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#Post#: 65171--------------------------------------------------
42cc Poulan Clamshell Case Inserts
By: Chris-PA Date: November 19, 2015, 7:58 pm
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This is a continuation of a build of a 42cc Poulan clamshell
that I documented over at AS (
HTML http://www.arboristsite.com/community/threads/reducing-squish-on-a-poulan-clamshell.273087/<br
/>).
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.
#Post#: 65175--------------------------------------------------
Re: 42cc poulan Clamshell Case Inserts
By: Chris-PA Date: November 19, 2015, 8:35 pm
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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.
#Post#: 65199--------------------------------------------------
Re: 42cc Poulan Clamshell Case Inserts
By: 3000 FPS Date: November 20, 2015, 12:57 pm
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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.
#Post#: 65200--------------------------------------------------
Re: 42cc Poulan Clamshell Case Inserts
By: Chris-PA Date: November 20, 2015, 2:07 pm
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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.
#Post#: 65201--------------------------------------------------
Re: 42cc Poulan Clamshell Case Inserts
By: David Young Date: November 20, 2015, 3:07 pm
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excellent job Chriss did you ever toy with the idea of a full
circle crank?
#Post#: 65204--------------------------------------------------
Re: 42cc Poulan Clamshell Case Inserts
By: Chris-PA Date: November 20, 2015, 3:21 pm
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[quote author=David Young link=topic=5393.msg65201#msg65201
date=1448053676]
excellent job Chriss did you ever toy with the idea of a full
circle crank?
[/quote]
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.
#Post#: 65205--------------------------------------------------
Re: 42cc Poulan Clamshell Case Inserts
By: aclarke Date: November 20, 2015, 3:56 pm
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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?
#Post#: 65223--------------------------------------------------
Re: 42cc Poulan Clamshell Case Inserts
By: The Ripper Date: November 20, 2015, 7:07 pm
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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.
#Post#: 65228--------------------------------------------------
Re: 42cc Poulan Clamshell Case Inserts
By: Chris-PA Date: November 20, 2015, 7:36 pm
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[quote author=aclarke link=topic=5393.msg65205#msg65205
date=1448056611]
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?
[/quote]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.
[quote author=The Ripper link=topic=5393.msg65223#msg65223
date=1448068071]
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.
[/quote]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.
#Post#: 65236--------------------------------------------------
Re: 42cc Poulan Clamshell Case Inserts
By: Chris-PA Date: November 20, 2015, 9:26 pm
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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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