Chainsaw Repair
Husqvarna - Stihl - Poulan - Jonsered - Dolmar chainsaws and more => Modified Ported Work or Race saws => Topic started by: Chris-PA on November 19, 2015, 08:58:52 pm
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This is a continuation of a build of a 42cc Poulan clamshell that I documented over at AS ( http://www.arboristsite.com/community/threads/reducing-squish-on-a-poulan-clamshell.273087/ ).
The cylinder was cut down by 0.040" and the crank/bearing pockets relocated, and I cut a pop-up on the piston. The exhaust and intake ports are modified, with the the exhaust at 58% of the bore and the intake at 56%. The port timing is at (duration):
E = 150
I = 165
T= 106
However, that was not what I found interesting. I have wondered about the design of this engine since I first opened one up, specifically the very wide, flat crankshaft counterweights. They are much wider than they need to be, and there is no attempt to round them off like I would have expected. It always seemed intentional but I didn't know why. After making a cut away engine out of a scrap cylinder, it occurred to me that the counterweights should push a high pressure wave in front of them. Given the clamshell design, the transfers extend all the way down to the bearings and have a hook shaped profile, which I figure should partially catch some of this pressure as the transfers are opening. So I decided to try to enhance that effect.
I designed and 3D printed some inserts for the lower cap to more closely match the shape of the counterweights. I had to modify the shape of the crank to remove the square shoulders at the pin. The intent was to try to block some of the air pushed by the leading edge of the counterweights, creating a high pressure at the base of the transfers as they open. Basically, an inefficient and weak case blower helping to force air up the transfers. The thin con rod and narrow slot help some too.
Whether all of this actually works is hard to know for sure, but the saw ran like crazy. It fast became my favorite, easily pulling a 19" bar with lo pro in anything. However, the inserts failed twice, and clearly the 3D printed PETG material just isn't appropriate for the temperatures in an engine. The last attempt held out for maybe 8 tanks, including running the heck out of it on 90deg+ days, but I think the heat soak killed it. The attached picture shows the remaining intact insert after the last failure - it shows that the leading edge was pushed back and flattened out. There is no sign of mechanical impact, rather that there was considerable air pressure pushing on that edge. The trailing edge is fine.
So that is where the project has been at for a bit while I decided what to do.
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I decided that the only appropriate material I have ready access to is JB Weld, but forming it was a problem. I wanted it to fit decently close to the crank all the way around the bottom cap. Then I thought that while the 3D printed parts might not hold up in the engine, they should work as a mold/form for the epoxy. I also decided to use the same cap I had modified for the first attempts, as it had pins inserted that would help hold the inserts. I also cut some brass window screen strips to embed into the epoxy. I did one side at a time, after smearing the form and other parts of the cap with silicone grease as a mold release - which basically didn't work at all, so the form got destroyed each time. After googling it I found that car wax is better.
I had a lot of excess to clean/grind/chip off.
Then I decided that the inserts really should extend further up toward the transfers in the cylinder, so I made another form to add an insert there. The car wax worked better on that one. I also did a little grinding to open the access from the barrier to the lower transfers.
So that's where the project is at the moment - the inserts are made and the engine is cleaned up. I just have to find the time to put it back together.
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That is some very nice work Chris. I am curious as to how the JB Weld will hold up.
On the Poulan Clamshell I did I added some finger ports at an angle going into the transfer ports. I pointed them in such a way that when the crank was turning it would push the fuel mixture into those ports. I have no way of knowing what affect it had because I did all my mods at one time. But I am of the same mind as you that the fuel mixture must be turning in the same direction as the crank.
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I forgot to mention it, but that picture I posted of the two crankshafts above are a 42cc Poulan and a 46cc Poulan like the one you did. I've no doubt there is a high pressure pushed in front of all the crank parts, but the difference in width and the shape of the edges is very pronounced.
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excellent job Chriss did you ever toy with the idea of a full circle crank?
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excellent job Chriss did you ever toy with the idea of a full circle crank?
Yes, but that would defeat the effect I'm trying to enhance here. It needs the flat edge at the front of the counterweight which would not exist with traditional crank stuffers.
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Chris, nice work with the stuffers. I think the low speed (fps) of the outer portion of these small diameter crank bells limits the ability to create much pressure. There a lot of info on the sled forums about this, turbo vane cranks, etc. One study suggested a theoretical 2% pressure rise from a 5"diameter Crank bell at 8500rpm
take into account losses and you may be back to ground zero?
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Iv'e used JB weld years ago to reshape intake ports in motorcycle heads , it has the same expansion rate as aluminum but surface preparation for it to adhere is the key or weld in wedges to anchor the epoxy to keep it place . Hopefully you hard work pays off.
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Chris, nice work with the stuffers. I think the low speed (fps) of the outer portion of these small diameter crank bells limits the ability to create much pressure. There a lot of info on the sled forums about this, turbo vane cranks, etc. One study suggested a theoretical 2% pressure rise from a 5"diameter Crank bell at 8500rpm
take into account losses and you may be back to ground zero?
Thank you for giving it a name and some direction I can start to search under - I figured it must be a known thing but had not found any references. I agree the small diameter is a limitation, but I'm really only hoping for a modest effect. Do you know if anyone tried reducing the clearance to the crank weights?
Also I know there isn't any practical way for me to measure if it's effective either - at least nothing I'd take the time to do. I do know the saw ran very well, and I would not have taken it apart again if the previous parts hadn't failed.
Iv'e used JB weld years ago to reshape intake ports in motorcycle heads , it has the same expansion rate as aluminum but surface preparation for it to adhere is the key or weld in wedges to anchor the epoxy to keep it place . Hopefully you hard work pays off.
I scarred the surface with a Dremel grinder bit and sandpaper, and cleaned it with brake clean. I'm not too worried about the fillers in the bottom cap, as they have pins and some mechanical restraint in addition to the epoxy adhesion.
The ones up in the cylinder are the bigger risk, as the jug is plated and that is a harder, smoother surface even down at the lower edge, plus there isn't much mechanical support. I almost decided to skip those, but curiosity won out. Still, I did prep it as best I could.
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I found some interesting stuff on turbo vane cranks, including the attached pictures. They all are similar in concept but seem to work just a little differently than what I'm trying here - I'm trying to push the pressure wave on the front edge of the counterweight into a reduced volume cavity in the case cap, hoping to create a higher pressure zone right as the transfer open. Then (hopefully) a lower pressure zone at the back edge of the spinning weights will be in position as the intake opens.
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Perhaps a better designed vane system WITH a diffuser would have more effect, but would be a nightmare to build
I think you gains are a result of less crankcase volume. Maybe take it farther and see how it runs...
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Those were interesting pics Chris.
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Perhaps a better designed vane system WITH a diffuser would have more effect, but would be a nightmare to build
I think you gains are a result of less crankcase volume. Maybe take it farther and see how it runs...
That's possible, but the volume of these fillers is not very large - especially compared to what a traditional crank stuffer would be. Since I did them at the same time I raised the compression, it may be they do nothing and I'm just seeing the compression increase. Still, it ran pretty good and I'm going to experiment further.
I tried putting it together yesterday but it needs more clearance to the fillers I added in the cylinder, so I had to take it apart again.
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Well it's back together and running nicely! I had just enough time to buck a couple pieces from a standing dead ash I dropped last weekend. It will take a little more time to form an opinion as to whether it is improved from before, but it sure hold revs well. It's certainly no worse and I was happy with it before, so hopefully it will hold together this time.
I replaced the fuel line while I was waiting for the sealant to set, and I installed 5mm helicoils on the recoil and coil mounting screws - this plastic case is 21 years old and has been taken apart so many times while I experimented that several of those were stripping out.
I'm hoping to get a video over the coming holiday break so I can see what rpm it is holding under load.
Attached are a couple of shots, one showing how I cut apart an old clamshell cap so I could clamp the crank down fully and be able to see in. The other shows how the cylinder fillers fit to the crank. It is really the passage formed by outer bevel of the crank and the cylinder cavity radius that I'm trying to block off
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Wow :o nice 8) R&D 8)
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Nice work.
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Nice work .
I have toyed with the concept of vain crankshaft counter weights, full circle .Thinking out loud I wonder if by some method the internal shape might be made such as a flexable vain pump like either an air compressor or hydraulic pump uses .
In my mind that idea might work on a rotary valve engine but by the same token probably would not on either a piston port or reed valve design .It would seem to me using either design the internal crankcase pressure would merely short circuit during the transfer portion of the cycle .That thing would definitely move air though .
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More .I don't claim to be a wizard on port designs but one thing caught my eye .On I think the 281 and 2100 Husqvarnas' the intake port to me is oddly shaped .It's never been mentioned but to me I think the design might have been used to take the rotation of the counter weights to boost the crankcase pressures .Acting in a way kind of like a pump of sorts .
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Nice work .
I have toyed with the concept of vain crankshaft counter weights, full circle .Thinking out loud I wonder if by some method the internal shape might be made such as a flexable vain pump like either an air compressor or hydraulic pump uses .
In my mind that idea might work on a rotary valve engine but by the same token probably would not on either a piston port or reed valve design .It would seem to me using either design the internal crankcase pressure would merely short circuit during the transfer portion of the cycle .That thing would definitely move air though .
Those vane type air pumps were very much on my mind, and I'm sure something could be designed with various types of pumps incorporated into the case/crank - but not unless starting from scratch. The offset shaft of those vane air pumps creates a positive displacement pump as the volume of the chambers changes throughout the rotation, with a port at each side. That is not all that different from what is happening here as the intake and transfers open and close.
Here the slot for the con rod will always be a giant leak. Still, I think it is possible that at 10,000rpm once can make use of the the high pressure area that must exist at the front edge of those counterweights. So I am trying to enhance it by having the opening area constrict to just clear the spinning weights as the transfers open.
Naturally it will be bypasses heavily by the con rod slot, but if any pressure increase can be created the flow will be improved.
More .I don't claim to be a wizard on port designs but one thing caught my eye .On I think the 281 and 2100 Husqvarnas' the intake port to me is oddly shaped .It's never been mentioned but to me I think the design might have been used to take the rotation of the counter weights to boost the crankcase pressures .Acting in a way kind of like a pump of sorts .
The shape of these transfers has a pronounced hook that extends all the way down to the bearing and sits just off the spinning crank.
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Finally got to make a couple of cuts. This is ash, and it's turning around 10,500rpm under load.
https://youtu.be/2ETNfsNAD9w
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That is not bad. 10,500 in the cut is pretty good. What is the compression on it now.
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That is not bad. 10,500 in the cut is pretty good. What is the compression on it now.
Thanks! I felt like it was turning a bit faster later on but was too lazy to set up the camera for another video to check. The wood was reasonably hard stuff.
I actually don't have a compression gauge, as I don't really think that low speed cylinder pressure readings mean much of anything other than to tell you if it's shot (which is usually obvious anyway). You just can't infer much about the cylinder pressure at 10,000rpm by taking a pressure reading at 100rpm.
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What I liked was that it hold the rpm well under load. The images are audio spectrum plots of the entire 1st and 2nd cuts. The lowest spike is quite narrow, showing that the rpm did not vary much during the cut.
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I would have to disagree with you on the compression testing.
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I would have to disagree with you on the compression testing.
That's OK - I'm sure most everyone else does too!
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I would have to disagree with you on the compression testing.
That's OK - I'm sure most everyone else does too!
We both like to modify our saws and I like to know how those modifications affect the final out come in specific ways. I do not use a compression gage to tell me how much torque I am getting, but if I raise the exhaust port I would like to know how much compression difference there is. Just an example of not wasting time.
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I would have to disagree with you on the compression testing.
That's OK - I'm sure most everyone else does too!
We both like to modify our saws and I like to know how those modifications affect the final out come in specific ways. I do not use a compression gage to tell me how much torque I am getting, but if I raise the exhaust port I would like to know how much compression difference there is. Just an example of not wasting time.
That's a good illustration of why I made the claim I did though - I have several saws that have a notch in the top of the exhaust port. It bleeds off pressure at low starting rpm, but doesn't really do anything at operating rpm. The time and pressure at operating speed are too different from what you see on a gauge.
There's no doubt that raising the top edge of the exhaust port must reduce cylinder pressure and shorten the power stroke (at least if it's more than a notch), but you can't measure what the effect is at operating speed with a typical compression tester. So for example a 10% change at low speed can't be assumed to make a 10% difference when it's running.
Without dynos it's tough to quantify the results of our modifications - we're left with cut times, rpm under load and "feel".
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That is true.