Reducing Squish on a Poulan Clamshell

[Moparmyway]

Great idea with the 3D printer !
When you drop the jug, do you think the counterweights will have enough clearance so they wont hit the casting by where the cylinder begins ?
How would you measure that clearance ?
The only way I could imagine would be clay or putty, then slice and measure

 

[Chris-PA]

Great idea with the 3D printer !
When you drop the jug, do you think the counterweights will have enough clearance so they wont hit the casting by where the cylinder begins ?
How would you measure that clearance ?
The only way I could imagine would be clay or putty, then slice and measure

Yes, that was one of my concerns. However, since I have the cutway engine in the first picture I can see in there pretty well. Just eyeballing it I'd say I've got around 0.070" until it hits, so I should be OK with the amounts I'm going to be cutting here.

 

[Moparmyway]

Yes, that was one of my concerns. However, since I have the cutway engine in the first picture I can see in there pretty well. Just eyeballing it I'd say I've got around 0.070" until it hits, so I should be OK with the amounts I'm going to be cutting here.

You dont miss much .............. great ideas and another well put together thread !

 

[HuskStihl]

Chris could probably end world hunger and eradicate disease if he didn't spend so much time on cheap clamshells.

 

[Chris-PA]

You dont miss much .............. great ideas and another well put together thread !

Thanks - you've got me looking at that gap now on the cut way engine. That whole bottom cap is perfectly symmetrical to the crankshaft weights all the way around, and there is case volume there I'd like to get rid of!

Chris could probably end world hunger and eradicate disease if he didn't spend so much time on cheap clamshells.

Thank you, but you're the one with skills in that area and those are far more important than anything that can be done playing with little engines!

 

[Wagnerwerks]

I have got to get a 3d printer.

 

[RedneckChainsawRepair]

Well the piston should arrive tomorrow and I'd like to disassemble the saw so I could get moving on this project. But not in an unheated barn at 10deg and blowing (assuming it actually gets that warm).

-10F this morning and 0F as I type. Drifted shut right now. need to hook plow up later. Enjoy and good talking to ya.

 

[Moparmyway]

Thanks - you've got me looking at that gap now on the cut way engine. That whole bottom cap is perfectly symmetrical to the crankshaft weights all the way around, and there is case volume there I'd like to get rid of!

If it was easy to grind/sand for proper bearing clearance on the block, then make an adapter for the cap and spin her up on the lathe - take off some material, re-shape the bearing bore, case volume reduced !!

 

[Chris-PA]

If it was easy to grind/sand for proper bearing clearance on the block, then make an adapter for the cap and spin her up on the lathe - take off some material, re-shape the bearing bore, case volume reduced !!

Yeah, but all the cool guys are using epoxy these days, and I don't think that lowly Poulans should be neglected! Actually, dropping the cap as you suggest would only reduce that clearance in the center. However, if I print up a spacer and epoxy it into the cap I can reduce the gap all the way around. I just finished taking measurements and I'll try to print it tomorrow. Then I just have to JB Weld them into the cap.

 

[Chris-PA]

I looked at the bottom cap for the clamshell, and realized that there were more differences between the earlier and later caps than I thought. I've always preferred the later caps as they added a groove on the ends to help seal to the cylinder. however, they also changed the shape of the part a bit:


For the purpose of reducing case volume I decided the earlier cap would work better. I drew up a filler - there was 1.5mm clearance to the counterweights, and I kept 0.5mm clearance:

I will glue them in tonight, but here is how they fit:


The material is PETG, which is pretty soft, so hopefully if there is a collision it will not be catastrophic.

 

[Moparmyway]

 

[Paragon Builder]

Pardon my ignorance... Why do you want to reduce case volume? This is pretty cool!!!

 

[jackjcc]

Pardon my ignorance... Why do you want to reduce case volume? This is pretty cool!!!

Boost transfer velocity. If there is no idle space than all movement of the engine results in fuel/air mix being forced into the transfer and into the combustion chamber, or drawing it into the bottom end from the carb.

 

[Chris-PA]

Pardon my ignorance... Why do you want to reduce case volume? This is pretty cool!!!

It's been a topic of discussion for a while - it's the purpose of "crank stuffers" - basically plastic fillers that round out the crankshaft counterweights. The idea is that with a 2-stroke the engine must draw air into the case, and then squeeze it up the transfers into the cylinder. But of course air is compressible. If you imagine some ridiculously large volume (like say the case was as big as your room, with the inlet on the other side from the piston). It's still sealed, so if the piston goes up it will eventually draw a volume of air equal to the cylinder into the big case, but it may take a while. If the piston is moving very fast then it will just reduce the pressure locally a bit and not much air will be drawn in.

So if you can reduce the volume of the case relative to the displacement, you can improve the pumping efficiency and move more air into the case, as well as increase the pressure pushing it up the transfers.

 

[Paragon Builder]

Ahh, I get it! Like trying to pull a vacuum, pressurize a space. The less volume the quicker either can happen. Correct?

 

[Paragon Builder]

Thanks! While I was writing you explained my thoughts with more elegant words. [emoji106]

 

[HuskStihl]

But if you reduce the volume too much you can lose power

 

[Paragon Builder]

But if you reduce the volume too much you can lose power

Because too small means not enough room to "store" the charge?

 

[Hedgerow]

Why wouldn't you just use a piston with a taller deck height?
Maybe I missed a post?

 

[Hedgerow]

But if you reduce the volume too much you can lose power

More case volume = more HP to an extent..