Porting tools what's out there for the poor man like me.

[MG porting]

I'm pretty ignorant about such things, but my understanding is that one would rather leave the "throat" of the transfer tunnel alone as widening decreases transfer velocity (visualize putting your thumb over the end of a garden hose to increase the power of the stream of water).

Someone wanna school me???

That's pretty much how I see it that's why I ask him how much to widen but I'm always willing to lern something new. Lol.

 

[odin]

Same with lowers. Open them up too much and you start to lose bottom-end pressure and problems ensue. Or so I've thought...

 

[Terry Syd]

The 'choke' point is actually the port opening. The piston spends most of its' time partially blocking the port.

When you have a choke point before the port, the FLOW slows down (think of a kink in a water hose). I emphasis the word flow as that is what you are aiming for. - That is why it is called a 'flow bench' and not a 'velocity bench'.

The transfers should have a decreasing cross sectional area as the flow heads towards the port opening. In other words, the lowers are bigger than the uppers.

 

[MG porting]

The 'choke' point is actually the port opening. The piston spends most of its' time partially blocking the port.

When you have a choke point before the port, the FLOW slows down (think of a kink in a water hose). I emphasis the word flow as that is what you are aiming for. - That is why it is called a 'flow bench' and not a 'velocity bench'.

The transfers should have a decreasing cross sectional area as the flow heads towards the port opening. In other words, the lowers are bigger than the uppers.

So in theory if I open the choke point twords the intake side that should increase speed and flow. Make sense if it doesn't hert the case pressure to much.

 

[Terry Syd]

When you refer to 'case pressure' I'm assuming you are referring to the crankcase compression. Crankcase compression on chainsaws is kept relatively low compared to other two-strokes. The reason why is that a higher crankcase compression will NARROW the powerband.

You can increase the crankcase compression and it will push the maximum delivery ratio of the transfers up higher in the RPM range, however it will also tend to narrow the powerband. Chainsaws need a broader powerband, so lower crankcase compression ratios are used.

Increasing the crankcase compression does not make the crankcase more efficient, that is, it doesn't make it flow any more mixture. Racers have indicated that going to a full-circle crank will push peak power up about 400 RPM. Having the maximum delivery ratio up higher in the RPM range will create more horsepower for cutting cookies and they are willing to accept the tighter powerband for racing.

 

[Bigmac]

When you refer to 'case pressure' I'm assuming you are referring to the crankcase compression. Crankcase compression on chainsaws is kept relatively low compared to other two-strokes. The reason why is that a higher crankcase compression will NARROW the powerband.

You can increase the crankcase compression and it will push the maximum delivery ratio of the transfers up higher in the RPM range, however it will also tend to narrow the powerband. Chainsaws need a broader powerband, so lower crankcase compression ratios are used.

Increasing the crankcase compression does not make the crankcase more efficient, that is, it doesn't make it flow any more mixture. Racers have indicated that going to a full-circle crank will push peak power up about 400 RPM. Having the maximum delivery ratio up higher in the RPM range will create more horsepower for cutting cookies and they are willing to accept the tighter powerband for racing.

Good info. I know it’s a whole other subject but on a tuned pipe, reed valve motor, where scavaging is huge..having a large crankcase volume can yeild bigger gains in the top end, where the pipe will pull more mixture even after bdc, and also pulls more mix through the reeds when it really shouldn’t, in terms of crank rotation. It’s cool stuff!

 

[drf256]

So I did it can't say that it's pretty but heck it works. Lol. View attachment 133397 View attachment 133398

Looks like you did damn well. They look great.

Dont mess with the intake side hump in the intake tunnel, other than smoothing casting flash.

If you want a full out race saw with max rpm and peaky power, widen the uppers and throat.

If you want a worksaw that will keep pulling when you dog it in, and not bog down, leave the throat and the uppers stock width.

 

[Lightning Performance]

Good summation here of quality work.
I take the choke off of both sides mid port in different locations. The overall idea is to not kill velocity at the port center. Most novice grinders make this mistake and actually kill flow with a "ballooned" port. Not a term you will hear often. That one very small area on the short side turn is critical. Measure as you go. It takes about one or two seconds to ruin the port shape. Choking the port twice will kill your top end power. It does so many bad things you need a whole page to list them and how they affect each other.
Ask the port shops if you don't think this is critical.

 

[MG porting]

Looks like you did damn well. They look great.

Dont mess with the intake side hump in the intake tunnel, other than smoothing casting flash.

If you want a full out race saw with max rpm and peaky power, widen the uppers and throat.

If you want a worksaw that will keep pulling when you dog it in, and not bog down, leave the throat and the uppers stock width.

Thank you Drf256 this saw is definitely a work saw so I'm going to stay clear of the bottle neck I'm always trying to look for more torque than rpm and that can be hard sometimes. Lol.

Good summation here of quality work.
I take the choke off of both sides mid port in different locations. The overall idea is to not kill velocity at the port center. Most novice grinders make this mistake and actually kill flow with a "ballooned" port. Not a term you will hear often. That one very small area on the short side turn is critical. Measure as you go. It takes about one or two seconds to ruin the port shape. Choking the port twice will kill your top end power. It does so many bad things you need a whole page to list them and how they affect each other.
Ask the port shops if you don't think this is critical.

I understand what you are saying but I'm not building a racing saw. Lol.

 

[srcarr52]

Thank you Drf256 this saw is definitely a work saw so I'm going to stay clear of the bottle neck I'm always trying to look for more torque than rpm and that can be hard sometimes. Lol.

I understand what you are saying but I'm not building a racing saw. Lol.

These are only recommendations, not laws. It's all about balance and proper aiming of the transfer inlet. You can have big transfers if you have more blowdown, good exhaust duration and keep the intake in check so you have the crankcase compression. If these recommendations were laws this cylinder would have no torque.



But it has plenty of torque... running a 9 pin here.

 

[Lightning Performance]

Thank you Drf256 this saw is definitely a work saw so I'm going to stay clear of the bottle neck I'm always trying to look for more torque than rpm and that can be hard sometimes. Lol.

I understand what you are saying but I'm not building a racing saw. Lol.

These are only recommendations, not laws. It's all about balance and proper aiming of the transfer inlet. You can have big transfers if you have more blowdown, good exhaust duration and keep the intake in check so you have the crankcase compression. If these recommendations were laws this cylinder would have no torque.

View attachment 133495

But it has plenty of torque... running a 9 pin here.

You were just shown and told sumtin. Suck it up biches.
Very nice transfer port! Case closed.

 

[MG porting]

You were just shown and told sumtin. Suck it up biches.
Very nice transfer port! Case closed.

Lol. Where I live we still have a few big trees and require much bigger bar than that just saying but yes very cool.

 

[srcarr52]

You were just shown and told sumtin. Suck it up biches.
Very nice transfer port! Case closed.

If you are going to widen the transfer outlet, you better widen it under the bridge and all the way up. It's hard to see but if you draw a line from the back corner behind the bridge through the outlet exhaust side it points at the intake side of the opposite transfer. Ideally you'd want the two transfers exhaust side to collide just shy of center on the intake side of the cylinder. If they collide to far on the exhaust side the loop scavenging is significantly hurt and peak torque will suffer.

 

[srcarr52]

Lol. Where I live we still have a few big trees and require much bigger bar than that just saying but yes very cool.

Not to worry, they normally run long bars. But large test logs are hard to get to my property.



Here is a normal day in the life of another one I've built. It's done a lot of oak milling.

 

[brshephard]

If you are going to widen the transfer outlet, you better widen it under the bridge and all the way up. It's hard to see but if you draw a line from the back corner behind the bridge through the outlet exhaust side it points at the intake side of the opposite transfer. Ideally you'd want the two transfers exhaust side to collide just shy of center on the intake side of the cylinder. If they collide to far on the exhaust side the loop scavenging is significantly hurt and peak torque will suffer.

View attachment 133496

Am I understanding this correctly?

 

[smokey7]

@srcarr52 that 2100 sounded very strong running that setup. Maybe im a little bit of a chicken but i am scared to turn a old saw up to that high of a rpm. I worry that a 20 year old crank cant handle the extra 4000 rpm. Atleast that is what it sounds like to me. I assume the 298 1100 2100 2101 all are about a 9500-10500 saws. It sure sounds like about 13500-14000 to me. Very nice builds you do and id love to run something you have done especially one of your infamous 394s.

 

[srcarr52]

Am I understanding this correctly?
View attachment 133573

They are pointed a little further toward the intake side but you got the idea.

 

[srcarr52]

@srcarr52 that 2100 sounded very strong running that setup. Maybe im a little bit of a chicken but i am scared to turn a old saw up to that high of a rpm. I worry that a 20 year old crank cant handle the extra 4000 rpm. Atleast that is what it sounds like to me. I assume the 298 1100 2100 2101 all are about a 9500-10500 saws. It sure sounds like about 13500-14000 to me. Very nice builds you do and id love to run something you have done especially one of your infamous 394s.

The cranks are pretty overbuilt, they can handle a lot. It’s probably turning around 12k in the cut.

 

[smokey7]

They must be as the saws i see round here sure are running some high revs. In my world of modded skis cranks sure dont tolerate too many extra revs before they come unglued. They are roughly from the same era but 2 or 3 cylinder. Got a hotrods crank here from a 61x based yamaha motor. It was doing 1500 ish rpm over stock and the dang thing walked apart at the center destroying the whole bottom end cases and all. Got a whole 3 totes full of kawi 440 and 550 cranks that are out of phase bent or ruined in some way all from ported race skis.

 

[Bigmac]

They must be as the saws i see round here sure are running some high revs. In my world of modded skis cranks sure dont tolerate too many extra revs before they come unglued. They are roughly from the same era but 2 or 3 cylinder. Got a hotrods crank here from a 61x based yamaha motor. It was doing 1500 ish rpm over stock and the dang thing walked apart at the center destroying the whole bottom end cases and all. Got a whole 3 totes full of kawi 440 and 550 cranks that are out of phase bent or ruined in some way all from ported race skis.

Multi-cylinder cranks are another story! They need a lot more attention, really need to trued phased and welded, but getting quality work, and fully welded cranks is expensive and time consuming!