help a little. A modded muffler Will help to get more pulling effekt. The muffler on the picture make a realy Strong pulling effect.
The Best exhaust system on a 2-stroke engine i a expensionchamber. It Will help pulling fuel from the crankcase and push back unburned fuel in the cylinder before the exhaust port close. But you need to have more blowdown on the engine. if you use a expensionchamber. Around 25-35 degrese. So the pipe gets time to work correct. My Hybrid have 30 degrese blowdown. regular muffler.
Here it is with pipe.

 

[link]

Hello, What I was meaning in my first post. Is that the burning mix ceratet a pulling effect in the cylinder vacuum, when the gases is leaving The cylinder out in the exhaust port. An then to the muffler.
A original muffler is restrectiv and Will not make a strong pull, But it wil help a little. A modded muffler Will help to get more pulling effekt. The muffler on the picture make a realy Strong pulling effect.
The Best exhaust system on a 2-stroke engine i a expensionchamber. It Will help pulling fuel from the crankcase and push back unburned fuel in the cylinder before the exhaust port close. But you need to have more blowdown on the engine. if you use a expensionchamber. Around 25-35 degrese. So the pipe gets time to work correct. My Hybrid have 30 degrese blowdown. regular muffler.
Here it is with pipe.

Hybrid or what, that is not the tenacious long lasting diesel kinda engine you need for milling now is it Bjonn?
That's more of a saw living on borrowed time suitable for cutting cookies in order to impress people...

 

[huskihl]

Hybrid or what, that is not the tenacious long lasting diesel kinda engine you need for milling now is it Bjonn?
That's more of a saw living on borrowed time suitable for cutting cookies in order to impress people...

Keep tipping that bottle back….

 

[EchoShindaiwaMakitaDolmar]

Expert porting of chainsaws is worth every penny- for the satisfaction alone of pi**ing off the malcontents, Nancys, and Karens.

Owner satisfaction, increased performance, and higher productivity, are bonuses.