Let's Talk Transfers

[Deets066]

You do what ever it takes to get the best cylinder fill. This is figured out with testing. 2 port or 4 port transfer motor?

How bout some pics of that 046 two port that you put the epoxy divider through the whole transfer

 

[Stihlbro]

I posted in another thread the pic I had on my phone. I will look to see if I have another.

 

[Brewz]

When i ported my 026 I removed a fair bit of material from the lowers, blending them into the cylinder with the intention of making it easier for the fresh charge to make its way into the transfer.
I left the throat of the transfer the same size, just smoothed it out.
The uppers got raised with minimal material removed

I was worried it would be gutless in the cut due to the increased volume in the bottom of the case but it is actually the opposite. It hauls some serious ass and can handle a lot of load in very hard wood.

It was initially said that the removal of a lot of material around the lowers would increase case capacity and slow the transfer charge but my thought was that increased case capacity would allow me to fit more fuel/air mix in the case and the more streamlined lowers would aid in velocity as the charge was pushed into the uppers by the piston.

And I mean, realistically, a bit of shaping on the edges of the lowers wouldn't add more than 1 or 2% to the case volume at worst.

I am liking the theory behind getting the main charge sweeping the top of the piston first to prevent mixing and then keeping the transfer throat the same size should keep velocity up and hopefully more volume with the less restrictive lowers.
It may not be correct but is sure seemed to work.

The black mark is the original width of the lower transfer. I rounded it off towards the intake and also towards the base of the cylinder


I then raised the transfers by about 4 deg and also rounded the intake side out by a couple mm to help encourage the charge direction to the intake side. I left the exhaust side as it was.


It is running factory squish, standard piston, factory gasket, .020 squish and about 160psi of compression with 2 tanks of fuel under its belt and like I said, it really hauls!

I think there is more than one way to skin a cat, its getting all the changes to work in harmony and balance that is the tricky bit.

I still have questions about the pressure created in the combustion chamber which is still falling as it vents into the exhaust port. How does this effect the low pressure charge trying to get in from the transfer?
I understand the pressure will drop fairly fast during the blow down period if the downward cycle, but do we know how long it takes for it to drop to a point where it is no longer restrictive to the incoming charge?
At 15000 rpm, we are talking 0.004 of a second per engine cycle. That gives us 0.002 seconds on the downwards stroke to do everything we are talking about efficiently.

Are we looking at this too closely?

Great thread!!!!!!

 

[Terry Syd]

it's not about how fast the charge is moving, but how much charge your moving in a given time. volume, air pump

Yep, there's a reason it is called a 'flow bench' and not a 'velocity bench'.

OK, let's talk transfers. Back in the days when Timberwolf used to post on AS he put some transducers in the transfers to see what the velocity was and in what direction. When the saw was running at max power there was a slight reversion as the residual exhaust gas pressure in the cylinder would push back down in the transfers. You can see this on your engines by noticing the bit of exhaust carbon near the top of the transfer ports.

Then as the piston continued to drop (and the crankcase compression increased) the flow changed and headed into the cylinder. The maximum flow occurred just before BDC (5-10 degrees), the flow would continue into the cylinder even as the piston was now moving upward (there was still pressure in the crankcase). In the case of maximum power, the flow would STILL BE FLOWING INTO THE CYLINDER as the piston closed off the port.

There is a thing called 'maximum delivery ratio' - this occurs when the transfer flow stops just as the piston closes the transfer ports. For all practical purposes we can call the maximum delivery ratio point - maximum torque.

So, what happens when we drop below maximum torque? The rpm is lower and there is more time for the mixture in the crankcase to get into the cylinder. In fact, it gets in there too soon - with the piston now rising, it starts sucking the mixture BACK OUT OF THE CYLINDER.

Thus, you can see there is only one point where the maximum delivery ratio will be at the maximum. Above that rpm and there isn't enough time/area to get all the mixture into the cylinder - and below that rpm the mixture is getting sucked back out of the cylinder.

When you increase the time/area of the transfers you will raise the point of maximum delivery ratio. In other words, you move the powerband up higher.

Now about raising the transfers on a chainsaw - if it is a quad port, why in the Hell would you do it? This is not a high performance two-stroke with multiple transfers wrapped around the cylinder. The required time/area of transfers on a saw is the LOWEST of all the two-strokes. There is ample room on the side of the cylinder to WIDEN the ports.

So why widen them? - Well, you can either make the transfer wider to flow the mixture, or you can increase the time they stay open - either way you will increase the time/area and can flow the same amount. HOWEVER, if you raise the transfers you are making the engine SMALLER.

A two-stroke only begins to suck in mixture when the transfers are closed. The swept volume of the engine is from the point of TRANSFER CLOSING to TDC. If you raise the transfers you are effectively shortening the STROKE.

 

[Backyard Lumberjack]

What about piston Windows? The Charge starts out exiting through the piston windows in to the lowers. The piston windows are the first restriction the mix hits.

I'm totally just thinking out loud, but the mix is restricted at the piston Windows then kind of dumped into the lower transfers which are wider and more open than the piston Windows. Then it's redirected and compressed more before the upper transfers/fingers opens.
I built a 024 and opened the piston Windows. I was thinking at that time the benefit would be the windows were open to eliminate the first restriction.

I was thinking let the velocity increase come at the last point which is the upper transfers shape and size.
Think about it like a garden hose leave the end open the water doesn't go very far but put a small nozzle on the end and at It will spray far.
You would not put a nozzle internally mid hose and then expect the water to spray out of the end as if it was placed on the end.

Another benefit I thought about when I did this was being able to shed a lot of weight off of the piston.

I could be all wrong... But the saw runs great!! It seems to hang with the 026 I build.. I can't explain why??

I was going to read all the posts in this thread and then maybe comment. great! thread... rolls my sox up and down!! but sharpshooter here (czar800) is echoing my thots in his post, and I cannot resist. btw - for the most part, I am reading these posts with cylinder and piston in it in hand... great thread, no doubt!

* I'm totally just thinking out loud, but the mix is restricted at the piston Windows then kind of dumped into the lower transfers which are wider and more open than the piston Windows.

I do not think you can talk transfers effectively as a subject without including the piston in the conversation(s). I would not say the 'mix' is restricted at all. but I would say it will flow up and thru the piston windows (044) and that open area is significant. besides, on either sides of the piston at the pin boss are large openings. to me, these openings and windows do not designate a restriction other than possibly the piston not being there, but then... we don't have a saw! as I see it, there is a lot of opportunity to blend these openings into the cyl and related areas, lots of flat sharp spots, mill ridge type stuff. I will give u an example. mocking up the squish set as to cyl to crankcase, the piston mocked up at BDC... I can clearly see, at least on my 044 cyl.. that where the piston window is at BDC, there is a mill ridge on the topside of the upper portion of the lower transfer port. both sides. I plan to blend these in to 'port match' the piston windows to this area of the transfer port. one only has to look inside cyl with piston at BDC to see numerous add'l port blending opportunities. I will tell u this, from my exp on my flow bench... all else remaining =, contouring and blending will pick up the cfm capacity of the column of air.

* I was thinking at that time the benefit would be the windows were open to eliminate the first restriction.

imo, it is not the first restriction. as I see it, the first restriction is in the area of crankcase to gasket to bottom flange/shape if cyl. but over all, the piston windows contribute to transfer filling, but a gas will take the path of least resistance, and the huge area on either side of the pin boss is wide open...

* Think about it like a garden hose leave the end open the water doesn't go very far but put a small nozzle on the end and at It will spray far.

imo, good analogy. I see the upper transfers are smaller, perhaps for this reason. acceleration and to keep the gaoline in suspension as best as possible. but I see both upper transfer ports while they do have a directionally focused design on one side of the transfer port, both point towards the intake port area...

*Another benefit I thought about when I did this was being able to shed a lot of weight off of the piston.

not so sure this is a good idea. not on a working saw. case of the 044... with stuck rings in OE piston, the meteor piston w/o rings still weighs 12 grams more. with a focus on piston windows blending and opening... I would like to see my meteor piston about OE weight, and maybe 5-1o grams less just for fun of it. lends itself to a lot of cool contouring, porting and blend shaping of the windows, piston boss etc as the meteor is brought back into OE spec weights. maybe a bit more than 10 grams, but doubt more than 15. piston/rod is balanced by crankshaft counterweights, etc... more or less. imo, there is lot of room to work over the meteor piston to improve transfer port filling and cfm flow. but in any event, I do not think in terms of shedding ' a lot of weight', but some for sure.... how much weight removal is a lot to you and at same time acceptable? wondering. I have mulled over idea of removing lower piston skirt bosses. the upper is substantial. most of the force the piston is subjected to is thru compression and that is a vertical loading and resultant shape imo should support it. in considering piston windows I am reminded of how important the blowdown function is, effect and timing... and that all this 'basement' stuff is at about 5-6 psi. not a lot of pressure, but significant. it is hunting for path of least resistance, and there are many areas in 'basement' to further help the dawg to hunt!

* I could be all wrong... But the saw runs great!!

no... and I, for one, do not think u r wrong at all. good to hear ur saw runs great! ok, back to reading this thread. prob read it several times. good stuff!

 

[Backyard Lumberjack]

I think 10 is optimistic.

Velocity is direction that's why u lose when velocity is lost.

5-6 psi in crankcase.

 

[Backyard Lumberjack]

What would be a good indication of a saw that has low transfer velocity? Bog off idle? Slow throttle response?

I do not think, short of porting work damage, wrong shapes, no concern for port timing that a saw will have low transfer velocity... at 200-225 frames per second... a lot happens fast and often. I would ponder more so does the column of air start and stop, or one continuous flow? despite mechanical ports opening closing and 'closing' the doors. a gas is compressible. a fuel laden column of air has density and kinetic energy... and the oil even adds more. to me while the crankcase psi is low... both velocity and volume are important. caring, well planned and smartly executed porting can and will improve the volumetric efficiency of these engines. on a chevy V8, for example, small cam, and some upgrades... econo port work in the cyl head valve bowl and port matching the intake runners to head... are always good for better higher rpm performance and... an improved throttle response and vehicle driveability.

 

[Backyard Lumberjack]

The ones I've seen will have good acceleration and will four-stroke while not under a load to a high RPM range, but fall on their face in the cut. Walker's saws from the early 2000's were that way. They would hog out the transfers and gut the muffler and the saws had no torque band at all.

If you look at the Sachs-Dolmar designs from the 70's and 80's- they had long, narrow transfers with a small upper. Those saws consistently had a fat torque band and would hold decent RPM in the cut with a long bar.

torque bands matter on a 2-stoke also called the power band...

 

[Backyard Lumberjack]

it's not water, air doesn't flow the same

you are right, in that water is a liquid and the air/fuel mixture in a saw is in a gaseous state. but, I tend to think of the two having very similar characteristics especially when flowing thru ports... ie cfm, velocity, eddies, tumbling, pooling etc...

 

[Backyard Lumberjack]

Your right Scott I'm just explaining how I see it.
You build a strong saw so do i, I seriously doubt there night and day inside.
I don't think there's a black and white answer.

>I don't think there's a black and white answer

 

[Backyard Lumberjack]

So if you see grinding in the lowers or case matching as enhancing flow, that is more important to you?


Sent from my iPhone using Tapatalk

case matching - I do. easy free simple mods to help flow out of crankcase and into transfers. even if not significantly material. but 'free' works for me... works well!

 

[Backyard Lumberjack]

it's not about how fast the charge is moving, but how much charge your moving in a given time. volume, air pump

I am reminded of large engine performance when I read this in that slowing down of the column or air often times can lead to pooling and/or gasoline droplets yet needing to atomized (squish function as a last resort) falling out of suspension...

 

[Backyard Lumberjack]

That would insinuate larger ports are better. But doesn't seem to be the case...Sent from my iPhone using Tapatalk

always room for improvement... OE mfgr optimize casting, casting cores, designs etc for optimal performance... ha, often as not a compromise to facilitate the manufacturing process and give a reasonably suitable product.

 

[Backyard Lumberjack]

No. It doesn't ignite til it's under compression and the plug fires.


Sent from my iPhone using Tapatalk

well, so long as detonation is not taking place... or pre-ignition.

 

[Backyard Lumberjack]

So what's the concensus on ex side fingers in a two port?

I have magic markered them in to see them in my 044 cyl work, my consensus is to leave them out. I don't know enough about them... shape, where to start where to end, directional paths... I did see on one cyl a set that started right out of the top corner of the bottom transfer port... I think I will go with one of this thread's themes:

don't remove more than you need to!

 

[mdavlee]

Yep, there's a reason it is called a 'flow bench' and not a 'velocity bench'.

OK, let's talk transfers. Back in the days when Timberwolf used to post on AS he put some transducers in the transfers to see what the velocity was and in what direction. When the saw was running at max power there was a slight reversion as the residual exhaust gas pressure in the cylinder would push back down in the transfers. You can see this on your engines by noticing the bit of exhaust carbon near the top of the transfer ports.

Then as the piston continued to drop (and the crankcase compression increased) the flow changed and headed into the cylinder. The maximum flow occurred just before BDC (5-10 degrees), the flow would continue into the cylinder even as the piston was now moving upward (there was still pressure in the crankcase). In the case of maximum power, the flow would STILL BE FLOWING INTO THE CYLINDER as the piston closed off the port.

There is a thing called 'maximum delivery ratio' - this occurs when the transfer flow stops just as the piston closes the transfer ports. For all practical purposes we can call the maximum delivery ratio point - maximum torque.

So, what happens when we drop below maximum torque? The rpm is lower and there is more time for the mixture in the crankcase to get into the cylinder. In fact, it gets in there too soon - with the piston now rising, it starts sucking the mixture BACK OUT OF THE CYLINDER.

Thus, you can see there is only one point where the maximum delivery ratio will be at the maximum. Above that rpm and there isn't enough time/area to get all the mixture into the cylinder - and below that rpm the mixture is getting sucked back out of the cylinder.

When you increase the time/area of the transfers you will raise the point of maximum delivery ratio. In other words, you move the powerband up higher.

Now about raising the transfers on a chainsaw - if it is a quad port, why in the Hell would you do it? This is not a high performance two-stroke with multiple transfers wrapped around the cylinder. The required time/area of transfers on a saw is the LOWEST of all the two-strokes. There is ample room on the side of the cylinder to WIDEN the ports.

So why widen them? - Well, you can either make the transfer wider to flow the mixture, or you can increase the time they stay open - either way you will increase the time/area and can flow the same amount. HOWEVER, if you raise the transfers you are making the engine SMALLER.

A two-stroke only begins to suck in mixture when the transfers are closed. The swept volume of the engine is from the point of TRANSFER CLOSING to TDC. If you raise the transfers you are effectively shortening the STROKE.

After cutting the base and chamber for compression if you don't raise them back up you don't get the gains. I've never tried to widen them to get the time area but the exhaust is usually getting raised 2-10 degrees so that added to blowdown would make most a turd.

 

[Backyard Lumberjack]

Mains open first directing charge across the piston towards the intake side....then boost port opens helping the charge up the backside of cyl towards the head also helping exhaust out....hence loop scavenged...

ah-ha! a wizard has spoken... and one can see the light now. well, some of it... good comment.

 

[Backyard Lumberjack]

Most new strato designs are using fresh air to purge spent gas from chamber before transfers recharge with fresh charge. The concept can be the same with booster ports at the expense of fresh charge out the exhaust. Delaying boost ports can aid in less out the exhaust and better cylinder fill. Better cylinder fill=more power. So the long and short of my interpretation, if auxiliary boost ports are found effective, what are you leaving on the table with factory transfers. Is it time area, opening points, roof angle, transjectory angle?

>Most new strato designs

I assume you mean stratified scavenging?...

 

[Backyard Lumberjack]

When i ported my 026 I removed a fair bit of material from the lowers, blending them into the cylinder with the intention of making it easier for the fresh charge to make its way into the transfer. It hauls some serious ass and can handle a lot of load in very hard wood. and like I said, it really hauls!

I think there is more than one way to skin a cat, At 15000 rpm, we are talking 0.004 of a second per engine cycle. That gives us 0.002 seconds on the downwards stroke to do everything we are talking about efficiently. Are we looking at this too closely? Great thread!!!!!!

>Are we looking at this too closely?

no, I do not think so. all this stuff is in part a compromise. so, to know what to trade off for something else, one needs to understand. and the more we can 'see' things, related to things... the better we can modify and yest still control the variables that must remain in line with each other... tweaked or not...

to me, its in the details is the same as looking closely, but never too closely! can one ever know too much about an engine? (generic statement)

I like seeing what u did and hearing about your results. for the most part, what you say you did is what I have done in my 044 'deflashing' efforts. enjoyed the pix. you always have lots of pix, and they always are great! thanks... I do like knowing your 026 can and will kick some tail... as 026s r near and dear to me... no doubt quite a saw! lots of power n umph! right from the git go, ie stock socks form!

so much to think about, so little time!

 

[Terry Syd]

After cutting the base and chamber for compression if you don't raise them back up you don't get the gains. I've never tried to widen them to get the time area but the exhaust is usually getting raised 2-10 degrees so that added to blowdown would make most a turd.

So you have dropped the exhaust port by cutting the base, great, that gives you more trapping efficiency and compression. Then why not take the approach of widening the transfers to pick up the required time/area on them to get them to flow. Admittedly, there may be problems with getting enough flow through the transfer tunnels to feed the ports, but it seems like widening should be the preferred approach. Then if the power can't be realised with widening, then as a last resort raise them.

Crikey, take a look at high performance two-strokes and see what extremes they go to (multiple transfer ports) to get the area for flow versus resorting to an increase in timing.

One of the great things about widening the ports is that it is a LINEAR relationship. If you widen the ports 10% then the time/area is also raised 10%. - and widening the transfers 10% on a saw wouldn't take that much work.