Question about Crank Stuffers Case Volume and Primary Compression

[DavidP71801]

I really appreciate all the help understanding this I even made a cut a way from junk parts I had laying around and it kinda makes sense but I think there is something I’m not taking into account or I’m overlooking. Honestly I feel like I’m 10 watching a magic show.

 

[DavidP71801]

You would need a slo mo camera since 10,000 RPM is 166 revs per second. Even at idle speed of 3000 RPM that is 50 revs per second. All you would see is a blur.
Maybe these pics can help with visualization:
View attachment 450757View attachment 450758

That does help I actually made a cut a way yesterday I’m sure there is some detail I’m missing or not taking into account and when I have the DUH moment I’ll feel really dumb for a while. As far as the acrylic the high speed camera is something else I forgot about. The good news is that my brother said that there is a type of acrylic they use that should work to make see through cases, transfer caps, and intake. It can withstand temperatures up to 450 F and up to 5psi at 450. Just something to think about

 

[Mastermind]

I look at primary case compression like this… Grab 2 16 ounce solo cups and a 24 ounce solo cup. Put 16 ounces of water into a 24 ounce cup and a 16 ounce cup. Take the other 16 ounce cup and push it down into each one flush with the top. Which example pushes out more water?
Higher case compression pushes more charge quicker out through the transfers.
Chainsaws need to be lightweight and don’t have the stroke length to take advantage of bigger cases like in a motorcycle engine

Great analogy.

 

[huskihl]

I guess where I get confused or have trouble understanding is if the 24oz cup is the cylinder and the 16oz cup is the cases how can the cylinder ever get all the way filled? I understand the idea that stuffers increase the compression ratio which increases pressure and therefore velocity.

1 16 ounce cup is the smaller case and the 24 ounce cup is the bigger case. When you push the 16 ounce cup into the smaller cup, more of it gets pushed out than if you push the smaller cup into the bigger cup

The difference in this analogy obviously is water versus air. Air is more compressible. When you have a big case, there is less vacuum created in the case from bottom dead center until the intake opens so that air is slower to move into that case. And the same thing happens on the downstroke as the piston closes the intake port and starts to build pressure. In a larger case, that pressure is lower and the charge moves through the transfer ports slower

 

[DavidP71801]

1 16 ounce cup is the smaller case and the 24 ounce cup is the bigger case. When you push the 16 ounce cup into the smaller cup, more of it gets pushed out than if you push the smaller cup into the bigger cup

The difference in this analogy obviously is water versus air. Air is more compressible. When you have a big case, there is less vacuum created in the case from bottom dead center until the intake opens so that air is slower to move into that case. And the same thing happens on the downstroke as the piston closes the intake port and starts to build pressure. In a larger case, that pressure is lower and the charge moves through the transfer ports slower

I understand that it’s essentially The same as changing the combustion chamber volume in a small block Chevy, adding stuffers would be the same as reducing the combustion chamber volume just for easy numbers to work with if you have a cylinder volume of 100 cc At bdc and 10cc At TDC, You have 10 to one compression If you remove 2 cc of chamber volume then you have 98 at BDC and eight at TDC, which should be 12.25 to one compression Which will Increase pressure at TDC stuffers are doing the same thing. What gets me is the best way I can describe it is think of the cases like a scuba tank That holds 10 ft.³ of airit’s under great pressure, but What’s in the tank is all that’s gonna be in the tank just like when the intake closes what’s in the cases is all that’s gonna be in the cases now. Think of the cylinder. Now I think of the cylinder as a balloon that takes 20 ft.³ of air to fill. How could that scuba tank ever fill? It wouldn’t that be the same as having a 80cc of case volume on a 100 cc saw

 

[Mastermind]

I understand that it’s essentially The same as changing the combustion chamber volume in a small block Chevy, adding stuffers would be the same as reducing the combustion chamber volume just for easy numbers to work with if you have a cylinder volume of 100 cc At bdc and 10cc At TDC, You have 10 to one compression If you remove 2 cc of chamber volume then you have 98 at BDC and eight at TDC, which should be 12.25 to one compression Which will Increase pressure at TDC stuffers are doing the same thing. What gets me is the best way I can describe it is think of the cases like a scuba tank That holds 10 ft.³ of airit’s under great pressure, but What’s in the tank is all that’s gonna be in the tank just like when the intake closes what’s in the cases is all that’s gonna be in the cases now. Think of the cylinder. Now I think of the cylinder as a balloon that takes 20 ft.³ of air to fill. How could that scuba tank ever fill? It wouldn’t that be the same as having a 80cc of case volume on a 100 cc saw

Dear sir.

You are seriously overthinking this.

Sincerely, Randy

 

[huskihl]

You’re way overthinking it in my opinion.

Piston goes up and makes a vacuum in the bottom until the intake opens. Then it sucks air and fuel through the carburetor. Piston comes down and closes off the intake floor and builds pressure for 40–50°. And then the transfer is open and whatever percentage of it goes up to the transfers. And then it happens all over again.

If you have a tight case, it builds more vacuum on the way up until the intake opens. And then you build more pressure when the piston is on its way down after the intake floor closes

 

[Mastermind]

You’re way overthinking it in my opinion.

Piston goes up and makes a vacuum in the bottom until the intake opens. Then it sucks air and fuel through the carburetor. Piston comes down and closes off the intake floor and builds pressure for 40–50°. And then the transfer is open and whatever percentage of it goes up to the transfers. And then it happens all over again.

If you have a tight case, it builds more vacuum on the way up until the intake opens. And then you build more pressure when the piston is on its way down after the intake floor closes

Simple.

Seriously.

 

[Ketchup]

@DavidP71801, you’re on to something with volume relationships, but you don’t need 100% of displacement.

On top of the piston things are going boom and creating pressure. You don’t need to fill the whole space above the piston. You just need enough for a big boom when the piston goes up again.

Everything doesn’t happen in a single cycle. Each cycle has a mix of fresh charge and leftovers from the last cycle. We want less leftovers but there are always some.

And you aren’t filling full displacement. You’re only filling the upstroke after the exhaust. Roughly 100 degrees of 180. That’s not quite 60% of displacement. Maybe that’s where the case volume guidelines come from.

 

[Outback]

I have added primary compression vs. secondary compression and a line to reflect total displacement. As ketchup stated there is not 100% efficiency and there never will be. The goal is to move closer to it. You can see that primary compression time is very short compared to transfer time. As stated the stuffers take up space and increase bottom end compression. Actual top end compression is a little over half the total displacment. More squish on the bottom is more bang on the top. Everybody already said this, but I hope the picture helps.

 

[Mastermind]

I have added primary compression vs. secondary compression and a line to reflect total displacement. As ketchup stated there is not 100% efficiency and there never will be. The goal is to move closer to it. You can see that primary compression time is very short compared to transfer time. As stated the stuffers take up space and increase bottom end compression. Actual top end compression is a little over half the total displacment. More squish on the bottom is more bang on the top. Everybody already said this, but I hope the picture helps.

View attachment 450966

Great post.

 

[bwalker]

I look at primary case compression like this… Grab 2 16 ounce solo cups and a 24 ounce solo cup. Put 16 ounces of water into a 24 ounce cup and a 16 ounce cup. Take the other 16 ounce cup and push it down into each one flush with the top. Which example pushes out more water?
Higher case compression pushes more charge quicker out through the transfers.
Chainsaws need to be lightweight and don’t have the stroke length to take advantage of bigger cases like in a motorcycle engine

Another way to look at is in terms of pressure.

 

[MustangMike]

Everything has to match the saw, and different porting can change things.

I believe some 044/046 hybrids benefit by increasing case volume because the displacement is larger than the original 044.

If the saw sucks in more air / fuel due to the increase, transfer speed will not fall off so much.

Enlarging the case volume (usually by increasing the size of the lower transfers) will often increase low end torque and reduce peak RPM resulting in a saw with a broader power curve.

Of course, so much depends on other factors, but everything has to work together. Port timing, muffler modes, timing advance will all factor in.

IMO, results will always be somewhat based on experimentation, and if you find "lightning in a bottle", remember it!

 

[DavidP71801]

Dear sir.

You are seriously overthinking this.

Sincerely, Randy

I tend to do that especially when it’s something that really interests me I’m very sorry if I’m being a bother how all this works really has me interested it’s just so amazing how something that seems so simple at first glance is actually so complicated. Porting is kinda like a chess game where every move affects everything else and to be successful you have to be able to think the whole game in advance.

 

[DavidP71801]

And you aren’t filling full displacement. You’re only filling the upstroke after the exhaust. Roughly 100 degrees of 180. That’s not quite 60% of displacement. Maybe that’s where the case volume guidelines come from.

Here is my DUH moment I knew was coming. Wow that makes a lot of things make way more sense.

 

[Ketchup]

Everything has to match the saw, and different porting can change things.

I believe some 044/046 hybrids benefit by increasing case volume because the displacement is larger than the original 044.

If the saw sucks in more air / fuel due to the increase, transfer speed will not fall off so much.

Enlarging the case volume (usually by increasing the size of the lower transfers) will often increase low end torque and reduce peak RPM resulting in a saw with a broader power curve.

Of course, so much depends on other factors, but everything has to work together. Port timing, muffler modes, timing advance will all factor in.

IMO, results will always be somewhat based on experimentation, and if you find "lightning in a bottle", remember it!

This seems to be the conventional wisdom and as a principle I agree but…

The first part I find hard to understand is how the saw accesses a volume of charge greater than the displacement of the piston. Is some burning while the transfers are open? Does a saw consume more fuel under load? How? I have had a few saws that “wake up” under medium/heavy load and you could actually feel them pulling harder. One was a ported 7900, the other a ported 020t. Quite different engine designs and displacement.

Second, there’s all these new saws with stuffers and case fillers. When you remove them you lose power and you don’t gain any bottom end. They do all seem to be of the front fed transfer design.

Third, looking at dyno graphs, we don’t see a distinct pattern of higher torque at lower rpm in saws with greater case volume compared to similar saws with tighter cases. Compare an 044 to a 462 for example. Or 572 to 372. It’s actually the opposite. The newer, tighter saws hold a flatter torque curve and the older saws have more of a distinct power band hump. Typically the older saw doesn’t even reach the same torque numbers and if it does it’s in a more distinct RPM range.

On the 440/460 hybrids, they don’t seem all that peaky until you port them. Then adding some volume to the lowers does seem to widen out their range. What does that tell us?

All anecdotal and with endless variables, but it’s stuff that nags at me.

 

[MustangMike]

This seems to be the conventional wisdom and as a principle I agree but…

The first part I find hard to understand is how the saw accesses a volume of charge greater than the displacement of the piston. Is some burning while the transfers are open? Does a saw consume more fuel under load? How? I have had a few saws that “wake up” under medium/heavy load and you could actually feel them pulling harder. One was a ported 7900, the other a ported 020t. Quite different engine designs and displacement.

Second, there’s all these new saws with stuffers and case fillers. When you remove them you lose power and you don’t gain any bottom end. They do all seem to be of the front fed transfer design.

Third, looking at dyno graphs, we don’t see a distinct pattern of higher torque at lower rpm in saws with greater case volume compared to similar saws with tighter cases. Compare an 044 to a 462 for example. Or 572 to 372. It’s actually the opposite. The newer, tighter saws hold a flatter torque curve and the older saws have more of a distinct power band hump. Typically the older saw doesn’t even reach the same torque numbers and if it does it’s in a more distinct RPM range.

On the 440/460 hybrids, they don’t seem all that peaky until you port them. Then adding some volume to the lowers does seem to widen out their range. What does that tell us?

All anecdotal and with endless variables, but it’s stuff that nags at me.

First, comparing a 044 to a 462 when the transfer designs are about 50 years apart is, IMO, not practical.

Second, I think most of the time if the factory adds crank stuffers, it is because they knew the saw needed them. The case was likely a bit big for the displacement to begin with.

Third, ported 460s are often very strong and not usually peaky ... when you put a 460 cylinder on the smaller 440 case, it usually will benefit from a little additional case capacity (everything should be in balance). Otherwise, the hybrid will often be higher revving but more "peaky" than the 460.

Fourth, I'm sure there are a lot of factors determining how much air/fuel will enter the crankcase, one of them being case capacity. Also keep in mind that we are dealing with an air/fuel mixture, not a fluid, so it can be denser or compressed. If case volume is increased, backpressure on the incoming mixture will be delayed (at least that is the way I visualize it). And, within limitations, the more air/fuel available for transfer the better.

Usually, anything that can be done to improve performance can also be overdone to detract from performance. It is all a balance.

 

[huskihl]

I would rather just leave the exhaust and transfers a couple degrees lower on the hybrid to tame it down. You get the best of both worlds that way.

Adding case capacity doesn’t add torque. Adding case capacity lowers peak horsepower which gives the feeling of being more friendly on the dogs. If the horsepower isn’t there making it scream 2000 more RPM, it’s less likely to fall off and feels more forgiving. Like a pipe on a trail bike as compared to one on a race bike

 

[Mastermind]

I would rather just leave the exhaust and transfers a couple degrees lower on the hybrid to tame it down. You get the best of both worlds that way.

Adding case capacity doesn’t add torque. Adding case capacity lowers peak horsepower which gives the feeling of being more friendly on the dogs. If the horsepower isn’t there making it scream 2000 more RPM, it’s less likely to fall off and feels more forgiving. Like a pipe on a trail bike as compared to one on a race bike

The best running hybrid I've seen was one Deets brought to my gtg. He actually filled in a little of the lowers with epoxy on it...making the case volume even smaller. I'm sure you know the saw I'm talking about.

 

[MustangMike]

The best running hybrid I've seen was one Deets brought to my gtg. He actually filled in a little of the lowers with epoxy on it...making the case volume even smaller. I'm sure you know the saw I'm talking about.

Yes, but it had a velocity stack and no air filter. I'm referring to real "woods port" saws.

He also said he did not touch the upper transfers.