Question about Crank Stuffers Case Volume and Primary Compression

[Ketchup]

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.

044 is quite different from 462 no doubt. I’m still trying to get my head around 462. 572 as well.

With most of the front fed jugs I think a lot of the case capacity is in the transfer tunnels, especially the ones with dual lowers and quad uppers. I agree that the stuffers are a compensation for the added volume in the tunnels. I do wonder if putting the volume closer to the uppers has inherent benefits.

I haven’t messed with 44/46 hybrids in several years. Memory may not serve. I just remember being surprised on the first one that it wasn’t a screamer at all. Ran just like a 460. My subsequent port work just muddied the waters further. I think the low and wide exhaust approach might have not been the best.

You say above,
“If case volume is increased, backpressure on the incoming mixture will be delayed…”
This interests me a lot and is something I’ve thought about before. My current thinking is greater case capacity will increase the duration of transfer a little farther past BDC but with a weaker flow. (I’m not meaning the full duration of open transfers, just from open until the uppers no longer push charge). I used to think this was a behavior of dual uppers and indicated a need for longer blowdown. Now I’m less sure. Almost all saws with duals have relatively greater case capacity. One or both factors may benefit from longer blowdown. But then there’s 7900, which adds to the confusion.

I still wonder if the combustion in the chamber is able to draw more charge from the transfers. If that can happen, I could see how a larger case reserve would be a big performance factor.
@DavidP71801, I think you were talking about a clear two stroke engine. They exist.

This isn’t the video I was hunting for, but there’s a good one out there with slow-motion close-ups of combustion. Unfortunately, the engine is intentionally retarded so the camera can get the shot, so I don’t know how much more combustion is happening at full speed. But from the video, I don’t see any combustion leading into the transfer uppers.

@MustangMike, you’re dead on about too much of a good thing. The right amount of change is as much the question as which changes to make.

 

[huskihl]

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

You could have put 5 HD2 filters on it that day and Crouching Tiger Hidden Dragon still wasn’t beating it, Mike. That saw is just as much a work saw as anything else

 

[Mastermind]

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.

When I ran it, it was equipped with an air filter. And it was still the most impressive hybrid I'd ever ran.

My point....which you furthered, is that there is no "one size fits all" approach to porting a chainsaw engine.

 

[MustangMike]

No doubt that Hybrid was a screamer, but I don't think I've seen another match it, and I never ran it personally. But I believe when it was dynoed it had a velocity stack and no air filter.

Tiger Hidden Dragon

My Asian 660 did a respectable 8.6 Hp with a broad torque curve, so nothing to be embarrassed about. In fact, that saw is quite strong and still runs strong. It noodles large hardwood rounds easily.

I also have a Hybrid that Doc Al did for me that has tons of torque. My nephew (MechanicMatt) and I both love running that saw with 28" full comp in hardwood.

 

[bwalker]

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.

The newer saws are strato charged so cylinder scavenging is greatly improved. As such you can't compare than with traditional two strokes.
Any two stroke does consume more fuel under load.
The rest I can't speak too.

 

[bwalker]

There is something we need to consider in regards to case volume. Much of what is written on the subject is speaking of motors that use a tuned pipe. IE with a tuned pipe increasing case volume increases power curve width and torque and decreasing does the opposite. The reason for this is the increased case volume acts as a reservoir for the pipe to suck out off. This increased air/fuel mix can then be jammed back in once the piston closes the transfers.
With a muffler saw there is none of these resonance effects going on so increasing case volume just makes for lazy flow though your transfers.

 

[Ketchup]

The newer saws are strato charged so cylinder scavenging is greatly improved. As such you can't compare than with traditional two strokes.
Any two stroke does consume more fuel under load.
The rest I can't speak too.

You’re saying strato saws make more power than conventional now? I agree. I think it’s due to both better chamber conditions and tighter cases. And maybe more volume near the uppers.
I agree on fuel consumption as well.

There is something we need to consider in regards to case volume. Much of what is written on the subject is speaking of motors that use a tuned pipe. IE with a tuned pipe increasing case volume increases power curve width and torque and decreasing does the opposite. The reason for this is the increased case volume acts as a reservoir for the pipe to suck out off. This increased air/fuel mix can then be jammed back in once the piston closes the transfers.
With a muffler saw there is none of these resonance effects going on so increasing case volume just makes for lazy flow though your transfers.

This is the relationship I am thinking about, but instead of draw being caused by the pipe, it’s a smaller effect just by combustion. That might make an argument for extra volume in the case as a reservoir, but less is needed and it’s easier to tip over into slow transfers. Like there’s a sweet spot between 60-75% displacement.

 

[bwalker]

You’re saying strato saws make more power than conventional now? I agree. I think it’s due to both better chamber conditions and tighter cases. And maybe more volume near the uppers.
I agree on fuel consumption as well.



This is the relationship I am thinking about, but instead of draw being caused by the pipe, it’s a smaller effect just by combustion. That might make an argument for extra volume in the case as a reservoir, but less is needed and it’s easier to tip over into slow transfers. Like there’s a sweet spot between 60-75% displacement.

Strato charged saws make more torque/broader power curve than traditional two strokes because they scavenge better. More HP over all is debatable. This has nothing to do with the case volume... take the 372 are and XT for instance. Both have basicly the same HP but the XT has more torque.
Their is no "draw" from combustion in a non piped motor. It's all about pressure from the position of the piston.
The other thing is for get about v8 4 strokes. Nothing in common with a two stroke.

 

[Ketchup]

Strato charged saws make more torque/broader power curve than traditional two strokes because they scavenge better. More HP over all is debatable. This has nothing to do with the case volume... take the 372 are and XT for instance.
Their is no "draw" from combustion in a non piped motor. It's all about pressure from the position of the piston.
The other thing is for get about v8 4 strokes. Nothing in common with a two stroke.

Makes me want to try stuffers in an XT.

 

[bwalker]

Makes me want to try stuffers in an XT.

Do you know what the crankcase volume is of an XT vs a XP?

 

[Ketchup]

Do you know what the crankcase volume is of an XT vs a XP?

No but I expect it’s slightly larger than OE because the bore is taller. I don’t recall much difference between cases.

 

[bwalker]

No but I expect it’s slightly larger than OE because the bore is taller. I don’t recall much difference between cases.

Cases are the same as is the rod length.
Don't hold me to this as I sm guessing, but to my eye the OE appears to have more volume than the XT.
Maybe Randy ot Kevin can comment?

 

[Mastermind]

Cases are the same as is the rod length.
Don't hold me to this as I sm guessing, but to my eye the OE appears to have more volume than the XT.
Maybe Randy ot Kevin can comment?

XP likely has more, XT piston takes more space up I think.

 

[Ketchup]

XT on the left. Maybe 1.5cc more space under the crown. Crank stuffers would probably take up 4-6 times that. They did it on 575 IIRC, then on 572.


Edit: I did some measurements. I was way off. Measuring the height difference of the interiors (7.7mm) and multiplying by a radius of 20mm I get 9.67cc. That’s a fair extra bit.

 

[huskihl]

Cases are the same as is the rod length.
Don't hold me to this as I sm guessing, but to my eye the OE appears to have more volume than the XT.
Maybe Randy ot Kevin can comment?

I think the cases are exactly the same other than being cut out for the longer cylinder extension on the XT. But the XT would have more case compression just from the heavier piston. If the piston weighs more, it doesn’t have a choice but to take up more space unless it’s made of a different alloy

 

[Outback]

I've watched a lot of dyno video's. Has anybody made more power with 70cc's without a pipe than the oe 372xp at this time? Please correct me if I'm wrong(with links to video's, not opinions). I would think the definition of the most effecient scavenging would equal the most air fuel in the cylinder and then the most power. If so I would argue that it has the most efficient scavenging without stuffers of any saw. Strato's blow fresh air, so they are the most efficient at fuel to power, ie not wasting fuel, not total power, ie fuel used for combustion. I accept that a lot more development has gone into that platform over the years than any other and could skew results.

 

[drf256]

When I ran it, it was equipped with an air filter. And it was still the most impressive hybrid I'd ever ran.

My point....which you furthered, is that there is no "one size fits all" approach to porting a chainsaw engine.

That saw was and is impressive. Pretty sure it’s back in Deet’s hands. A total worksaw that he made a V stack for. It’s was impressive even with an air filter on it.

It’s a unicorn jug. Tight square uppers which were left stock/oem with ZERO grinding. Stock tunnels exc for some JB in the lowers. D chambered early 046 jug.

Made me rethink a lot of theory.

You can do 1000 saws the same way and one will stand out. This is the one.

 

[Ketchup]

I think the cases are exactly the same other than being cut out for the longer cylinder extension on the XT. But the XT would have more case compression just from the heavier piston. If the piston weighs more, it doesn’t have a choice but to take up more space unless it’s made of a different alloy

That would make sense in the same cylinder, but the XT bore is taller. The piston windows offset it some, but I’m relatively sure now XT has more case volume. Not that it matters much. OE and XT are different jugs and carbs for the same saw. I think they designed the XT to the same specs as the OE.

I think it’s interesting that grinding the covers on a 365xt makes gains. Is it the case volume, the proximity or reduced turbulence? 359 also gains from opening up the tunnels/covers.

 

[MustangMike]

That saw was and is impressive. Pretty sure it’s back in Deet’s hands. A total worksaw that he made a V stack for. It’s was impressive even with an air filter on it.

It’s a unicorn jug. Tight square uppers which were left stock/oem with ZERO grinding. Stock tunnels exc for some JB in the lowers. D chambered early 046 jug.

Made me rethink a lot of theory.

You can do 1000 saws the same way and one will stand out. This is the one.

Al, I'm not saying it is anything like Deets saw, but after my busy tax season I've got to make some time so you can get some trigger time on the hybrid you did for me, which (IIRC) also has a D jug.

It is a torque monster, and very pleasant to run.

I know you did some pretty special work to it ... it worked!

 

[bwalker]

I've watched a lot of dyno video's. Has anybody made more power with 70cc's without a pipe than the oe 372xp at this time? Please correct me if I'm wrong(with links to video's, not opinions). I would think the definition of the most effecient scavenging would equal the most air fuel in the cylinder and then the most power. If so I would argue that it has the most efficient scavenging without stuffers of any saw. Strato's blow fresh air, so they are the most efficient at fuel to power, ie not wasting fuel, not total power, ie fuel used for combustion. I accept that a lot more development has gone into that platform over the years than any other and could skew results.

When I mentioned stratos scave ginger better I am talking about the ability to clear the cylinder of exhaust gas. A traditional two stroke does this well over a pretty narrow band compared to a strato charged engine. And torque is a measure of cylinder filling efficiency of fresh charge.