Let's Talk Transfers

[Backyard Lumberjack]

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.

interesting post, to say the least! very interesting. I like knowing, and assume it's true since measured and I am firm believer one can believe all he reads on the internet... that mx flow from crankcase due to pressurization/compressed effect of piston coming down... happens just before BDC. at that point the bottom of the piston, at least on my 044, that is the lower area of casting encompassing the bottom ring land is just above the top edge of bottom transfer port. ie, a mill ridge. I had planned on some contouring there to augment piston window reshaping, but now I have a better idea of where the max flow out of crankcase is taking place and just how much the piston's postion in the bore affects that mill ridge height...

*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.

I would like to know more about the sweep volume to better understand it based upon your comment. I understand stroke, etc... and in general sweep volume... actually to include cam timing specs effects on it effectively in a V8... but with my 044 when I put a light in from the exhaust port, it lites up well the cylinder and the piston in it. when I look thru the plug hole I can see inside very well. when I place the piston so that the top ring would have just closed top transfer... the piston skirt has completely still blocked off the intake port. and the bottom of the piston skirt is only 1/16th or so up from flange of cyl base... and when I place the bottom ring land so as to have just sealed the upper cylinder... the upper transfer port is still cracked open, with an OE intake port would be even wider... and when the top ring is above the transfer port, the exhaust port is still open albeit closing... and even once the exh is closed the intake port is not open, although the rings are above top top of intake port and it still is a few more degrees rotation before the bottom of the piston skirt begins to open the intake port.

am I missing something here? seems to me the intake charge is not going to enter the engine until the intake port begins to open. and then as far as intake charge is concerned, all h*ll breaks loose and crankcase begins to fill due to atmospheric pressure augmented by piston signal strength as it rises.

I do not see how a 2-stroke begins to 'suck in mixture' the moment the transfer closes.... since when the transfers r closed the exh is open and the intake closed.

and I would like to fully understand it....especially if I am missing some important event or action.

 

[mdavlee]

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.

My thinking is a saw like a 372 runs best with 18-20 of blowdown and 158-162 of exhaust duration. You drop the base puts you down around 148-150 of exhaust. You will gain torque doing that but it won't be as snappy or as quick in small wood. I've tried 18-25 degrees of blowdown on 372xpw. The shorter blowdown ran the best all around. I've widened and raised the transfers in them since the mains are restricted compared to the 48 and 50mm bore. On those I just raise them and don't widen.

 

[Terry Syd]

I do not see how a 2-stroke begins to 'suck in mixture' the moment the transfer closes.... since when the transfers r closed the exh is open and the intake closed.

That's when the pressure in the crankcase starts dropping. With a reed valve or rotary valve engine, it can immediately start sucking mixture. However, with the piston port engine it can only start dropping pressure so that when the piston clears the port the engine can 'gulp' the mixture with the low pressure that has developed in the crankcase. The strength/amplitude of the 'gulp' will depend up upon the delay before the port is open. The bigger the 'gulp' the move mixture you move.

 

[Backyard Lumberjack]

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.

do u mean the lowers? the uppers? and on 2 port or 4 port are you talking about.

I have read with a keen interest... all of the posts in this thread. nearly every one, I had my 044 cyl in hand, piston I numerous locations and light in hand... seeing just what the many posters were saying in terms of the mechanical aspects of my 044 as a 2-stroke saw engine... seeing is believing ( I mean better understanding)

I like the term, don't remove what you don't have to. paraphrase it, don't remove what you are not sure about.

for my 2-cents worth... lol... I would say be careful with what you do. understand it thouroughly! and look at the piston windows and the transfer ports components as they interact. even at 6 psi when coaxed a lot of fuel moves fast... very fast. its all about cylinder filling. so make it easy to fill the cylinder. hunt down with a vengeance all the mill ridges, casting lines and flash, compare AM meteor to OE piston... and whittel away. it is safe territory, and easy to do, again, I like freebies. its a given: genuine freebies. gotta get the air/fuel mixture out of the crankcase... and into the transfers. there are stumbling blocks along the way, even though the path is 'very slippery' stock or AM. these mods can easily be done with you average $39.95 dremel kit. if you like porting threads then you will have read the AS thread on porting 101... and imo, the tool bit mentioned in there is ideal to do a lot... a lot of very effective contouring.



yes, indeed!

 

[drf256]

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.

What's your feeling on 2 ports Terry?

Widening them can cause one to catch a ring. It also can affect the direction of flow if done improperly.

 

[Backyard Lumberjack]

That's when the pressure in the crankcase starts dropping. With a reed valve or rotary valve engine, it can immediately start sucking mixture. However, with the piston port engine it can only start dropping pressure so that when the piston clears the port the engine can 'gulp' the mixture with the low pressure that has developed in the crankcase. The strength/amplitude of the 'gulp' will depend up upon the delay before the port is open. The bigger the 'gulp' the move mixture you move.

ok, thanks! I see your explanation... hadn't considered scavenging, vacating, venting... the crankcase part of the incoming intake charge. I would have thot more so preparing crankcase maybe for incoming charge. in any event, semantics perhaps. I would still take issue with the term: that is when the 2-stroke begins to 'suck in the mixture'. because the ports that let that happen are either open venting to atmosphere or closed to it... one thing I have recenlty learned learned about blowdown is that its job is to get the cyl venting the combustion by-products (exhaust) out exh port so as to ideally get cyl down to -0- psi above piston or even better slight negative... before transfers crack open. this then helps prepare crankcase for better filling. and in the case of transfer better cyl filling since crankcase pressures are fighting less upper pressures still in aboe piston once exh port opens...

thanks again...

 

[Terry Syd]

My thinking is a saw like a 372 runs best with 18-20 of blowdown and 158-162 of exhaust duration. You drop the base puts you down around 148-150 of exhaust. You will gain torque doing that but it won't be as snappy or as quick in small wood. I've tried 18-25 degrees of blowdown on 372xpw. The shorter blowdown ran the best all around. I've widened and raised the transfers in them since the mains are restricted compared to the 48 and 50mm bore. On those I just raise them and don't widen.

Disclaimer: I haven't done a bunch of porting on chainsaws, I just look at the principles of two-strokes and see what makes the most sense to me. Maybe your approach works the best.

I like the idea of dropping the blowdown figure. The larger blowdown numbers seem to be a left over from the days of restrictive mufflers. However, increasing the area of the transfers also appeals to me. It may be that an increase in transfer area could be better realised by using some finger or boost ports. Not only could the time/area be improved, but the flow in the cylinder could also be improved.

 

[mdavlee]

The longer blowdown works fine on a 7900. 372 or even a stihl with 2 transfers doesn't like it. On stihls I don't widen the transfers since they get raised .080-.125" on average.

 

[drf256]

I've been doing some research in Bell's book.

On transfer flow, he stated that the move in the 70's but the Jap manufacturers was a tighter and tighter crankcase with less intake duration. He said the engines made more power with lower transfers, but the engines got really peaky.

I've never run a Dolly. I imagine the tight case is what allows the smaller transfer duration these have to work.

Are Dolmars peaky?

 

[Terry Syd]

What's your feeling on 2 ports Terry?

Widening them can cause one to catch a ring. It also can affect the direction of flow if done improperly.

Yeah, the two-port is a problem. You can only go so wide before you run into problems with ring support. One of the great things about the stratos is the incredible 'gulp' factor you can get on the intake. The multiple ports of both the strato intake an carb intake can give you an effective width of more than 100% of the bore. Not only is there a great increase in width, but because of the multiple ports giving support to the piston or rings, the opening is IMMEDIATE. You don't have to have a slow taper on the ports, they can be flat and open quickly.

 

[drf256]

Yeah, the two-port is a problem. You can only go so wide before you run into problems with ring support. One of the great things about the stratos is the incredible 'gulp' factor you can get on the intake. The multiple ports of both the strato intake an carb intake can give you an effective width of more than 100% of the bore. Not only is there a great increase in width, but because of the multiple ports giving support to the piston or rings, the opening is IMMEDIATE. You don't have to have a slow taper on the ports, they can be flat and open quickly.

When one "guts" the stratos for perfomance gains, does one make them provide fuel/air mix or just change the amount of fresh air delivery they provide?

 

[Terry Syd]

On transfer flow, he stated that the move in the 70's but the Jap manufacturers was a tighter and tighter crankcase with less intake duration. He said the engines made more power with lower transfers, but the engines got really peaky.

Perhaps not "less intake duration", but less transfer duration. Yes, a higher crankcase compression will move the powerband up higher, but it will also NARROW the powerband. That's why saws generally have a lower crankcase compression - they need a broader powerband.

 

[mdavlee]

The 6400-7900 family isn't peaky. The 6400 runs strong for an open port cylinder.

 

[Backyard Lumberjack]

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.

I would like to know more about this. I understand the mods and effects. but more clarity could be helpful. I noted in some AS threads.... stepping on the squish via base gasket being thinner might raise exh port timeing 020 or so. wasn't mentioned in degrees. and poster, a porter... said that amount is not a problem. I can see where 2-degrees of change to blowdown should be ok... but 10-degrees? not sure how you would keep squish in 020 range and get 10 more degress to blowdown. as to running bad, I assume u mean top of transfer is opening 10 degrees later, too?

 

[drf256]

Understood.

But with a tight case, a lot of intake duration would cause a lot of spitback. You can't overstuff a case.

So a tight case has to have some intake duration limit above a larger case saw.

Am I off here?

 

[Terry Syd]

When one "guts" the stratos for perfomance gains, does one make them provide fuel/air mix or just change the amount of fresh air delivery they provide?

It is AIR that is hard to get into the cylinder. You can always tweak the carb to provide a bit more fuel, but getting that air in is the hard part - think turbos or superchargers.

My feeling is you want to maximise anything you can to get more AIR in the engine. Although they talk about 'stratified' mixture, when the loop-scavenging and squish band squishing gets finished, you pretty much have a homogenous mixture to burn.

 

[drf256]

The 6400-7900 family isn't peaky. The 6400 runs strong for an open port cylinder.

What's the difference in the timing on the 64cc vs the 79cc jug?

It's the same case, right?

 

[mdavlee]

I would like to know more about this. I understand the mods and effects. but more clarity could be helpful. I noted in some AS threads.... stepping on the squish via base gasket being thinner might raise exh port timeing 020 or so. wasn't mentioned in degrees. and poster, a porter... said that amount is not a problem. I can see where 2-degrees of change to blowdown should be ok... but 10-degrees? not sure how you would keep squish in 020 range and get 10 more degress to blowdown. as to running bad, I assume u mean top of transfer is opening 10 degrees later, too?

395 after machine work is around 110 degrees on exhaust opening. I run them at 100. When you raise exhaust you gain blowdown. Raise the transfers back you lose it.

 

[mdavlee]

What's the difference in the timing on the 64cc vs the 79cc jug?

It's the same case, right?

Never timed a 6400. Same case is correct.

 

[Terry Syd]

But with a tight case, a lot of intake duration would cause a lot of spitback. You can't overstuff a case.

So a tight case has to have some intake duration limit above a larger case saw.

Obviously there must be some limits, but I've found that with some carb tuning/modding there is a fair bit of leeway on making the engine run acceptably. If you can rework the carb (especially through the first 1/4 travel, then you can work around a bit too much intake duration. If it wants to cough it guts out on too much fuel, you lean it out more than usual.