Case Compression, Let's Talk About It

[Ketchup]

Higher intake makes more vacuum in the crankcase before the piston open the intake, espesially if the crankshaft have stuffers.

I don’t see that as a bad thing. Do you? How much do you think the stuffers increase vaccum?

 

[Bjorn]

I don’t see that as a bad thing. Do you? How much do you think the stuffers increase vaccum?

higher vacuum is a good thing, it create a stronger pull through the carburator.

It is hard to say a number, how much the stuffers increace vacuum. But a small case Will build vacuum faster and harder before the intake open.

 

[Ketchup]

That’s how I see it too. More case vacuum should pull faster through the carb, fill the case faster and have more momentum after TDC.

But what about large volume cases pulling more torque? Is that a myth? I feel like some of my large case saws can really grind. My saws with stuffers are high strung cupcakes. Many people prefer the 357xp with the stuffers removed. Why more torque when fuel is moving with less velocity?

And what about case turbulence? The crank weights really stir things up compared to stuffers. Can turbulence add force to the transfer opening? @Red97 documented gains with a turbo crank mod. What’s the mechanism there?

 

[wcorey]

I think it's sort of accepted that less case volume pushes the peak power up higher in the rpm range, which begets more peak power but then typically narrows the power band/torque curve. This could account for the 'peaky' or 'torquey' thing...

Just spitballing here... The effect of seemingly excess case volume and why it's beneficial to have more than the piston displaces is a mystery to me. But...
A compressed gas has 'springiness' to it, so changing the case volume while maintaining the same amount of case compression alters the rate of that spring and therefore the harmonics/timing of the event.

 

[Red97]

That’s how I see it too. More case vacuum should pull faster through the carb, fill the case faster and have more momentum after TDC.

But what about large volume cases pulling more torque? Is that a myth? I feel like some of my large case saws can really grind. My saws with stuffers are high strung cupcakes. Many people prefer the 357xp with the stuffers removed. Why more torque when fuel is moving with less velocity?

And what about case turbulence? The crank weights really stir things up compared to stuffers. Can turbulence add force to the transfer opening? @Red97 documented gains with a turbo crank mod. What’s the mechanism there?

Was around 3% towards the peak rpm. I'm guessing more to do with reduced weight than any loss/pumping efficiency.

More torque "feel" may be just a lack of hp/rpm

Just a random example.

50cc saw that spins 16k free rpm 5hp and 2.6 ftlb

Or 50cc saw that spins 11.5k 4.2 hp and 2.6 ftlb

Same operator is going to pull the saws down to 7.5-8k max.

High strung saw dropped close to 5k rpm and the lower rpm saw only loses 3k in the cut. So it is perceived as having better tq or holding ability but it is more a lack of rpm as the peak torque on both is the same.

 

[huskihl]

More torque "feel" may be just a lack of hp/rpm

Very misunderstood concept, as we proved with the “torque monster” saws. They don’t really have any more torque than slightly smaller saws. They just simply lack the horsepower

 

[Ketchup]

I think it's sort of accepted that less case volume pushes the peak power up higher in the rpm range, which begets more peak power but then typically narrows the power band/torque curve. This could account for the 'peaky' or 'torquey' thing...

Just spitballing here... The effect of seemingly excess case volume and why it's beneficial to have more than the piston displaces is a mystery to me. But...
A compressed gas has 'springiness' to it, so changing the case volume while maintaining the same amount of case compression alters the rate of that spring and therefore the harmonics/timing of the event.

I guess I originally thought of the entire case as shifting from one pressure stage to another in a single rotation. Now I feel that based on the speed of rotation that becomes impossible. Localized areas of pressure pulses makes more sense. What’s happening at the transfer lower can be very different than what is happening at the intake, or at the crank lobes. A full circle or stuffed crank have a different dynamic again.

As far as harmonics, there’s even more to think about. Where is it possible? What is the effect? Do we even want a harmonic? Isn’t that the twin brother of cavitation?

 

[Red97]

Hope this shows what I was trying to describe. The sxlao is a 58cc reed valve "torque monster" type saw. Vs the 56-7cc "High strung " modern saw

The 357 is being pulled down 5k rpm before it matches and then starts to "lose" to the super xl.

.24 ftlb different peak to peak. Works out close to 8% but in the wood and listening to the rpm it is easy to believe the old saw has more tq.

 

[wcorey]

Do we even want a harmonic?

There are harmonics at play in pretty much everything that moves, sort of unavoidable.

 

[Maintenance Chief]

Its alot easier to see the homelite's torque start from 3 grand also ,so if you've got the saw bar resting on the log ,its gonna start walking through the log without having to build RPMs . Which is nice when cutting a face or blocking firewood against the spike.
Perception is that theres more torque but the fact that it available at lower rpms is nice.

 

[Ketchup]

Hope this shows what I was trying to describe. The sxlao is a 58cc reed valve "torque monster" type saw. Vs the 56-7cc "High strung " modern saw

The 357 is being pulled down 5k rpm before it matches and then starts to "lose" to the super xl.

.24 ftlb different peak to peak. Works out close to 8% but in the wood and listening to the rpm it is easy to believe the old saw has more tq.View attachment 343499

I see what you're saying. In this example the old saw actually does have more torque. I could see a case where the same model saw with different porting could have the same torque curve but one hits a higher peak HP well above cutting RPM. They perform the same but the slower saw seems more powerful because it doesn't drop as far in the cut.

Just looking at your graphs in general, it seems most show a rise in the torque curve and peak HP fom porting. Do you think individual elements of the port work effect the torque curve and HP curve seperately, or does nearly every mod effect both?

IE: All other port work being the same, does the difference between an intake at 76 effect torque and HP difderently than an intake at 80? Does it effect one more than the other?

There are harmonics at play in pretty much everything that moves, sort of unavoidable.

Definitely. I'm just thinking general chaos might be better in there than a dominant synchronized vibration.

 

[Red97]

I see what you're saying. In this example the old saw actually does have more torque. I could see a case where the same model saw with different porting could have the same torque curve but one hits a higher peak HP well above cutting RPM. They perform the same but the slower saw seems more powerful because it doesn't drop as far in the cut.

Just looking at your graphs in general, it seems most show a rise in the torque curve and peak HP fom porting. Do you think individual elements of the port work effect the torque curve and HP curve seperately, or does nearly every mod effect both?

IE: All other port work being the same, does the difference between an intake at 76 effect torque and HP difderently than an intake at 80? Does it effect one more than the other?



Definitely. I'm just thinking general chaos might be better in there than a dominant synchronized vibration.

Most normal "worksaw" porting will simply elevate the curve. Keeps mostly the same shape, just more power everywhere.

Now if someone is really pushing, or start with a low rpm saw then the actual curves will be stretched out further. Most of the echo are like this. Choked up and make peak power at a lower rpm. After porting power is made at higher rpm so the graphs are raised and stretched vs stock.

 

[wcorey]

Definitely. I'm just thinking general chaos might be better in there than a dominant synchronized vibration.

I guess general chaos would be kind of a neutral but synchronization in the right frequency, place and timing could be very beneficial, and just the reverse if it's out and fighting in the opposite direction.
Tuned pipes are an obvious example of synchronized harmonics at work but it's happening in a myriad of ways through the whole process, from the air filter to the exhaust outlet.
Way over my head to take advantage of other that incremental experimenting with timing and angles, part of the dance with compromise we all do with porting.

 

[Mastermind]

I just wanna say one thing.


I like turtles.

 

[Ketchup]

View attachment 343594

 

[Stump Shot]

I just wanna say one thing.


I like turtles.

That's plural,just saying.

 

[Bjorn]

Hello, all chainsaws have big piston diameter and short stroke, to make small vibration. It is very hard to make an engine like that to have Good torque. If you get an engine with a 50mm piston and 50mm stroke that Will give torque and power. Espesially with reeds, so the intake is open almost 180 degrese and the case compression gets almost 180 dregrese of compression. That will be a real monster.

 

[isaaccarlson]

More case volume means more opportunity to cram extra charge into the case, but your timing/port flow has to be right to make it happen. Smaller case volume means less extra charge but it's quicker to build pressure and vacuum. Either one if fine, depending on the saw. I think smaller saws benefit from less case volume and quicker/stronger case compression/vacuum and bigger saws benefit from bigger case volume and packing more charge in.

I have enough intake timing on my ms390 to be on the ragged edge of spit back. Intake velocity needs to be relatively high to pack that charge in there. That means clean high flow air filters and an open exhaust that flows well. Everything has to work together. It starts with the exhaust and ends with the intake. You can't go the other way. You can't put more in than you can get out.

I like ports that flow. Transfer velocity is no good if the ports can't move enough fast enough. Velocity is exponential in energy, so making something move twice as fast takes 4 times more energy.

 

[Wonkydonkey]

I just wanna say one thing.


I like turtles.

Can I ask…. Is that Baked or deep fried

 

[Mastermind]

Can I ask…. Is that Baked or deep fried

Yes.