Squish Ratios and Angles, Combustion Chamber Volume and Shape

[Stump Shot]

There are a lot of in-depth 2-stroke tuning references out there that are mentioned here. While they are very interesting, it seems that only some of the information directly translates to a saw's engine when fitted with a muffler as opposed to a tuned pipe.

The ratio of squish area to total bore area is interesting, as is the angle (if any) between the squish band and the crown of the piston, how that transitions into the combustion chamber and the volume of that chamber.

I am building an 066 to eventually use, but this is the first saw I will build that I don't need yesterday for firewood and other stuff, so I am doing it for fun when I have time. The variables mentioned above are fun to think about but I wonder if I am chasing a 15% gain or a 1% gain over just cutting the squish band parallel to the piston so its wider, cut the base to get the squish back and putzing with the ports.

Thats what I did on my first two saws and they run great, to me at least. Has anyone experimented with optimizing the chamber volume or its shape? Are different diverging angles used on the squish band?

The more I read about this stuff the more I want to just cut the head off this cylinder so i can try all these things with different heads.

Good question. Just like a coin has two different sides to it, a piston has two very different jobs at each end of its stroke TDC & BDC.
I've yet to see a domed piston and squish band perform better at the bottom. It's much easier to effectively clear spent gasses with a flat top piston. This it seems outweighs any advantage at the top of the stroke.
I did just cut part of the dome on an experimental cylinder that needed such work done. Otherwise it would be counter intuitive to making more compression normally. Other than knocking down the corner edge if it turns out sharp between the band and the dome if the cast radius is removed.
These are just some of the opinions I came to from my experimentation, it shouldn't however discourage yours by any means. Some times something new can be learned if it was just tried out once. Even if that something doesn't work out as planned you have still learned what not to do.

 

[mdavlee]

Toroidal chambers are supposed to make the most power. A moto GP type chamber is easier to machine. You can play with all angles and end up with the same volume to keep compression as close to each other as you can. I know 235-240 will slow them down especially with timing advance. But it will run decent if you add some different fuels. There’s lots of stuff to try and you may find they feel more free and jut have a better personality at around 200 lbs on a 660. I like a higher exhaust and about 190-200 on them but some people don’t like that type of saw for firewood

 

[drf256]

I am not questioning it, more along the lines of wondering about its application to what we work on. Finding a more favorable geometry for the squish band, playing with its area % etc. Doing this alone I would think won't yield much if anything, but it may if the combustion chamber shape and volume is also changed to favor top end power.

I am not in here bringing this up cuz I'm bored and just wanna chat about saws, Randys right, most people are Dalls, 'specially these days.

The main question in the original post was if anyone had done this work, applied the research out there and came up with anything.

Applying all this stuff to a factory cast thing would be tedious if even possible, tons of compromises, definitely not practical.
I'm just going to cut the head off this cylinder and start there. Design a head and try it out, Make another one with one variable changed and see what it does. This is just a hobby for me.

Didnt think you were questioning any theory, just didn’t understand what your original inquiry was.

Mopar Kevin above has tinkered with angling both the squishband and the piston at the same angle. He has done flat vs angled.

If someone wants less % band, the answer is a tiny band and a popup to fill the chamber until desired static compression ratio is reached. Angling the band, along with the piston, is a way to pickup more real estate in the same size cylinder bore. Like Randy said, we are stuck with what the oem gave us. I question why they chose to give us what they did, they certainly have more toys and time to figure out what’s optimal. In general the oem has gone to higher temp applications to increase efficiency and reduce emissions. We aren’t worried about 10% less efficiency if we gain power.

It seems that Kev’s angles have improved power quite a bit. My question is why? Is the angle working because of higher SB velocity or is the secondary consequence of doing such that the transfer flow is increased.

 

[Moparmyway]

It seems that Kev’s angles have improved power quite a bit.

Maybe ........... but I dont know if I'd get too carried away with adding "quite a bit"

Could also be that:
- It funnels in the mixture as it begins to enter/exit any opening in the cylinder ?
- It increases surface area on piston crown ?
- Piston is lighter

Is the angle working because of higher SB velocity or is the secondary consequence of doing such that the transfer flow is increased.

Exhaust could be benefitting here too ..?

 

[Mastermind]

As I understand it, tighter the better. Just touching when cold to .005” clearance Note that’s running so it’s actually larger due to flex on the crank, rod and bearings. For saws that seems to be from .012” to .020”

rule of thumb is 50% area and 0-2 deg angle from piston. You can adjust the power band by changing the area, smaller tends to have less issues with detonation. There are limits to how fast you want the squish velocity as it raises the forces on the crank.

it seems to work by increasing turbulence which raises the flame speed. This makes the engine more efficient. Less timing is needed and more fuel is burned in the cylinder instead of out the exhaust. On sleds they see reduced egt’s after tightening the squish. It’s free hp. The only real downside is the extra work required to set it.

I remember years ago Dennis Cahoon telling me that the tighter the better on squish, as long as it wasn't touching. But....I've torn down engines that had accumulated carbon over the exhaust port to create a zero clearance situation. That didn't seem like a good thing....

 

[srcarr52]

I remember years ago Dennis Cahoon telling me that the tighter the better on squish, as long as it wasn't touching. But....I've torn down engines that had accumulated carbon over the exhaust port to create a zero clearance situation. That didn't seem like a good thing....

The carbon will only deposit so thick before it's squished and knocked off.

 

[Mastermind]

The carbon will only deposit so thick before it's squished and knocked off.

This stuff was hard......and you could see that there was no clearance. Sorta worried me. You could be right though.

 

[Mastermind]

Maybe ........... but I dont know if I'd get too carried away with adding "quite a bit"

Could also be that:
- It funnels in the mixture as it begins to enter/exit any opening in the cylinder ?
- It increases surface area on piston crown ?
- Piston is lighter




Exhaust could be benefitting here too ..?

Interesting stuff Kevin. By lowering the edge of the piston, you are opening the transfers more in some instances I would imagine.

 

[Moparmyway]

Interesting stuff Kevin. By lowering the edge of the piston, you are opening the transfers more in some instances I would imagine.

And directing the charge upwards

 

[drf256]

This is what I wonder about. Matching the roof angle results in a functionally larger and faster port opening.

 

[drf256]

Anyone else having a hard time finding this thread? I go to “Technical Area” under “Chainsaws” and can’t find it. I can only get to it through notifications, but there is also a highlight as least thread posted in tech area. Is it a hidden thread or something?

@Mastermind

 

[Moparmyway]

Same

 

[drf256]

 

[drf256]

Same

Thought it was me...

 

[SOS Ridgerider]

Anyone else having a hard time finding this thread? I go to “Technical Area” under “Chainsaws” and can’t find it. I can only get to it through notifications, but there is also a highlight as least thread posted in tech area. Is it a hidden thread or something?

@Mastermind

Same

View attachment 271342 View attachment 271343 View attachment 271344

It’s there, but almost at the bottom of the page because of all the stickies in that category.

 

[Moparmyway]

I knew I felt pain, cheek got stuck - by stickie

 

[Mastermind]

Y'all want it moved to the regular chainsaw forum?

 

[Deets066]

Toroidal chambers are supposed to make the most power. A moto GP type chamber is easier to machine. You can play with all angles and end up with the same volume to keep compression as close to each other as you can. I know 235-240 will slow them down especially with timing advance. But it will run decent if you add some different fuels. There’s lots of stuff to try and you may find they feel more free and jut have a better personality at around 200 lbs on a 660. I like a higher exhaust and about 190-200 on them but some people don’t like that type of saw for firewood

Hard shape to get consistent on the manual lathe. Have halfassed done it though.

 

[Deets066]

Well, my brother did it. I just had to talk him into it lol.

Swiveling the tool holder in the cut isn’t really something I’d recommend.

 

[srcarr52]

Well, my brother did it. I just had to talk him into it lol.

Swiveling the tool holder in the cut isn’t really something I’d recommend.

I just made a large rounded cutter to cut the majority of a toroidal chamber at once.