Part Three: The Transfer Ports

[MustangMike]

Probably not as much as you think, as I don't think the piston will ever fully open it.

 

[farminkarman]

Does the size of the upper transfers matter, top to bottom? If I raise the upper transfers a lot and I end up with this huge hole, will it affect the way the saw runs?

I think it depends.... in some saws it probably matters, but who knows how much? Food for thought, think about an open transfer cylinder... I would venture to guess that a person could get undesirable results from raising a transfer too much and not taking care to make sure the roof doesn't angle upward too much.

 

[Ronie]

Probably not as much as you think, as I don't think the piston will ever fully open it.

I didn't think about that.

 

[Ronie]

I think it depends.... in some saws it probably matters, but who knows how much? Food for thought, think about an open transfer cylinder... I would venture to guess that a person could get undesirable results from raising a transfer too much and not taking care to make sure the roof doesn't angle upward too much.

It seems the the open transfer saws don't run as good as the closed ones.

 

[MustangMike]

I think open transfer generally has better low end, while closed generally has better top end.

However, some of my strongest running saws have bridge ports, which is a step toward open transfers.

Probably depends on the saw, and what other stuff has been done.

 

[AKJonsereds]

200t is open port I believe. Lots of top RPM. 2139t is closed port and has more torque. I think timing matters more. Port style has an effect on efficiency of flow, but isn’t the full answer. Builders can get good results from large transfers and small, depends on the reset of the build.

 

[Ketchup]

You add case volume when you grind transfers. It doesn’t seem to matter on most saws. I could see some issues if you raised the uppers 0.1” or more. There’s always the risk of grinding to daylight too.

TreeMonkey did a build thread for a 2153. He epoxied extra thickness to the outside of the cylinder and then raised the transfers a bunch. I think he added some epoxy to the case to compensate for the added case volume.

 

[Nutball]

The ports on the 9010 are designed to easily direct flow at a steep angle away from the exhaust port. How much is too much? Based on water flow tests the exhaust side of the flow wants to impact the cylinder anywhere from dead center to as far over as the opposite side of the intake port. The lines drawn are between those points.

Is this too much of a back angle? Is it better for a 52mm X 42mm cylinder? Will it reduce peakiness and broaden the power curve? I want it to run like a short bar worksaw, like a scaled up 590 or 7900, but it will wear a 29" .404 full skip.

 

[Nutball]

I'm liking this digital degree wheel. I can just pop it on whatever and get some numbers.



10cc rc helicopter glow engine, runs on 30% nitro, rated for 2.1HP @17,000
EX: 103
TR: 120
IN: opens 150BTDC, closes 61ATDC

It has a much larger carb bore than most similar sized glow engines, and an unusual intake timing that suggests most of the transfer happens soon after the ports open since the intake opens 30deg before the transfers shut. Maybe that is why some chainsaws get away with even 9 degree blow down: because there may be plenty of time for transfer to occur before they close. In this engine, which is designed to maintain roughly 17,000rpm and not so much at lower rpm, transfer gets 90 degrees to happen before the intake opens. However, by opening the intake early, the transfers can continue to flow with minimal back flow pull caused by the piston starting to move up. I wonder what port timing would be used if they could close the transfers whenever they wanted to? One has to keep in mind that if 30% nitromethane methanol fuel is the intended fuel, then the intake to exhaust volume ratio will be much different that with a gas engine, right?

 

[00wyk]

200t is open port I believe. Lots of top RPM. 2139t is closed port and has more torque. I think timing matters more. Port style has an effect on efficiency of flow, but isn’t the full answer. Builders can get good results from large transfers and small, depends on the reset of the build.

I think open transfer generally has better low end, while closed generally has better top end.

However, some of my strongest running saws have bridge ports, which is a step toward open transfers.

Probably depends on the saw, and what other stuff has been done.

It all depends on the exact type of cylinder and chassis. And even then there are often many ways to skin a saw as we all know now.

Here is an open port example. I spent a lot of time staring at 420 and 4300 Dolmar/Makitas when Andrew asked me to port one. When I first ran them, I thought they were a lot like Echos. All timing and no 'porting'. But then I opened it up and saw it was all ports all over the place. It seemed to me they made all these ports mostly to save rings than to improve performance since they weren't all that great at directing flow. At least the squish band looked nice.

I watched all the videos on the tubes by all the guys you all know and love. I watched them machine the poor things and bump the compression all to hell and stuff them full of epoxy, put huge exhaust ports on them etc. But one thing most of them had in common is that lil 420 wants to cut, but it doesn't like to rev. When I see this in an open saw, I always think scavenging.

So I added these to the last 420 and 4300's I have done:

It is only a tiny little nick in the edge of the top of the primary directing the flow into the corner of the cylinder. RC model porters often call this an eyebrow port(many RC airplane cylinders are open port, which is why I spent a lot of time on their forums when I got this thing in). Now, I also mod the entire transfer just a bit to help facilitate this flow, but I am careful not to hog anything out, and I only just match the bottom of the cylinder to the chassis. I do hog out the exhaust, but I shape it like a horn on this model there is so much meat on it. The cat is removed and the muffler has only two medium sized exit holes. I leave in the gasket as this model produces upwards of 180 psi stock, and once you add that eyebrow port, you want to have that exhaust mostly where it is with a gasket anyways. And you almost can not give it too much intake. A 6-8* timing adjustment tops her off. I get about a 100% increase in cut time over stock with this set up. It is, by far, the most insane amount of waking up I have ever seen a saw do in my life.

Now instead of struggling to climb to it's rev limiter, it eagerly bounces off of it. Here it is in an 80 year old seasoned piece of red oak:

And here is Andrew using it on a defenseless Sycamore(you should allow comments on that vid, Andrew!):

 

[NPKenny]

@00wyk

Would you please share a clearer picture of the eyebrow port? It seems you posted one from inside the cylinder a while back.

 

[Wonkydonkey]

What were the numbers on that cylinder with the eyebrow's @00wyk

it looks very similar to a 200t or the 018, I guess cos it’s got open transfers

 

[00wyk]

What were the numbers on that cylinder with the eyebrow's @00wyk

it looks very similar to a 200t or the 018, I guess cos it’s got open transfers

I'm sorry, I didn't write them down. Kinda due to simple reasons - the exhaust is basically not changed much, just a few degrees. It has the torque it does because that exhaust is so tame. You can add 3-5* easy, but I keep a good arch in the exhaust because this is a work saw, not a GTG saw. The transfers only gain mebbe 1-2*, and only due to me angling the L port/Eyebrow port to prevent catching a ring - nothing to do with performance intentionally. I mean, look at all these ports. I am surprised the BD is around 18* to start. The intake I widen quite a bit as the slug on these is wide and there's loads of support all around the cylinder. I had/have an extra experimental cylinder to test on that Andrew gave me. So I tried adding 3-5* of intake in steps and it took it all with the gasket in place. But there is a caveat. I do not just lower the intake floor, I grind the entrance like a horn/curve. So the air flows around it and dumps into the chassis. So I only modify it near the cylinder, and do not hog out the intake at all. I dunno if you can tell in the images above, but I also removed something like 20 grams from the flywheel - they are huge on these models.

This is why #'s don't tell the whole story. I put a lot of thought and love into these to get them to wake up like this. Most of the work is shaping, not timing. Lemme see if I can add a few more pics:

OK, bear in mind this is shot close up with a wide angle lens, so everything is rather exaggerated:

It looks like a lot, but it is a very tiny little nick in the edge of it. I would grind...take a photo, check, grind...take a photo, check it, it was so hard to see.
Also, that is the last photo I took of it. I was enlarging it slowly step by step and testing it. I found you only need maybe 70% of what you see there to make a big difference. Less is more here. The "+" is the ring end.

Lemme see if I can find one I did the transfer tunnel on...

Here we are. I just barely tunnel the bottom of the xfers for more flow towards the primaries(I guess it kinda has two primaries? *shrug*). I marked the amount of port I added, it is just a few mm's. On later attempts I went more squared than pointy. And you can see the intake is widened, but the exhaust port isn't crazy wide. I do not bother to unshroud the secondary entrance, and in fact barely touch the bottom at all.

This is what I mean by shaping the exhaust like a horn:

It's not a huge port, but the exit is flared like a horn speaker, so is very large when it dumps into the muffler. There was just so much material to work with, and it has such a big flange(she said), I had to try it.
I think that contributes a bit to it's exhaust sound:

That puppy there is idling at something like 2400 RPM, and loving it. Like a freaking 288XP.

 

[00wyk]

OK, just realized I might have been a bit evasive with my answer, especially since many of you own 420's/4300's.

These things run something like 112 131 72. It's meant to recirculate the hydrocarbons a bit before they hit the cat, so the xfers also aren't great.
Once you fiddle with that primary like I did, you will not need much blowdown, especially if you open up the intake and muffler. I mean, look at all these ports everywhere. Unless you wanna try for a GTG saw, I guess.
75 intake is easily doable even with widening. As is 108 exhaust. If you look at my saw above, you'll see it has a hell of an arched roof on the exhaust, I just followed the stock profile.
108 is still way up there anyways, and I want it to last more than win cookie fights.
Having said that, I think it works great at 109/110 once you give it the treatment on the exit and muffler end I did.
Remember once you modify that primary, instead of having 6 mediocre transfers, you now have 2 effective mains with 4 back ups, all of which are aimed towards the intake, with 2 also aimed towards the roof.
If you give it a few degrees more intake timing, its gonna blow well at high RPM. That chassis isn't what I would call small for a 42cc saw, so it appears to like intake.
I mean, 22* of blow down is rather generous on a saw with 6 working transfers...

Third one's aimed at the spark plug.. so yeah OK:

BTW, if you try this, you have to make sure you introduce a laminar flow so you aren't introducing turbulence that might effect your results. So don't use water-saving faucets etc.
Second one up front across the piston and a bit up. Yayyyy:

Butter knife for scale, and PB&J sammiches.
And this guy is hugging the front and corner with a bit of a piston kiss, too. This is all shape happening here with just a couple of added degrees at most.
Stock, it almost crashes into it's brother across the cylinder. Which I am sure creates lovely turbulence for mixing hydrocarbons and reburning them.
But that's not us. We want them outside the muffler. Well, not all of it...just a touch. Enough to make the EPA upset.

So when you do this, it's kinda like having two sets of primaries. Which is prolly why it acts like it.

 

[telejed]

Good subject with a lot of opinions .In my way of thinking the transfers can assume to be like HVAC ducting .Quads would seem to distribute the mixture more evenly .Possibly help sweep the area above the intake port better . McCulloch on old reeders as well as some Homelites used three port per side transfers .Some models of kart engines had two extra ports above the intake .
Blow down,the faster it goes the longer it takes open which is why I like a long blow down,some don't though .
My way,right or wrong is using a raised dome piston which doesn't do a bloody thing for compression but is to help sweep the chamber of spent exhaust gasses above the intake combined with sweep upper transfers as opposed finger ports .Those I always worried about snagging a ring .

Sectioned transfers quad or like the open type on a Stihl 200T do not have a great rush hitting the end of the tunnel because they are sectioned .At the end remember it has to change direction by about 90 degrees .The normal Stihl design is one large tunnel .Which although they work well IMO not as good as the Husqvarna design or the 200T for that matter .
It's all a matter of opinion ,what works--works .

Longer blow downs would seem to blow some unused fuel out the exhaust port .If it were an over bored and stroked 454 Chevy I'd be concerned about but not on a chainsaw engine .

"Longer blow downs would seem to blow some unused fuel out the exhaust port .If it were an over bored and stroked 454 Chevy I'd be concerned about but not on a chainsaw engine ."

Care to expound?

Maybe I already understand what you are saying about blowing out unburnt charge out the exhaust port. I'll go through it as it is in my head:

In a 4 stroke engine Suck, Squeeze, Bam, Blow are all discrete (don't have a better word than "they all happen separately"). In 2 stroke engines, they all "shake hands with each other." (It's a wonder the damned things run at all).

In a 2 stroke, the exhaust gasses trapped in the muffler act as a "valve" to keep the charge in the combustion chamber until the exhaust port closes.

Are you saying you think that charge will escape because of the longer time it takes to close the exhaust port, and therefore the exhaust gas has left the muffler allowing charge to enter the muffler?

 

[huskihl]

"Longer blow downs would seem to blow some unused fuel out the exhaust port .If it were an over bored and stroked 454 Chevy I'd be concerned about but not on a chainsaw engine ."

Care to expound?

Maybe I already understand what you are saying about blowing out unburnt charge out the exhaust port. I'll go through it as it is in my head:

In a 4 stroke engine Suck, Squeeze, Bam, Blow are all discrete (don't have a better word than "they all happen separately"). In 2 stroke engines, they all "shake hands with each other." (It's a wonder the damned things run at all).

In a 2 stroke, the exhaust gasses trapped in the muffler act as a "valve" to keep the charge in the combustion chamber until the exhaust port closes.

Are you saying you think that charge will escape because of the longer time it takes to close the exhaust port, and therefore the exhaust gas has left the muffler allowing charge to enter the muffler?

I think he misspoke. The opposite would be true.

 

[Ketchup]

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.

 

[srcarr52]

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.

There is no one size fits all here. What seems long at 160 ex duration would probably be ideal at 175 ex duration.

 

[Ketchup]

There is no one size fits all here. What seems long at 160 ex duration would probably be ideal at 175 ex duration

I’m not sure what you’re saying.

 

[Al Smith]

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 .