Part Four: Compression

[Ketchup]

I don’t see any reason that a wide squish band would be optimal, just a necessary evil on machined worksaw builds. Need to reduce volume, there’s a few ways to do so, but a band cut makes sense for piston replacement down the road and no worry of crown to ring land issues. Band is going to get wider to make the chamber smaller.

If it were possible I always prefer a narrow band, wide bands trap more fuel and are more prone to detonate.

You think fuel doesn’t have as much chance to burn at the edges of the piston crown if the band is too wide?

I’ve only had one saw detonate. It did have a wide band.

 

[Dolkitafreak]

You think fuel doesn’t have as much chance to burn at the edges of the piston crown if the band is too wide?

I’ve only had one saw detonate. It did have a wide band.

Dunno. I think a wide band doesn’t allow as much fuel to reach the chamber. Same motor with two different heads, one had 180 psi and a wide band, seemed to run okay, cut another with 240psi with a narrower band and it woke up significantly…in a work saw we’re fighting it, so we make wide bands work. If I’m cutting a two piece head, and can get the same psi either way, why would I want a wider band?

 

[Stump Shot]

Thanks for your reply; as new chainsaw modifier I can try angle or flat for my style and likeing

Experiment and see what works best for you and the style of porting you come up with. Take good notes with every saw you do right down to how it felt when you cut with it. The more you do the better you will see where you are going. When you have an idea, try it, there's no sense leaving it in your head when it could be making power in a saw and if not it will leave new room for the next idea to come along to try out and won't still be lingering there holding you back.

 

[Moparmyway]

Dunno. I think a wide band doesn’t allow as much fuel to reach the chamber. Same motor with two different heads, one had 180 psi and a wide band, seemed to run okay, cut another with 240psi with a narrower band and it woke up significantly…in a work saw we’re fighting it, so we make wide bands work. If I’m cutting a two piece head, and can get the same psi either way, why would I want a wider band?

Do you think the compression had anything to do with the results ?

 

[Dolkitafreak]

Do you think the compression had anything to do with the results ?

Not really no, I would understand if it was stronger just in the cut, but it was more lively and snappier out of the cut, I would think 60psi more would slow it down more than make it snappier, not like 180 is low by any means. Could have been chamber shape possibly too? Hard telling…but I don’t see why a wide band would be preferred.

 

[Ronie]

I read on one site that 45-55 percent of the bore is optimal with a 1 degree slant upward towards the center for the squish band so that it directs the mix to the center and speeds up combustion, maybe the one with the small squish band did a better job of directing the mix to the center, caused more turbulence and that speed up combustion.

 

[Ronie]

The article I I was talking about, http://www.powercdi.com/manual/combustion/squish/en.html.

 

[pbillyi69]

i guess i dont really understand the relationship between the width of the band to the performance of the saw. if all other things are the same chamber volume squish gap timing numbers why would one be better. i realize that you cant really have the same volume with differen band width and the squish at the same time, but if you could why would one run better than the other? chamber shape?

 

[huskyboy]

Sounds like a task for the dynO. Port one saw but don’t add compression other than tightening squish… then cut the cylinder and keep the same timing numbers on portwork… retest. Or just keep adding compression in progressive amounts and retest each time on dynO. Not all of the variables would be eliminated but a lot of them would be. But it would be a good learning experience.

 

[huskyboy]

I do think there is a point where the saw can’t cool as effectively from the compression and starts to loose power when it gets hot. Especially in long cuts in hot weather. Gotta remember the cooling system is air cooled and only designed to cool stock power levels. I guess a race application it wouldn’t matter cause your only making a few cuts. But on a worksaw it could be something to consider.

 

[Ketchup]

I could see scavenging being less effective on a wide band…uh, maybe.

 

[Moparmyway]

Not really no, I would understand if it was stronger just in the cut, but it was more lively and snappier out of the cut, I would think 60psi more would slow it down more than make it snappier, not like 180 is low by any means. Could have been chamber shape possibly too? Hard telling…but I don’t see why a wide band would be preferred.

I'm not sure that I agree with everything here .....

-Having more power, wouldn't it display those attributes across the board, no ?

-60 psi more ...... might be just what it needs, no ?

-Chamber shape .......... might have to think about that, but the question I have is this: which is better, a larger or smaller chamber shape ?

-Wide band preferred.....I dont see the "large" advantage in it either..... How much percentage of the stroke does squish affect ?

 

[Ketchup]

-60 psi more ...... might be just what it needs, no ?

Seems awfully high…

-Chamber shape .......... might have to think about that, but the question I have is this: which is better, a larger or smaller chamber shape ?

Smaller Volume = better. Up until it doesn’t.

-Wide band preferred.....I dont see the "large" advantage in it either.....

A gain is a gain. 2% or 20% still seems worth it if it’s actually there.

-How much percentage of the stroke does squish affect ?

Lots of places to go with that last question. 100 Degrees of the up stroke? 200? How much and where are you thinking?

 

[Moparmyway]

-60 psi more ...... might be just what it needs, no ?

Seems awfully high…

What saw are you referencing ?

-Chamber shape .......... might have to think about that, but the question I have is this: which is better, a larger or smaller chamber shape ?

Smaller Volume = better. Up until it doesn’t.

At what point doesn't it ?

-Wide band preferred.....I dont see the "large" advantage in it either.....

A gain is a gain. 2% or 20% still seems worth it if it’s actually there.

Is there a gain due to this, or something else ?

-How much percentage of the stroke does squish affect ?

Lots of places to go with that last question. 100 Degrees of the up stroke? 200? How much and where are you thinking?

I don't believe the "up stroke" really has anything to do with this, why is it that you do ?


My quotes were towards someone else, however, I'm happy to oblidge ......... while waiting patiently for @Dolkitafreak to respond

 

[Dolkitafreak]

I'm not sure that I agree with everything here .....

-Having more power, wouldn't it display those attributes across the board, no ?

-60 psi more ...... might be just what it needs, no ?

-Chamber shape .......... might have to think about that, but the question I have is this: which is better, a larger or smaller chamber shape ?

-Wide band preferred.....I dont see the "large" advantage in it either..... How much percentage of the stroke does squish affect ?

It could, but if I pump it up to 300 I bet it slows down. I bet with even less compression it revs to the moon out of the cut…feels like more power in your hands but not the cut.

Might have been what it wanted, could need 60 psi more or 100 psi less…hard telling!

Also-I’m solely talking two piece heads, on single piece heads I’ll make a wide hand work because it’s all I can do to get that compression, in a two piece head I could make a wide band and large chamber to have the same psi as a narrow band and small chamber, don’t see any reason I would want the wide band if it can be avoided.

 

[Ketchup]

-60 psi more ...... might be just what it needs, no ?

Seems awfully high…

What saw are you referencing ?

-Chamber shape .......... might have to think about that, but the question I have is this: which is better, a larger or smaller chamber shape ?

Smaller Volume = better. Up until it doesn’t.

At what point doesn't it ?

-Wide band preferred.....I dont see the "large" advantage in it either.....

A gain is a gain. 2% or 20% still seems worth it if it’s actually there.

Is there a gain due to this, or something else ?

-How much percentage of the stroke does squish affect ?

Lots of places to go with that last question. 100 Degrees of the up stroke? 200? How much and where are you thinking?

I don't believe the "up stroke" really has anything to do with this, why is it that you do ?


My quotes were towards someone else, however, I'm happy to oblidge ......... while waiting patiently for @Dolkitafreak to respond

I was referring to Dolkitafreak’s 240psi saw. Seems on the margin.

At some point the volume gets so small the saw detonates once it heats up.

I don’t really know if there if distinct gain to a wider band. That’s what I was getting at to begin with. I think it might. Just like I think a diagonal band creates a small gain. Small gains are harder to identify.

Up stroke is where resistance is going to slow the piston. Eventually more compression will create more resistance than force gained on the down stroke. That’s my thinking. Is that baseless? I think you’re saying more compression is usually better, smaller volume is better. It’s that simple. I’m open to that. Maybe my high compression concerns are unfounded.

 

[Dolkitafreak]

I was referring to Dolkitafreak’s 240psi saw. Seems on the margin.

At some point the volume gets so small the saw detonates once it heats up.

I don’t really know if there if distinct gain to a wider band. That’s what I was getting at to begin with. I think it might. Just like I think a diagonal band creates a small gain. Small gains are harder to identify.

Up stroke is where resistance is going to slow the piston. Eventually more compression will create more resistance than force gained on the down stroke. That’s my thinking. Is that baseless? I think you’re saying more compression is usually better, smaller volume is better. It’s that simple. I’m open to that. Maybe my high compression concerns are unfounded.

240 is the highest I ever like to see, this is a 3 cube saw, in my experience smaller motors like the compression (gain from it) more than larger motors. For a work saw, I wouldn’t push them quite that high, you’re asking for some problems imo. This is an 8x8 saw, I would never intent to cut firewood with it and expect it to hold up or not detonate.

I don’t think your opinions are baseless, too much compression will restrict rpm’s from what I’ve found. I think it’s a common misconception that more compression is always better.

 

[Stump Shot]

I read on one site that 45-55 percent of the bore is optimal with a 1 degree slant upward towards the center for the squish band so that it directs the mix to the center and speeds up combustion, maybe the one with the small squish band did a better job of directing the mix to the center, caused more turbulence and that speed up combustion.

For discussion sake lets automatically agree that the angle is best for squish, it wouldn't necessarily be the most powerful in a small, simple two stroke like a chainsaw has. They don't have the benefit of multiple ports like a larger modern motorcycle or snowmobile engine has. Also, there are plenty of examples out there of factory angled squish bands already in existence, which (we) can make run better by flattening them back out.
Just like there is two sides to every story, there's two ends to every stroke. So in other words, taking away a large advantage at one end to gain a small advantage at the other end isn't a very good trade. And trust me on this one, there's always a trade off when we work an engine over, everything we do has a positive and/or negative affect. The more positives you can get in your corner, the better off you will be.

 

[huskihl]

240 is the highest I ever like to see, this is a 3 cube saw, in my experience smaller motors like the compression (gain from it) more than larger motors. For a work saw, I wouldn’t push them quite that high, you’re asking for some problems imo. This is an 8x8 saw, I would never intent to cut firewood with it and expect it to hold up or not detonate.

I don’t think your opinions are baseless, too much compression will restrict rpm’s from what I’ve found. I think it’s a common misconception that more compression is always better.

Hard to prove without a 2 pc head. And even then it could be the smaller combustion chamber that caused the change in running characteristics rather than the increased compression.

It never made much sense before but in this context it does. Build a 2-ring saw with 240 lbs or so compression and run it with the top ring only. And then run it with the bottom ring only. Most of the other parameters stay the same this way as far as port height, shape, and area

 

[Dolkitafreak]

Hard to prove without a 2 pc head. And even then it could be the smaller combustion chamber that caused the change in running characteristics rather than the increased compression.

It never made much sense before but in this context it does. Build a 2-ring saw with 240 lbs or so compression and run it with the top ring only. And then run it with the bottom ring only. Most of the other parameters stay the same this way as far as port height, shape, and area

Even then it’s tough. Maybe a saw with a low exhaust won’t benefit from the added compression as much as a high exhaust because the low already has the torque, which is partially due to the likely higher compression.

It was put to me like this: is 180psi the same with an exhaust at 105 vs 180psi at 90?

These damn variables are always popping up…