Part Three: The Transfer Ports

[telejed]

I could maybe see excessive blowdown causing a short circuit to the scavenge loop. Especially with an open muffler and wide exhaust. Can’t say I’ve ever seen it though.

How much does the flow rate out the exhaust change related to transfers opening?

But yeah, SHORT blowdown could overfill the chamber if the transfer duration is long. I don’t see a performance problem there, just fuel economy. I’ve always tried to fill the chamber with 100% fresh charge. I’d rather some fresh fuel go out the can than have spent exhaust remain in the chamber.

I don't know much when it comes to this stuff. . . I'm really just trying to get some "rules of thumb" to get started grinding on my own. However, the short blow down/long duration part. . . are you sure?

Here is another wrinkle in all of this. . .
The maximum charge volume you can get into the combustion chamber is limited to the volume change under the piston from TDC to BDC. The only time that I know that you can actually count on that volume is a saw with reed valves on the intake. Most saws that are in production or are relatively easy to get parts for and keep running don't have those (at least for what I know).

Hey, I'm just trying to figure out if I'm even playing the same game to be in the same ballpark about this stuff.

 

[telejed]

All I can say rather than argue about is there's more than one way to skin a cat .My references are from MacDizzy and A Graham Bell and a couple more .Then again I'm talking about a modified work saw that can run all day every day . Certainly not a racer that only runs full tilt about 4 seconds plus cost a couple of grand .Not my cup of tea so to speak .Those are way above my pay scale----but they are a hoot to watch .

I know that post I quoted was a while back, I'm not asking you to defend the idea. . . just what were you thinking about when you wrote it. Whether or not it is a correct idea, concept or theory is mostly irrelevant to me at this point in time.

 

[Al Smith]

Obviously you know more than I do so I will concede to your vast knowledge .Having said that ,here's the deal you do it your way and I'll do it . mine .Simple as that .

 

[Ketchup]

I don't know much when it comes to this stuff. . . I'm really just trying to get some "rules of thumb" to get started grinding on my own. However, the short blow down/long duration part. . . are you sure?

Here is another wrinkle in all of this. . .
The maximum charge volume you can get into the combustion chamber is limited to the volume change under the piston from TDC to BDC. The only time that I know that you can actually count on that volume is a saw with reed valves on the intake. Most saws that are in production or are relatively easy to get parts for and keep running don't have those (at least for what I know).

Hey, I'm just trying to figure out if I'm even playing the same game to be in the same ballpark about this stuff.

Nope, never sure. My thinking is that longer transfer duration is the most opportunity to max out flow into the chamber. Pair that with a limit to how much I can raise the exhaust and still have torque, and I usually end up with shorter blowdown and longer duration. Seems to work.
I usually have a wide exhaust, so I think that allows me to shorten the blowdown a bit without a lot of change. I think if I shortened the blowdown too much I would get exhaust flow into the transfers, but I haven’t seen that.

I guess I have some general rules, but it’s more stylistic than hard science. I think that’s kind of true for everyone. It’s a lot of trial and error.

As far as piston travel limiting transfer volume, it’s important to remember that pressure doesn’t begin to build until the exhaust port is close to closing. The volume of transfer the piston can move is greater than the volume above the exhaust port by a good bit. I don’t know diddly about valves, but it seems to me that a rotary exhaust valve would be used for trapping transfer volume.

 

[thedude74]

This is a fascinating topic. If there is voodoo to porting...surely it's in the transfers. Im no expert...not even close. It does appear blowdown numbers that make one saw run like a scalded dog will turn another saw into a turd, at least from what I've read from the many experienced builders here.

Seams likes it tough to have a hard rule on transfer duration. Lots of variables like transfer cross sectional area, crankcase volume, intake and exhaust height, width and duration not to mention carb and intake area, muffler exit opening area....and probably much more I'm not even aware of.

I do know one thing though...it's easier to grind a little more material off...than to try and to put it back on.

 

[Al Smith]

It all works together .The transfers not only transfer the fuel/air they also sweep the unburned previous charge from the cylinder . The most difficult is the area above the intake port opposite the exhaust port .If the exhaust has been widened or raised because the exhaust allows it to get out faster the transfer portion ,time wise will be longer and it will blow fuel .The only way to make power is to use fuel or waste it depending on how you look at it .Most people are not concerned about fuel ecomomy if or when they modify the engine .It's not rocket science .

 

[Al Smith]

Now then the worry about short circuiting the transfer .In a normal porting scheme becuse the transfers are aimed opposite the exhaust port plus some multi channel tunnels and open port producing slightly timed transfer that's already been taken care of or be slightly improved .McCulloch figured that out decades ago with some of the 10 series saws .Take a look at the little Stihl 020T or MS 200T .Those are huge compaired to the bore size and obviously work very well .Husqvarna addresses it with multi tunnels .Many ways to skin that cat and they all work .More modern saws use another method with a fresh air air type purge during the first portion of the transfer mainly to satisfy EPA regulations .Being an old school type I have not subscribed to that method and doubt I'll ever own a low emmisions type saw .It's a damned dinky little saw engine not a railroad locomotive slurping fuel at 200 gallons per hour under load .

 

[Ketchup]

Anyone want to weigh in on transfer tunnel volume? Should it stay below a certain percentage of total case volume? Does a long tunnel behave the same as a short tunnel of the same volume?

Just thinking that increasing case volume (or decreasing but that’s more obvious) could potentially improve transfer behaviors by changing the ration between transfer and total case volume.

I’ve been cutting a lot out of the way in my transfer lowers lately. Seems to be working. Could just be a flow thing, but it changes case volume a fair bit too. Opinions?

 

[smokey7]

I also port things looking at flow smoothness direction and shape. I will also remove some stuff in the way below the piston. I so it sparingly tho as I've had other built 2 and 3 cylinder 2 cycle engines with way too much cround out of the cases.

 

[NateSaw]

Transfers...... depends a lot on the saw. On duals I'm not gunna open the lowers at all. That lip you speak of on the bottom.

View attachment 68614


I like to fill it in and smooth out that transition instead of grind it off

I'm late to the game. @Deets066 ,ive been reading... I've heard from many your hybrid is in fact the strongest running.. It has been my slow realization that more often than not, taking less from transfers gains more for a work saw. I intend to fill the lowers on mine. The things I have been wanting to ask you, are, what's the best epoxy? And, how do you match the filled area to the bore plane? Honing? Three stone? Or are you filling so the epoxy does not reach the bore plane?

 

[NateSaw]

I'm late to the game. @Deets066 ,ive been reading... I've heard from many your hybrid is in fact the strongest running.. It has been my slow realization that more often than not, taking less from transfers gains more for a work saw. I intend to fill the lowers on mine. The things I have been wanting to ask you, are, what's the best epoxy? And, how do you match the filled area to the bore plane? Honing? Three stone? Or are you filling so the epoxy does not reach the bore plane?

Never mind. I'm all over this and the other parts threads. You've already answered in here

 

[MustangMike]

Going from memory, and what was done to a hybrid I have that runs very strong ... I believe Deets said he left the uppers stock and modified the bottoms.

I believe many of the builders open up the bottoms to give the saw more case capacity and take the peakiness away (gives them a broader torque curve so they don't die in the cut). (A 460 has more case capacity than a 440. If you don't open the lowers you can end up with a saw that screams RPMs, but won't hold them in the cut.)

I also believe Deet's saw was not really a "work saw" as it had a velocity stack and no air filter.

Just FYI.

 

[huskihl]

Sometimes certain saws just run. You can duplicate everything on his hybrid 100 times and may never get one that runs any different than any other. I’m not going to chase what has been done to his thinking mine will run like that.

 

[Mastermind]

Going from memory, and what was done to a hybrid I have that runs very strong ... I believe Deets said he left the uppers stock and modified the bottoms.

I believe many of the builders open up the bottoms to give the saw more case capacity and take the peskiness away (gives them a broader torque curve so they don't die in the cut). (A 460 has more case capacity than a 440. If you don't open the lowers you can end up with a saw that screams RPMs, but won't hold them in the cut.)

I also believe Deet's saw was not really a "work saw" as it had a velocity stack and no air filter.

Just FYI.

This is pretty much opposite of my thinking.

 

[wcorey]

I believe Deets said he left the uppers stock and modified the bottoms.

IIRC why he didn't alter the uppers was because the timing was good where they were so he didn't need to. One of those 'unicorn' cylinders or whatever...

I've mostly messed with lowers for direction as opposed to volume but there's something to be said there as to velocity also.
I've always been tempted to fill in the floor of the uppers to improve/smooth the flow path of the initial opening where most of the action likely takes place, particularly when they get raised by a lot.