Squish Ratios and Angles, Combustion Chamber Volume and Shape

[Wisconsin Welder]

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.

 

[huskihl]

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.

Most of us cut the squish band to gain some compression and/or get the exhaust down. In my experience it’s worth about 10-15% of the gains

 

[Wisconsin Welder]

Yes, but is everyone cutting it parallel to the piston? there are reasons not to, I am wondering if they will yield more gains in these engines.
The idea behind the angling of the squish band, at least how I understand it, is to gain velocity of the charge being shot towards the combustion chamber

 

[Deets066]

Yes, but is everyone cutting it parallel to the piston? there are reasons not to, I am wondering if they will yield more gains in these engines.
The idea behind the angling of the squish band, at least how I understand it, is to gain velocity of the charge being shot towards the combustion chamber

If it is angled then it really isn’t the squish band anymore. Unless of course your piston is angled the same.

 

[Wisconsin Welder]

The math I am getting out of this shows in a 54 mm bore, with a 50/50 squish ratio, the difference from the edge of the bore to where it transitions into the chamber being about .005" for a 1 degree angle.

Very slight angles, not like rise/run level angles.

From what I understand, if I am understanding it at all, that diverging angle, along with the height and volume of the dome, and the squish ratio can move the peak output of the engine around.

 

[Lightning Performance]

Yes, but is everyone cutting it parallel to the piston? there are reasons not to, I am wondering if they will yield more gains in these engines.
The idea behind the angling of the squish band, at least how I understand it, is to gain velocity of the charge being shot towards the combustion chamber

Remember every little bit you close down the band area saves unburnt fuel. A lot of these guys go 50% of the total piston surface area with zero issues. You don't have a bike muffler exit in front of you so cleaner is better here. Tighter is better till you smack the head. Carbon will play a part latter if you have it. I have a small work saw 47mm set at 0.012. I won't try it with a 50mm piston. As your piston bulk increases so does expansion of the piston top. OEM expands at a different rate than say a Wisco. Longer con rods "grow" more when heated. All standard stuff most builder consider. Rule of thumb is 0.010 per inch of bore in most peoples notes. Ask around some.
Flat and straight is also easier than playing games of no return mostly.

 

[huskihl]

I’ve never really played around with an angled band unless the piston had a dome. And even then I usually cut the dome off and make a flat band.

 

[srcarr52]

If it is angled then it really isn’t the squish band anymore. Unless of course your piston is angled the same.

Not true. There are all sorts of research out there done on the angle of the squish band with flat pistons. I think there is even some data in Bells book.

I normally cut mine at 2deg.

 

[drf256]

This topic has intrigued me. Not certain if the OP is questioning the squish band being slightly angled and getting looser as it approaches bore or just angling both.

Cutting both the piston crown and the band at an angle makes more sense to me. It will allow one to get more % of band to bore and also increase surface area of the entire compression area to possible allow more cooling from combustion. Creates more surface area in the entire cylinder.

Cutting the piston crown is what is find the most interesting. Think about the angle of the transfer roof opening into a square edge of a piston crown vs an angle that nearly matched to the roof. You will get more port area faster and possibly more overall port area.

My cheesy Chinese lathe DRO’s do not allow me to swing the compound over enough to do minor angles. It’s frustrating that I can’t play with this.

I’m wondering why you guys think the OEM dome some pistons and in other saws they don’t?

 

[srcarr52]

This topic has intrigued me. Not certain if the OP is questioning the squish band being slightly angled and getting looser as it approaches bore or just angling both.

Cutting both the piston crown and the band at an angle makes more sense to me. It will allow one to get more % of band to bore and also increase surface area of the entire compression area to possible allow more cooling from combustion. Creates more surface area in the entire cylinder.

Cutting the piston crown is what is find the most interesting. Think about the angle of the transfer roof opening into a square edge of a piston crown vs an angle that nearly matched to the roof. You will get more port area faster and possibly more overall port area.

My cheesy Chinese lathe DRO’s do not allow me to swing the compound over enough to do minor angles. It’s frustrating that I can’t play with this.

I’m wondering why you guys think the OEM dome some pistons and in other saws they don’t?

I believe Randy @Mastermind posted a while back that he switched from large squish bands to keeping them more narrow and had better results. Probably has something to do with lost squish velocity, or the flame front probably never reaches the cylinder wall corner before the piston is moved significantly down the bore.

 

[Mastermind]

I believe Randy @Mastermind posted a while back that he switched from large squish bands to keeping them more narrow and had better results. Probably has something to do with lost squish velocity, or the flame front probably never reaches the cylinder wall corner before the piston is moved significantly down the bore.

Most of the time we are sorta limited to what we can do in relation to what the factory leaves us with.....but yeah, I've tried and re-tried about everything you can imagine. These days I'm sorta staying with what is working well, and just doing the job.

Well.....I am trying a few things on these 500is to see if I can wring a little more out.

Talking about saws ain't something I'm into much though.....too many personalities involved to make it an enjoyable topic. LOL

 

[Moparmyway]

Cutting both the piston crown and the band at an angle makes more sense to me. It will allow one to get more % of band to bore and also increase surface area of the entire compression area to possible allow more cooling from combustion. Creates more surface area in the entire cylinder.

Cutting the piston crown is what is find the most interesting. Think about the angle of the transfer roof opening into a square edge of a piston crown vs an angle that nearly matched to the roof. You will get more port area faster and possibly more overall port area.

 

[Lightning Performance]

I have a saw here that Doc would enjoy messing with

Hemi head chamber, narrow angled band with a full dome piston. The piston is specifically shaped to fit the pocket in the chamber. The transfers are strange as you will ever see imo. It's cool to get something different to work with. Small bore and long stroke should be a nice one to figure out. The transfer angle is steeper on the one feed port than the piston crown angle. It is aimed directly at the combustion chamber. It's been years since I've seen one of these types.
No extra piston to mess with on this build

 

[Wonkydonkey]

I was gonna add about what I’ve read, but I’ve no way to even test the theory, it’s the squish band velocity if it’s to fast you get detonation and the way you reduce it, is to angled the squish and also the amount of area it covers. But I guess it also matters what fuel you use.
This could be one of the reasons why you can get different thickness base gaskets

And as I say I’ve no idea if it really comes into play on chainsaws in the modern western world of good fuel. But if your into racing and stuffs then it’s something to look at.

http://www.dragonfly75.com/moto/SquishVelocity.html

 

[Mastermind]

I have a saw here that Doc would enjoy messing with

Hemi head chamber, narrow angled band with a full dome piston. The piston is specifically shaped to fit the pocket in the chamber. The transfers are strange as you will ever see imo. It's cool to get something different to work with. Small bore and long stroke should be a nice one to figure out. The transfer angle is steeper on the one feed port than the piston crown angle. It is aimed directly at the combustion chamber. It's been years since I've seen one of these types.
No extra piston to mess with on this build

Pictures or it didn't happen.

 

[av8or3]

I’d like to see it too!

 

[Wisconsin Welder]

This topic has intrigued me. Not certain if the OP is questioning the squish band being slightly angled and getting looser as it approaches bore or just angling both.

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.

 

[Mastermind]

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.

Well I wanna see what you come up with. Keep me in the loop?

 

[Wisconsin Welder]

Well I wanna see what you come up with. Keep me in the loop?

Absolutely, you're the ring leader of this wonderful *s-word show, I'd still be rubbing all my firewood in half with my old 290 if isn't wasn't for you guys just showing us how to do this.

Maybe I find nothing, whatever, then WE know. And I will have somehow furthered this exercise that, I don't about you guys, for me, keeps me from going nuts, drinking, drugs, prison, quitting my job and selling everything and moving to Montana to live in a tool shed and write letters to government, ya know? LOL

 

[Slotracer577]

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.