Reducing Squish on a Poulan Clamshell

[Hedgerow]

This is an earlier one I did, but transfers are the same:
View attachment 404279
They did change the shape in later years.



There should be 0.018" to 0.020" clearance to the filler.

I think there should be a pressure wave in front of the big, flat counterweights of this saw, which was the subject of this thread: http://www.arboristsite.com/community/threads/clamshells-transfer-shape.232407/page-2#post-5194065

It is a secondary reason I'm filling in that gap at the bottom of the engine, to help build pressure before the transfers open. Of course I cannot get rid of the big slot in the middle where the rod goes - though I have tried to figure out what I could do there too!

The squish improvement will help your cause, but you'll have to deal with your transfers .. They're simple, but not efficient... Of course that may not be the objective here.. But you should raise them the same amount you dropped the jug.

 

[Chris-PA]

Just remember to cut it in the lathe and don't file or sand it axially.
Don't want the really badly oriented stress lines that would create.

I have to measure again more carefully, but something like that. The OD at the crank pin boss is smaller than at the counterweights. One benefit of the wide weights is that the pin boss is considerably wider than on other cranks

I'll take your advice on not grinding - I had not thought about that aspect.

The squish improvement will help your cause, but you'll have to deal with your transfers .. They're simple, but not efficient... Of course that may not be the objective here.. But you should raise them the same amount you dropped the jug.

Yeah, I know, and I've not really been able to do an acceptable job on raising transfers before, but I'm going to have to deal with that. I'll have to come up with something. I have a bunch of scrap cylinders to practice on.

 

[Hedgerow]

1.2 to 1

And there you have it...

 

[Chris-PA]

1.2 to 1

Wow, that's tight! Considering it has to be at least 1:1 that gives you only 20% of the displacement for case volume.

 

[angelo c]

Just loggin in to follow along with my "buddy from up the hill"

 

[tree monkey]

120% of displacement

 

[Hedgerow]

Wow, that's tight! Considering it has to be at least 1:1 that gives you only 20% of the displacement for case volume.

But you'll have to ask him what the desired outcome of that ratio is...



Different strokes for different folks...

 

[Hedgerow]

120% of displacement

Is that for max HP? Rpm's be damned??
And why am I thinking that's not all that tight??

 

[Hedgerow]

transfer rate. Higher ratios will move more fuel/air

In most cases, yes..
Unless you lose your velocity..

 

[dirt_19]

looking forward to reading any further progress . Subscribed

 

[Chris-PA]

120% of displacement

I'm confused as to what that means - the case should be 120% of displacement or 20%?

This is 42cc and the piston has a simple flat lower edge. That edge sweeps 42cc in the case. There is volume under the piston that is considerably more than 20% of the swept volume, so just the volume under the piston crown at TDC to the lower edge of the skirt at BDC is way more than 120%

On this saw the volume of the crank/counterweight cavity plus the transfers is something like 2.5 X the displacement. If I add the swept volume plus the volume under the piston it's around 4 X the 42cc. Then again it has enormously thick flywheel weights.

My little cap fillers will reduce the total case volume by less than 2%. It would require crank stuffers to make any significant difference. But I'll probably try it anyway.

 

[LegDeLimber]

Actually you should be able to remove the stock in about any fashion that's handy.
Just remember to polish things out to remove any valleys or grooves that run axially
or even on a helix.
Keep everything running radially for the final finish work.
Believe I'd take a good close look a the holes in the outer walls of the crank weights too.
Just in case there are any voids, pitting or manufacturing dings from ham-handed workers.

 

[Hedgerow]

The more

huh? If the case compression is increased how can it decrease velocity? Oh never mind...[/QU

If the time/area of the upper transfers is increased and you have less degrees of crank case compression from dropping your intake down, you could end up with a net loss in velocity..
Though I doubt that will be a problem with this saw in question..

 

[Moparmyway]

It would require crank stuffers to make any significant difference.

Print them up and JB them in.
It has to be lighter than making a full circle crank out of pure JB

 

[kz1000]

Why not just buy a real saw?

That was an attempt at humor that failed Chris, I'm always in the back ground watching what you achieve out of these critters. Sorry

 

[Chris-PA]

Print them up and JB them in.
It has to be lighter than making a full circle crank out of pure JB

I have been going over and over that! I think you would need to turn the OD of the crank counterweight/pin boss to allow a retaining ring, or a notch for a wire, or something. The stuffer would likely have to be in 4 parts as I'm not sure there is enough clearance to go around the pin boss, and eve if there were it would be really thin. At first I thought it was doable, but now I don't think it would be so easy.

Besides, I still think the wide crank weights were intentional and they were trying for some sort of pressure wave effect at the transfers - they make other cranks that are much narrower. The whole saw design was effected by how wide the crank/engine is. Anyway, I'm still exploring that and don't want to defeat it by filling out the crank weights.

That was an attempt at humor that failed Chris, I'm always in the back ground watching what you achieve out of these critters. Sorry

And I darn well should have known that given your comment history. Sorry for being too touchy!

 

[HuskStihl]

Touchy bastard. So according to TM a 42cc (it's funny to even type a number that small in front of "cc") saw should have a crankcase volume between 42 and 50cc for maximum power. Smaller volume will increase compression and speed they the transfers, but at some point, it will blow the unburned charge straight out the exhaust port

 

[weimedog]

First, I really appreciate the fact you are willing to push boundaries to make things happen...
Second, I think you with an obvious engineering background should continue to follow your instincts as you have despite what you may see in the commentary.
Three...an interesting read relative to case volume & intake timing:
(Note some applies and some doesn't but what it does do is help with defining directions to explore.)
http://www.bridgestonemotorcycle.com/documents/crankcase_volume6.pdf

 

[weimedog]

Touchy bastard. So according to TM a 42cc (it's funny to even type a number that small in front of "cc") saw should have a crankcase volume between 42 and 50cc for maximum power. Smaller volume will increase compression and speed they the transfers, but at some point, it will blow the unburned charge straight out the exhaust port

All fun aside, How do you calculate or measure that volume? If the piston volume and transfer volume is included, that's a very small area. Are you just considering the volume of the case itself? Or the entire system when the piston is at BDC? Seems to me there are practical limits based on crank, case, transfer port, and piston design. And in those less than optimal designs, intake size & timing along with Transfer size & timing can help mitigate issues. Would rather see this as a baseline all things perfect and then push the other numbers as a discussion when they can't be. I guess a random number doesn't compute for me. A number in context with the other "numbers" defining a system does.

( Now if I had a "Solid Modeling Based Cad System... that calculation might be a bit easier.. Even that would be "plus or minus" some based on the accuracy of the models and the way that system does Booleans & calculates volume)

 

[Grande Dog]

Howdy,
Good read. I like the stuffers. Depending on how well it scavenges, you might want to put a rougher surface on them. Chris, it looks like you don't have any lack of tools. If you have the tools to do it, a cam ground crank pin will do the trick for adjusting squish on a clamshell.
Regards
Gregg