Clamshells & Transfer Shape

Chris-PA · Mar 23, 2013

Since my saws are all cheap homeowner stuff, most of what I have worked on are Poulan clamshells. I've gotten used to how they work and the shape of the transfers. Recently I've gotten to do some work on a mag cased pro saw with a removable cylinder and wondered about the lower transfer area. LOL - not really, it's my Earthquake clone of a RedMax G3800, but the rest is real.

What I have noticed on the Poulans is that the (open) transfer runner extends down until it is sealed off by the rubber cap of the bearing/seal. That means the outer edge of the crank counterweight spins right past the transfer, and the lower runner appears to be shaped to help capture that. But with the split case/removable cylinder the lower end of the transfer is well above the crank counterweights and separated from them.

When I first read of crank "stuffers" I thought maybe it was something like this, but I then learned it was not. Does anyone know what effect the spinning counterweights actually have? Do they help blow the mix up the transfers?

Giles · Mar 23, 2013

"Does anyone know what effect the spinning counterweights actually have? "
They balance the engine--counteract the weight of the piston. All engines that I have worked on, have counterweights, both Two and Four stroke.

Chris-PA · Mar 23, 2013

Well thank you, but I kinda knew that part.

What I should have asked was: Does anyone know what effect the spinning counterweights actually have in regard to forcing mix up the transfers?

brokenbudget · Mar 23, 2013

WoodHeatWarrior said:
Well thank you, but I kinda knew that part.

What I should have asked was: Does anyone know what effect the spinning counterweights actually have in regard to forcing mix up the transfers?

all i'm going to say is only open them up only a little bit. also, i have tried grinding a small 'scoop' on the outside outer edge to feed the transfers, but that saw so far is still in a box waiting to be unpacked.
the airfilter is a pretty big power robber also.
i'm pretty sure this one is a 38cc, so said the epa tag.

Chris-PA · Mar 23, 2013

I was just looking at a 42cc Poulan again out in the barn. The counterweight is wide and has a large flat front edge, which sweeps right past the entrance of the transfer runner. They have some features there that just happen to be placed right outside the diameter of the counterweight. While the piston is providing the pressure to drive the mix into the cylinder, I have to believe the counterweights drive a pressure wave in front of them.

Chris-PA · Mar 23, 2013

brokenbudget said:
all i'm going to say is only open them up only a little bit. also, i have tried grinding a small 'scoop' on the outside outer edge to feed the transfers, but that saw so far is still in a box waiting to be unpacked.
the airfilter is a pretty big power robber also.
i'm pretty sure this one is a 38cc, so said the epa tag.

So does that mean this is a known effect? I could not figure out what terms to search on.

brokenbudget · Mar 23, 2013

WoodHeatWarrior said:
So does that mean this is a known effect? I could not figure out what terms to search on.

no idea:msp_tongue:just something i was going to try out one night. however my house ended up selling faster than i thought it would and the saw was boxed up for the move. it's sitting out back in a shed right now under a bunch of other stuff.
i got the idea from the little nitro engines from my rc days. the end of them little cranks had a little curved scoop machined into the end to help feed the ports. all the higher end engines had them, but i'm sure all them little nitro engins have them now.

Chris-PA · Mar 23, 2013

I'm glad I'm not the only one who saw it then. I dunno, but it's pretty obvious they've shaped the transfers for it. As the piston comes down it's squeezing the air right into the area where the counterweights are approaching, and the transfer runners are right there shaped to catch it.

Chris-PA · Mar 25, 2013

So to look at this further, I decided to make myself a new toy! In this image, the crankshaft is rotaing counterclockwise, the piston is coming down, and the counterweights are just about to cross into the transfers:

Notice how wide and flat the front of the counterweights are, and the rod is narrow as is the slot for it. The crank throw is also wide and flat. The volume inside the case is split into two separate regions by the counterweight assembly, with only the rod slot and the gaps around the outside:

Here is the situation just before the intake opens up - the piston is going up and the counterweights are moving away from the piston:

I have to believe that at 8000 rpm that spinning crank is helping to blow charge up the transfers as well as to pull mix into the case.

Also, these engines have a lot of squish, which is something I have expected. My suspicion is that they intentionally kept the compression down for easier starting by the inexperienced. I think it is why I just cannot get them to run like the other Poulan design used in my 2775 (and similar to the Husky 142).

half_full · Mar 28, 2013

I have a few of the 42cc clamshells that are trashed. And I happen to have a metal cutting band saw. This would make a great educational aide when explaining to others motor function. It would also make learning to use my new degree wheel much easier.

srcarr52 · Mar 28, 2013

Look at the crank position when the transfers are just opening. The crankshaft counter weight will be blocking the lower portion of the transfers. Really that lower portion is just dead space from when the cylinder was die cast.

Chris-PA · Mar 28, 2013

half_full said:
I have a few of the 42cc clamshells that are trashed. And I happen to have a metal cutting band saw. This would make a great educational aide when explaining to others motor function. It would also make learning to use my new degree wheel much easier.

LOL - it's a great toy! It only took a couple of minutes to make with a band saw and a belt sander. And it really is useful for understanding the operation.

srcarr52 said:
Look at the crank position when the transfers are just opening. The crankshaft counter weight will be blocking the lower portion of the transfers. Really that lower portion is just dead space from when the cylinder was die cast.

Yes, that is true from a static position point of view. But from a dynamic viewpoint there is only a short amount of time between the position below to when the transfers open, and I expect the transfer runners may still be pressurized by the passing of the front edge of the counterweight when they open. The designers would have considerable leeway in choosing the width and the thickness of the counterweights - these are much thicker than the weights in my 46cc Poulan engine.

half_full · Mar 28, 2013

WoodHeatWarrior said:
LOL - it's a great toy! It only took a couple of minutes to make with a band saw and a belt sander. And it really is useful for understanding the operation.

Yes, that is true from a static position point of view. But from a dynamic viewpoint there is only a short amount of time between the position below to when the transfers open, and I expect the transfer runners may still be pressurized by the passing of the front edge of the counterweight when they open. The designers would have considerable leeway in choosing the width and the thickness of the counterweights - these are much thicker than the weights in my 46cc Poulan engine.

I hope you realize that without a piston ring on that, you'll never get decent compression :msp_tongue:

Chris-PA · Mar 28, 2013

half_full said:
I hope you realize that without a piston ring on that, you'll never get decent compression :msp_tongue:

I figured the compression would be cut in half.....

ttyR2 · Mar 28, 2013

Now, take the half you cut off and quarter it vertically. Then you can set a quarter back on and see the transfer port from the side.

Chris-PA · Mar 28, 2013

ttyR2 said:
Now, take the half you cut off and quarter it vertically. Then you can set a quarter back on and see the transfer port from the side.

Hmm - I may have to look into that.

Chris-PA · Feb 5, 2015

Given some of the recent discussions about engine design I wanted to revive this old thread and see if there were any more thoughts about this - I replaced the pictures too. Basically my contention is that the flat front edge of the spinning counterweights is intended to push a high pressure wave into the transfers and aid flow. Also that the flat trailing edge of the counterweights creates a low pressure as it passes the cylinder and the intake is opening.

It's hard to imagine any other reason that the crankshaft is made this way, as it added considerably to the width of the engine.

Moparmyway · Feb 5, 2015

One way to tell if your theory is correct would be to grind away at the leading and trailing edges, then drill the crank, and fill with Mallory metal to rebalance the assembly. You could also thin out the counterweights, then rebalance using the Mallory metal as well. Anything you do to the crank will also have an effect on case volume .............

My main thought is that of the pressure wave you suggested might be happening .......... at higher RPM's the charge thats trapped within and around the crank would probably tend to stay closer to the crank .......... I have seen videos of high RPM crankshafts, and the amount of oil that stays trapped between the counterweights and the rod end of the crank is significant ............ hence the use of crank scrapers to reduce reciprocating weight by scraping the oil off of the outer edges and outside walls of the counterweights and return the oil back to the sump. The oil falls off just fine at lower RPM's ................. but at higher RPM's the rotating crankshaft holds the oil

Chris-PA · Feb 5, 2015

Moparmyway said:
One way to tell if your theory is correct would be to grind away at the leading and trailing edges, then drill the crank, and fill with Mallory metal to rebalance the assembly. You could also thin out the counterweights, then rebalance using the Mallory metal as well. Anything you do to the crank will also have an effect on case volume .............

My main thought is that of the pressure wave you suggested might be happening .......... at higher RPM's the charge thats trapped within and around the crank would probably tend to stay closer to the crank .......... I have seen videos of high RPM crankshafts, and the amount of oil that stays trapped between the counterweights and the rod end of the crank is significant ............ hence the use of crank scrapers to reduce reciprocating weight by scraping the oil off of the outer edges and outside walls of the counterweights and return the oil back to the sump. The oil falls off just fine at lower RPM's ................. but at higher RPM's the rotating crankshaft holds the oil

Well, that would be quite a project, and hard to quantify without dyno time - I don't want to know that bad!

As far as crankshaft mass, they've clearly not worried about that and may have wanted to increase it. The flywheels are big in addition to the high crank mass. I don't think they were going for high rpm, rather good low rpm power.

Moparmyway · Feb 5, 2015

Chris-PA said:
Well, that would be quite a project, and hard to quantify without dyno time - I don't want to know that bad!

As far as crankshaft mass, they've clearly not worried about that and may have wanted to increase it. The flywheels are big in addition to the high crank mass. I don't think they were going for high rpm, rather good low rpm power.

True about them looking for a good low RPM, but even the rather low RPM saws turn close to 9000 RPM's .............
and thats considered high RPM to some........... consider NASCAR runs "high RPM" @ 8-9k, IIRC the video was showing an engine crankshaft at 7500 RPM

Clamshells & Transfer Shape

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