Reducing Squish on a Poulan Clamshell

[Chris-PA]

it's funny to even type a number that small in front of "cc"

Think of it as 4200mm3!

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

Thanks for that - it's going to take some digesting!

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?

This is what confuses me - what is included and what is not, and is it at TDC or BDC (max or min)? The only thing that makes sense to me is that it excludes the volume of the cylinder.

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.

Thanks, but other than the printer I actually don't have much to work with other than the usual drill presses, Dremels and such. I can use the shop at work, but that mostly just gets me an old lathe. All the CNC mills and lathes are set up for production. So taking apart the crank and cam grinding the pin is out of my capability.

I'll have to give the surface finish some thought - it had not occurred to me.

 

[Chris-PA]

I was working from home today and the temperature got up high enough to work in the barn, so I took a break and took the engine out and apart. I also tried my hand once again at grinding the transfer outlets. I cut down a cut off wheel and that did seem to work OK, but still a bit hard to control.

The new piston should arrive tomorrow, but first I have to try turning the crankshaft. After that I can attach the piston and do a test fit to measure the squish and port timing, and then cut the cylinder appropriately.

 

[Hedgerow]

This is what confuses me - what is included and what is not, and is it at TDC or BDC (max or min)? The only thing that makes sense to me is that it excludes the volume of the cylinder.

Should consist of what's under the piston and in the crank case and transfer ports when piston is at BDC...

 

[cobey]

Should consist of what's under the piston and in the crank case and transfer ports when piston is at BDC...

dang thats what i was thinking.... I might be understanding this stuff

 

[mdavlee]

I was working from home today and the temperature got up high enough to work in the barn, so I took a break and took the engine out and apart. I also tried my hand once again at grinding the transfer outlets. I cut down a cut off wheel and that did seem to work OK, but still a bit hard to control.

The new piston should arrive tomorrow, but first I have to try turning the crankshaft. After that I can attach the piston and do a test fit to measure the squish and port timing, and then cut the cylinder appropriately.

Have you thought of 2 small grooves a little higher on the transfers to start flowing a little earlier than the whole thing to see if it gains rpms? Just thinking out of the box here with the open port set up.

 

[Chris-PA]

Should consist of what's under the piston and in the crank case and transfer ports when piston is at BDC...

OK, using that definition and the measurements I took last night, it looks like this engine starts off around 2:1. My filler blocks will reduce the case volume by 3.6% - not much.

 

[Chris-PA]

Have you thought of 2 small grooves a little higher on the transfers to start flowing a little earlier than the whole thing to see if it gains rpms? Just thinking out of the box here with the open port set up.

That would probably be easier to do.

 

[tree monkey]

1.2 to 1 is supposed to be optimal, for what ever that's worth. I don't think it can be achieved.
full circle cranks will reduce volume more then anything.


it's painted to seal the epoxy

 

[heyduke]

just a question. i'm stoked with what you're trying to do, but are you working at the wrong end? might it be easier to cut the top off the cylinder and machine a new head with the desired squish/compression ratio? it would make it easier to mod the transfers too. my understanding was that the mission was to reduce the .040" squish. we all get a little crazy at this time of year.

 

[Hedgerow]

1.2 to 1 is supposed to be optimal, for what ever that's worth. I don't think it can be achieved.
full circle cranks will reduce volume more then anything.
View attachment 404592

it's painted to seal the epoxy

It just wouldn't be a genuine pic from Augusta without a mtn dew can in it...
Has it ever got "real" cold up there yet?

 

[Homelite410]

I want to do a jb weld popup n see how long it lasts in a compression/fire environment!

 

[tree monkey]

It just wouldn't be a genuine pic from Augusta without a mtn dew can in it...
Has it ever got "real" cold up there yet?

yes it's cold and windy

 

[Homelite410]

I give it a tank!

 

[Hedgerow]

yes it's cold and windy

I wish someone woulda got a pic of Alan and I trying to start that stupid frost bit splitter with the recoil and electric start at the same time...
I can only imagine what it's like now...
We ***** when it gets below 10 here..

 

[tree monkey]

-35 below with wind chill

 

[cobey]

-35 below with wind chill

wow!...... im not gonna complain about our 7 deg. tonight

 

[Hedgerow]

-35 below with wind chill

Yuk...

 

[LegDeLimber]

I want to do a jb weld popup n see how long it lasts in a compression/fire environment!

I'm curious as to what results some of this type of aluminum brazing rod give.
durafix dot com
=

just a question. i'm stoked with what you're trying to do, but are you working at the wrong end? might it be easier to cut the top off the cylinder and machine a new head with the desired squish/compression ratio? it would make it easier to mod the transfers too. my understanding was that the mission was to reduce the .040" squish. we all get a little crazy at this time of year.

That had me thinking about this Wild Thing parts saw, For about 5 mins.
And then I looked at the prices for copper shim stock.
Hmmm, maybe I could weave a loop from some of that 22g dollar store speaker wire.

Picked this one up a couple yeras ago, off C'list. Yeah another victim of running with the brake on.
Surprisingly the crank seal wasn't melted, Saw actually cranked pretty easily.
It's all there, just got that melted problem.

 

[Moparmyway]

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.

full circle cranks will reduce volume more then anything.
View attachment 404592

it's painted to seal the epoxy

Maybe not so complex ........... ???

 

[heyduke]

I'm curious as to what results some of this type of aluminum brazing rod give.
durafix dot com
=

That had me thinking about this Wild Thing parts saw, For about 5 mins.
And then I looked at the prices for copper shim stock.
Hmmm, maybe I could weave a loop from some of that 22g dollar store speaker wire.

Picked this one up a couple yeras ago, off C'list. Yeah another victim of running with the brake on.
Surprisingly the crank seal wasn't melted, Saw actually cranked pretty easily.
It's all there, just got that melted problem.

yeah, i wish i had $5 for every time someone brought a saw to the shop with the chain "jammed." just gently explain the function of the chain brake and send them on their way. i've also encountered a totalled poulan where the hombre had re-installed the clutch cover with the chain break on. didn't know his own strength.