Porting

[Sierra_rider]

I'll also add that my opinion on intake duration has changed somewhat as I've experimented. I used to keep my intake duration in the 150* range, just because I've heard that's good. I've since gone up to 160*+ on some of my builds with good success.

My current experiment is on my 500i and how to get 160* duration after machining, w/o use of epoxy. The experiment hasn't gone perfectly, but it shows promise.

 

[huskihl]

I'll also add that my opinion on intake duration has changed somewhat as I've experimented. I used to keep my intake duration in the 150* range, just because I've heard that's good. I've since gone up to 160*+ on some of my builds with good success.

My current experiment is on my 500i and how to get 160* duration after machining, w/o use of epoxy. The experiment hasn't gone perfectly, but it shows promise.

I used to use epoxy in the intake on the 500. It made the saw rev a little quicker and it ran a shorter bar better but long bar torque and forgiveness suffered. The intake lands at 178 duration after lathe work and I leave it there now

 

[link]

You guys talk chineese all day...

 

[ApePilot]

I used to use epoxy in the intake on the 500. It made the saw rev a little quicker and it ran a shorter bar better but long bar torque and forgiveness suffered. The intake lands at 178 duration after lathe work and I leave it there now

Thanks guys!
I'm pretty ignorant when it comes to the new saws, like strato-charge and electronic fuel injection, at least for 2T engines.
Does the 500i have more or less a throttle body injector system that monitors the incoming air volume through the intake? If so then I guess it pretty much relates to a carb system, except that the injection would likely occur during the actual intake impulse, rather than both intake and exhale like a carb/piston-port does?
My 3120 had been spitting a little crankcase spooge through the carb at times, but it completely stopped spitting after I had enlarged and radiused the lower transfers and piston windows. I don't see how that would have stopped the spitting, except that maybe it slightly reduced the primary compression with the removal of a very small amount of material and provided better flow, but perhaps something else was involved.
Perhaps getting the intake duration to at least 140 deg might be helpful for higher rpm torque and not worrying about the slobbering at low and mid-rpm ranges.
178 deg of duration sounds extreme to me, but if it provides better torque at higher working rpms on that particular saw platform, hey, why not?!
Just to verify that I'm reading you correctly, the 178 deg intake duration contributes to high-rpm torque? I would have thought that doing that would severely reduce primary compression. Maybe not?
I'm assuming the 500i has crank-stuffers too. I'm guessing that you can get away with a bit more intake duration with a higher primary compression due in part to the stuffers?
Thanks for all the little hints.

Oh, and thank you link for your such valuable insights.

 

[ApePilot]

I used to use epoxy in the intake on the 500. It made the saw rev a little quicker and it ran a shorter bar better but long bar torque and forgiveness suffered. The intake lands at 178 duration after lathe work and I leave it there now

Just did a quick calc, and found that I would have to lower my intake port by just over 8mm to achieve 178 deg duration.
What is the saw's stock intake duration?

 

[Sierra_rider]

Just did a quick calc, and found that I would have to lower my intake port by just over 8mm to achieve 178 deg duration.
What is the saw's stock intake duration?

I'd have to go through my notes, but my 500i was around 168* of duration stock. I'm playing around with machined 660 pistons and my current version of the build is down to around 164* even after cutting squish and decking the cylinder.

@huskihl can comment whether I'm off-base on this, but I speculate that the long intake timing has to do with crankcase compression and the long transfer tunnels on the newer saws. At BDC, the cylinder sits very close to the cases and even has a stuffer that takes up space on the intake side. Most of the newer Stihls I've taken apart seem to have a lot of intake duration.

 

[huskihl]

Just did a quick calc, and found that I would have to lower my intake port by just over 8mm to achieve 178 deg duration.
What is the saw's stock intake duration?

168-170 is stock. Opens at 84-85

 

[huskihl]

Thanks guys!
I'm pretty ignorant when it comes to the new saws, like strato-charge and electronic fuel injection, at least for 2T engines.
Does the 500i have more or less a throttle body injector system that monitors the incoming air volume through the intake? If so then I guess it pretty much relates to a carb system, except that the injection would likely occur during the actual intake impulse, rather than both intake and exhale like a carb/piston-port does?
My 3120 had been spitting a little crankcase spooge through the carb at times, but it completely stopped spitting after I had enlarged and radiused the lower transfers and piston windows. I don't see how that would have stopped the spitting, except that maybe it slightly reduced the primary compression with the removal of a very small amount of material and provided better flow, but perhaps something else was involved.
Perhaps getting the intake duration to at least 140 deg might be helpful for higher rpm torque and not worrying about the slobbering at low and mid-rpm ranges.
178 deg of duration sounds extreme to me, but if it provides better torque at higher working rpms on that particular saw platform, hey, why not?!
Just to verify that I'm reading you correctly, the 178 deg intake duration contributes to high-rpm torque? I would have thought that doing that would severely reduce primary compression. Maybe not?
I'm assuming the 500i has crank-stuffers too. I'm guessing that you can get away with a bit more intake duration with a higher primary compression due in part to the stuffers?
Thanks for all the little hints.

Oh, and thank you link for your such valuable insights.

It was more forgiving on the dogs, stayed on the pipe better with more intake.
But the 500 is different than other saws. It doesn’t need velocity through the venturi to pull fuel. The injector is about 1” from the bore.
I believe it has an impulse sensor, temp sensor, and crank position sensor and searches for the highest rpm in the cut via richening/leaning the mix every so often

 

[huskihl]

I'd have to go through my notes, but my 500i was around 168* of duration stock. I'm playing around with machined 660 pistons and my current version of the build is down to around 164* even after cutting squish and decking the cylinder.

@huskihl can comment whether I'm off-base on this, but I speculate that the long intake timing has to do with crankcase compression and the long transfer tunnels on the newer saws. At BDC, the cylinder sits very close to the cases and even has a stuffer that takes up space on the intake side. Most of the newer Stihls I've taken apart seem to have a lot of intake duration.

The whole top of the crankcase is a stuffer. There’s just a slot for the rod

 

[ApePilot]

It was more forgiving on the dogs, stayed on the pipe better with more intake.
But the 500 is different than other saws. It doesn’t need velocity through the venturi to pull fuel. The injector is about 1” from the bore.
I believe it has an impulse sensor, temp sensor, and crank position sensor and searches for the highest rpm in the cut via richening/leaning the mix every so often

whoa! whoa! whoa!.... hold up there... Did you say 'stayed on the pipe,' hinting that you have a tuned expansion chamber on it?

 

[ApePilot]

So, I don't see any crankshaft stuffers here. Interestingly different case top geometry, though. One transfer?

 

[Lightning Performance]

I used to use epoxy in the intake on the 500. It made the saw rev a little quicker and it ran a shorter bar better but long bar torque and forgiveness suffered. The intake lands at 178 duration after lathe work and I leave it there now

Numbers don't lie, period.
But....
Consider who built the port window profile here. Not a guy that did 486 of one model or 2 of everything. He has years of experience now.
Doc is damn good at doing the window profile and smoothly blending it into the 260 or 360 intake runners. I think Kevin has hit his own stride now building what he thinks it wants to pull more cutters not just go fast.
Down the rabbit hole we go

I tend to agree with this but not in all instances. Quad port vs a dual port has a tendency with good flow to not cause as much low speed spit back as a dual port does imo just by design. It should be able to handle a longer intake duration while still retaining it's manners on the low end.
Going to try answering another question someone asked on the last page.

 

[Lightning Performance]

I have a question that's been bugging me and I've only heard what copy-cats seem to indicate about the intake port.... that duration should be around 140 - 150 degrees. However, if you look here:

Dyno Joe's 3120xp build's torque seems to peter-out just after 8400 rpm, whereas the stock's torque curve began to nose-dive after 9200 rpm, even though of course the ported saw had far higher torque throughout the rpm range.
I have similar port numbers compared to Joe's except for my intake duration is stock at 135.8 vs Joe's 140 deg, but I also have a tighter squish (0.0228" vs joe's 0.027") and likely a bit higher true compression.

What does a longer intake duration do for the torque curve at higher rpms (8500-10,000)?

Right now the saw runs quite well and CHT hovers just below 395 deg F on 100LL at 24:1 on extended wot cuts, but I'm always looking for a bit more on top. Most of my long cuts are running at about 9600 - 9700rpm but if I could get it up a touch higher (10,000)...?
Thanks!
-doug

First things first.
What is your intended purpose for this 3120?
Is it modified under those covers?
Is it stock inside?
We need facts.
Numbers vary according to how and who took them.

If you need more air/fuel maybe you need to redesign the port window and possible fix the intake runner area, maybe not.

How many cutters are you pulling in the wood?

Let's see how far this goes for something to build on. Pulling ideas and numbers out of thin air is a waste of time, period. Combinations are the only thing that works. Miss one part and you missed it all.

Kinda knew no one would try to answer my question asked earlier. It's a real simple answer imo.

 

[Lightning Performance]

I'll also add that my opinion on intake duration has changed somewhat as I've experimented. I used to keep my intake duration in the 150* range, just because I've heard that's good. I've since gone up to 160*+ on some of my builds with good success.

My current experiment is on my 500i and how to get 160* duration after machining, w/o use of epoxy. The experiment hasn't gone perfectly, but it shows promise.

Why do you suspect it needs more air in?

 

[Lightning Performance]

Having nothing but trouble today to stay linked up in this thread. Drop link, error messages and not in service keep coming up.
My service here is just fine. Almost like someone doesn't want me posting here. That's wouldn't be the first time it's come up.

 

[Sierra_rider]

Why do you suspect it needs more air in?

I'm actually shrinking the intake duration compared to stock or what I would end up with if I just decked the cylinder with the stock piston. Stock opened at 84* BTDC, version 1 of my build opened at 80*, and version 2(which I haven't cut with yet) opens at 82*.

 

[Sierra_rider]

So, I don't see any crankshaft stuffers here. Interestingly different case top geometry, though. One transfer?
View attachment 1036427

It's not a stuffer like you'd find in the crank of a 2 stroke dirtbike, but that "appendage" and the "shrouding"(for lack of a better term) around the base serve to limit crankcase volume.

 

[huskihl]

whoa! whoa! whoa!.... hold up there... Did you say 'stayed on the pipe,' hinting that you have a tuned expansion chamber on it?

No, just a dirt bike reference. 300lb guy on a KX65. The bike needs to be on the pipe to go anywhere

 

[ApePilot]

This graph has more info in it than meets the eye.
Maybe this weekend or the next rainy evening I can catch up in this thread.

That graph shows a stable flow pattern. Notice how the power doesn't dive late in the game here in the higher RPM window. Who knows why this so and many ported simple chainsaws will not follow this pattern?


I'm betting most won't have an answer because they don't understand the why. Asking the right questions is how others can learn. Most just copycat others work, like MM, and don't have a clue about the real things going on inside the engine. If you at least grasp the basic concepts of flow your work will be better without hogged out ports or follow the "in" crowds from build to build. I've personally found the dynamic flow more important that any port size on the intake or exhaust in a dead simple two stroke piston ported engine. The total port area does not matter as much as the shape. If it has poppet valves with a cam shaft then your cross section and area becomes critical as RPMs increase. This carries over to these small engine s but not in a big way like valves do.

You can't get out what you can't get in.

Here's my feeble attempt at answering this question.... 'why does this power graph not dive at higher rpms?'
Clues: it's not all about port window area. Corners, sharp or kinda rounded, hurt flow. Square-ish corners through tunnels, corners in windows, sharp surface edges in inlet windows (inside exhaust port, lower transfer port, carb intake,etc), all hurt flow. Minimizing flow-drag is paramount to high-rpm torque, ei attempting to cram as much clean A/F charge into the combustion chamber as possible after efficient evacuation of the spent charge.
Perhaps the most efficient method of improving torque at high rpms (especially if not using a tuned expansion chamber) is to encourage smooth, laminar transfer flow. At higher rpms, the tunnel reynolds numbers become very high, creating more turbulence and therefore drag than at lower rpms, thus making far more resistance to efficiently filling the combustion chamber. The easiest way of reducing the high RNs is to slow the flow velocities down, while still providing more flow volume within the transfer duration.
How do we do all of that?
First, provide very smooth and radiused transfer entry windows (very large raduised corners) as well as smooth and open piston windows. Also provide means to smooth flow through the crankcase up into the transfers.
But if your cylinder has a dual-transfer system, the RNs will still be higher than a quad-transfer system, perhaps.
Are fingers the answer here for dual-transfer jugs? Prof. Gordon Blair mentioned this in his writings. Randy also used this to excellent effect in his 3120 build. I was surprised that Randy didn't really do much in the way of radiusing the surface corners in the lower transfers, nor did it appear that he did any massaging of the piston windows to facilitate smoother flow.
Also, by providing a larger transfer area with smooth, lower RN flow, we in effect create a longer blow-down period since the new A/F charge has a slower velocity but higher volume, something like a large open-ended water hose providing more volume but at a lower velocity vs the same hose with a tighter spray nozzle on the end.
https://www.arboristsite.com/threads/mastermind-meets-the-husqvarna-3120.202384/page-11

 

[ApePilot]

This article gives us an idea of the benefits (reduced drag and higher volume flows) of radiusing the inlets to ports such as lower transfers, exhaust port, carb inlets, etc.
http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdfSo area of a port per se is not the only prerequisite for efficient flow. It's the shape of the entry into that port that makes the largest difference in flow characteristics. Area is important, but is secondary to entry shape.