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Combustion chamber mods

thompsoncustom

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I was wondering what you guys thought about combustion chamber mods? I have seen where some people have made removable heads and I have filled in a chamber with brazing rod (haven't ran it yet) but both methods allow you to recreate the combustion chamber and set the compression without lowering the exhaust port which I would think would allow you to turn a higher RPM without giving up torque. Or you could even raise the exhaust port and increase duration without loss in compression as you could shrink the combustion chamber to make up the difference.

Any thoughts?
 

drf256

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Many thoughts.

First is the mechanics of it. Aluminum brazing rod melts at a lower temp than aluminum. There are some higher temp alloys, but you’re getting close to the entire jug melting or warping when you get in in there. Imagine a piece of it coming loose or melting in a running engine? Not gonna be good.

If youre making a 2 piece head, you’re already at an advanced stage of building. You can make that head to any chamber size and shape you want.

Ive tried tig welding cylinder chambers. It’s pretty rough to do. Shawn Carr is good at it. I’ve been tig welding for a decade and can’t come close to his skill at repairing plug threads.

I guess the biggest reason it’s not done is that there isn’t any need to. Combustion chamber shape is crucial to proper efficient combustion. Modifying it will likely only make things worse. Cutting the band nice and flat and then adjusting squish is the way to go. Then you can set the exhaust height where you want it.

I used to want everything to have monster compression, but now I fight to keep it lower. I’ve made two chamber cuts to reduce it with some success. I’m gonna try a small piston dish next. Too much compression adds heat and results in a saw that will get hot in the cut and lean out. It’s not always advantageous.
 

Funky sawman

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Many thoughts.

First is the mechanics of it. Aluminum brazing rod melts at a lower temp than aluminum. There are some higher temp alloys, but you’re getting close to the entire jug melting or warping when you get in in there. Imagine a piece of it coming loose or melting in a running engine? Not gonna be good.

If youre making a 2 piece head, you’re already at an advanced stage of building. You can make that head to any chamber size and shape you want.

Ive tried tig welding cylinder chambers. It’s pretty rough to do. Shawn Carr is good at it. I’ve been tig welding for a decade and can’t come close to his skill at repairing plug threads.

I guess the biggest reason it’s not done is that there isn’t any need to. Combustion chamber shape is crucial to proper efficient combustion. Modifying it will likely only make things worse. Cutting the band nice and flat and then adjusting squish is the way to go. Then you can set the exhaust height where you want it.

I used to want everything to have monster compression, but now I fight to keep it lower. I’ve made two chamber cuts to reduce it with some success. I’m gonna try a small piston dish next. Too much compression adds heat and results in a saw that will get hot in the cut and lean out. It’s not always advantageous.
To me, the worst pistons are those that are modified. Makes it a nightmare to keep saws running in commercial conditions if you happen to wear out a slug, or score it from aluminum transfer. In my years cutting timber in the rugged mountains of idaho, I've replaced probably 25 or 30 pistons in saws, either on the tailgate of the crummy or the kitchen table of a remote logging camp trailer, 120 miles from nowhere. I always carry spare pistons, rings, seals, carbs, fuel lines and oiler gears when I'm way out in the bush, one cannot afford to come down to civilization to do such work, and a lot of us idaho folk are saw poor. Heck some of the guys I cut with only had one saw, so broke they needed advances from the bull buck because they can't even live paycheck to paycheck!
 

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Filling the chamber could be useful to add compression, especially in clamshells. I have been meaning to thread some holes into the chamber of a clamshell and add some pan head bolts. Haven’t gotten around to it.

When you’re messing with the chamber, it’s important to consider shape. An overly flat or mis-shapen chamber won’t combust as well. A two piece head addresses that really well, but at that point you’re into advanced machine work.

The exhaust port is related to compression, but has a lot of other important dynamics. Same with torque. Yes you can buy exhaust height and rpm by increasing compression, but there are diminishing returns as you go higher and the performance of the engine becomes increasingly peaky.

I agree with @drf256. Too much compression can hurt as much as help and it won’t do much if other elements aren’t well designed.

And like @Funky sawman said, many performance mods aren’t really in the work saw wheel house. Showing up at a get together with some frankensaws and a truck full of tools is very different than making a quota so you can pay the bills.

But nothing is off the table. Braze the chamber, weld the rod, hack, grind, stuff, pour. That’s a fun part.
 

5155

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WHATT? Big comp is not the be all, top all?
I'm impressed you guys say that on the web.

I have a 55 with 22 squish. I don't work on it to un-squish it because, I don't run it because, it is a pita.
Be great for impressing a buddy, working in real world not so much.
 

thompsoncustom

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indeed it seems the most useful in a clamshell so you don't have to worry about recutting the bearing pockets.

I'm currently waiting on a different carb for the test saw as the one I ordered was junk, it's also a clamshell (wild thing)

I'm just imaging in my head that a saw that struggles with enough exhaust duration could benefit from this vs lower the jug. But I'm new to modding 2 strokes and only play with my own saws unlike some of you guys that do this for a living so I might not have a clue.

I also wonder how shape and size of the combustion chamber will effect things there have been a couple different designs in the automotive world over the years.


I don't think the brazing material coming loose is going to be a problem at that point your cylinder is so hot it's all over anyways. biggest issue I could think of would be cylinder wrapping but so far so good from what I can tell.
 
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Ketchup

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I'm just imaging in my head that a saw that struggles with enough exhaust duration could benefit from this vs lower the jug. But I'm new to modding 2 strokes and only play with my own saws unlike some of you guys that do this for a living so I might not have a clue.

I think you are right that adding compression will help. It’s possible your exhaust is just too high and you can’t build enough torque no matter how much compression you have.

I also wonder how shape and size of the combustion chamber will effect things there have been a couple different designs in the automotive world over the years.
There’s debate on shape. Small-ish domed chamber and a flat squish band seems the most popular among the porting crowd. Angled band is another common shape. The squish depth itself is debated as well. Most chambers seem to be tallest near the spark plug.

I don't think the brazing material coming loose is going to be a problem at that point your cylinder is so hot it's all over anyways. biggest issue I could think of would be cylinder wrapping but so far so good from what I can tell.

What product did you braze with? How did you prep the chamber?
 

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The combustion chambers on saws, like most other things about them, compromise some performance just for packaging. In an ideal world. Just look at bikes for an example...performances is the priority on most 2 stroke performance bikes. Compared to saws, they have shorter, but wider combustion chambers. The squish bands have less surface area in relation to the bore diameter than most saws. A wider(as in width, not depth) squish band should better combat detonation...in reality, I think the saws could run much narrower squish bands and be just fine.

Also performance builds have spark plug holes that are perfectly centered in the combustion chamber. The overall shape depends on the manufacturer. I've seen some of the newer 2 strokes with angled, rather than rounded, combustion chambers. Below is an example of a bike head...this is the head off my 300 that I milled to decrease the squish thickness, then removed material out of the dome to correct for compression. It has a rounded/ovaled shape compared to the angled heads on some bikes.

A note about bikes vs saws and how that relates to compression...compression is much more important on a non-piped saw motor. In a motor with a pipe, the pulse wave effectively negates potential loss of compression from a high exhaust port. In our saws, IMO a taller exhaust port needs a lower volume combustion chamber to negate this...this is all within reason. When it comes to saws, I'm a big proponent of increasing compression. On the bikes, they already come with so much compression, that I prefer to leave the head volume the same but correcting the squish band.

Disclaimer: I wouldn't call myself a pro or expert when it comes to 2 stroke tuning theory, I'm more of an educated idiot.

367331331_300head.jpg.8c396667a7e85e284c481ae5cd9a4835.jpg
 

thompsoncustom

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What product did you braze with? How did you prep the chamber
kt industries pre fluxed aluminum brazing rod, melt around 800 degrees.

only prep I did was to wash the cylinder with acetone no sanding at all. should also note that It was a brand new cylinder.
 
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drf256

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The combustion chambers on saws, like most other things about them, compromise some performance just for packaging. In an ideal world. Just look at bikes for an example...performances is the priority on most 2 stroke performance bikes. Compared to saws, they have shorter, but wider combustion chambers. The squish bands have less surface area in relation to the bore diameter than most saws. A wider(as in width, not depth) squish band should better combat detonation...in reality, I think the saws could run much narrower squish bands and be just fine.

Also performance builds have spark plug holes that are perfectly centered in the combustion chamber. The overall shape depends on the manufacturer. I've seen some of the newer 2 strokes with angled, rather than rounded, combustion chambers. Below is an example of a bike head...this is the head off my 300 that I milled to decrease the squish thickness, then removed material out of the dome to correct for compression. It has a rounded/ovaled shape compared to the angled heads on some bikes.

A note about bikes vs saws and how that relates to compression...compression is much more important on a non-piped saw motor. In a motor with a pipe, the pulse wave effectively negates potential loss of compression from a high exhaust port. In our saws, IMO a taller exhaust port needs a lower volume combustion chamber to negate this...this is all within reason. When it comes to saws, I'm a big proponent of increasing compression. On the bikes, they already come with so much compression, that I prefer to leave the head volume the same but correcting the squish band.

Disclaimer: I wouldn't call myself a pro or expert when it comes to 2 stroke tuning theory, I'm more of an educated idiot.

367331331_300head.jpg.8c396667a7e85e284c481ae5cd9a4835.jpg
Beautiful work.

Dynamic vs Static compression comes into play here. Pipes backstuff the cylinder before the exhaust port closes and increases the swept volume pushed into the combustion chamber. Sonic supercharging.

Saws are different for a variety of reasons. They are air cooled (and fuel mix cooled) and are run at WOT or idle 95% of the time, unlike a bike with gears. I can tell you from real world experience that too much compression adds too much heat and the saws can’t compensate. They will lean out in long cuts and score if they overheat. Autotune saws have a huge benefit, and I don’t mind upping the compression more on them. They will adjust to the best tune continually and offer a huge buffer.

I had a problem with an MS261CM one time. I was using Motul800 oil at 30:1 and the mix was cold. I had no idea that Motul800 was one of the more viscous 2T oils and that the 261CM has a small carb jet in the nozzle. The saw would run great with a 16” bar and short cuts. I added a 20” 3/8 and all hell broke loose. The saw could not compensate for the chamber heat and did all sorts of strange stuff.

I switched to Amsoil Sabre at 40:1 and the saw ran normally. No other changes. I have since begun to knock out the main nozzle and drill the jet larger. That has allowed me to go back to any oil I want at 32:1.
 

drf256

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indeed it seems the most useful in a clamshell so you don't have to worry about recutting the bearing pockets.

I'm currently waiting on a different carb for the test saw as the one I ordered was junk, it's also a clamshell (wild thing)

I'm just imaging in my head that a saw that struggles with enough exhaust duration could benefit from this vs lower the jug. But I'm new to modding 2 strokes and only play with my own saws unlike some of you guys that do this for a living so I might not have a clue.

I also wonder how shape and size of the combustion chamber will effect things there have been a couple different designs in the automotive world over the years.


I don't think the brazing material coming loose is going to be a problem at that point your cylinder is so hot it's all over anyways. biggest issue I could think of would be cylinder wrapping but so far so good from what I can tell.
Pistons melt when a saw is run hard on the exhaust side. They don’t have the same cooling potential as the jug, but if your heat gets anywhere near your chamber alloy’s melting temp, it wont be good.

it may not be possible to hit that temp with as large of a heat sink as the jug is.
 

thompsoncustom

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stock cylinder
20230206_223733.jpg
same one braze
20230207_173406.jpg

nothing has really been done to clean it up at this point I'm gonna run it first and see how it acts.
 

drf256

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Looks pretty good. Have you tried to smooth it? can try scotchbrite ball and thin oil.

I’d also chamfer the plug entry.
 

thompsoncustom

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Looks pretty good. Have you tried to smooth it? can try scotchbrite ball and thin oil.

I’d also chamfer the plug entry.
haven't done anything to it yet after I run the saw I might go back in and change things depending on how it runs. ya I thought the hole needed counter sunk too but didn't have a bit handy so it hasn't been changed yet.

Thanks for the scotchbrite ball idea may end up using that
 

thompsoncustom

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well got 4 tanks through this chamber now and here is the plug and the saw is on the rich side not lean at all. makes me think it's burning pretty clean or hot around the plug.

20230317_080243.jpg

according to my compression gauge it was around 165psi to start with and is the same now. First thing I need to do is check my gauge against the compressor and if it's right I'm gonna go back in and add more.


another upside to this method is it doesn't require machine work but you could also fill the chamber in 100% beside the plug hole and than recut the chamber how ever you want with machine work.

Whats your guys thought on squish band width? Bigger the better to push more into the combustion area?
 
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