Real talk about saw porting theory (no arguments)

[drf256]

Im still pondering on progressive opening in a 2 port to simulate a quad port. A bump to help push initial charge on through the loop?

I've done the exhaust side dead flat 90 opening and the intake half angled on a 262.

It works on a 262, cause there's a huge fin in top of the transfer roof. Tough to grind through.

Seemed to work.

 

[dall]

From reading the bike guys point out flatter transfer roofs are better for wider power and and roofs pointed to the plug is better for top end. Husky 372 have one set of each.

The last 46 I put together is a hodgepodge of parts with a D chamber cylinder with no machine work and creative grinding to match the piston used. In a cant it's the fastest one I've done. I haven't actually took it out and used it as a worksaw with a 28" bar or anything to see how it acts. I think it will work fine since it had great manners tuning and idling. Case volume has been reduced and transfers are stock width and uppers only leveled.

mike is the 046 you talk about the one i have or another one ?

 

[jmssaws]

I've done the exhaust side dead flat 90 opening and the intake half angled on a 262.

It works on a 262, cause there's a huge fin in top of the transfer roof. Tough to grind through.

Seemed to work.

I find flat transfers to be more peaky .
I prefer a 2 degree taper on duals or quads and on quads I open the secondaries 2 degrees after the primary.

 

[mdavlee]

mike is the 046 you talk about the one i have or another one ?

Different one[emoji6]

 

[jmssaws]

I find flat transfers to be more peaky .
I prefer a 2 degree taper on duals or quads and on quads I open the secondaries 2 degrees after the primary.

I also only use a popup anymore,it's easier to keep the transfer angles when you don't have to raise them as far.

 

[penzone]

Great thread! Thanks for the huge amount of info.

I'm in the very beginning stages, which is read a lot. I hope to be practice grinding on some cylinders this spring. This kind of info is invaluable to a newbie like me!

 

[Keith Gandy]

As I'm going around to my appointments, I'm having trouble seeing over the snow banks!!!

I feel for yall buddy

 

[jmssaws]

Great thread! Thanks for the huge amount of info.

I'm in the very beginning stages, which is read a lot. I hope to be practice grinding on some cylinders this spring. This kind of info is invaluable to a newbie like me!

Got to your dealor and get them to save there junk cylinders for you.
Practice material and 80% of the ones they junk clean up so it's a win win if you can get them to do it

 

[Keith Gandy]

I've done the exhaust side dead flat 90 opening and the intake half angled on a 262.

It works on a 262, cause there's a huge fin in top of the transfer roof. Tough to grind through.

Seemed to work.

I think I remember u and I discussing that drop in pressure in the loop creating a venturi effect just ahead of the main charge to purge the loop and draw the main charge in?

 

[penzone]

Got to your dealor and get them to save there junk cylinders for you.
Practice material and 80% of the ones they junk clean up so it's a win win if you can get them to do it

That's a great tip, thanks! I have a decent relationship with them, so I think they'd go for it. I'll stop by next week, buy some more bar oil & rim sprocket and talk to them about it.

 

[jmssaws]

That's a great tip, thanks! I have a decent relationship with them, so I think they'd go for it. I'll stop by next week, buy some more bar oil & rim sprocket and talk to them about it.

They don't clean up cylinders so most are good.

 

[penzone]

So, does anyone have any ideas on what the peak cylinder pressure is after ignition, vs the compression test reading? For example, if a saw is seeing 200 psi on a compression test, that would give a range of x-y max psi after ignition at tdc?

I'm wondering if I could estimate the cylinder pressure drop and the case pressure rise per crank angle, to look at the best time to open the transfers based on the fluid density and velocity through the ports. There will be a lot of assumptions, but would be fun to look at.

Also, want to confess I'm only about 60 pages into the 2 stroke handbook for the first time. So if it's in there, I'll get to it I guess.

 

[jmssaws]

So, does anyone have any ideas on what the peak cylinder pressure is after ignition, vs the compression test reading? For example, if a saw is seeing 200 psi on a compression test, that would give a range of x-y max psi after ignition at tdc?

I'm wondering if I could estimate the cylinder pressure drop and the case pressure rise per crank angle, to look at the best time to open the transfers based on the fluid density and velocity through the ports. There will be a lot of assumptions, but would be fun to look at.

Also, want to confess I'm only about 60 pages into the 2 stroke handbook for the first time. So if it's in there, I'll get to it I guess.

I read a few pages of it 20 years ago.
I learned by trial and error, 5 years ago I was certain that I was the only guy in the world who ported chainsaws until I started searching for port timing and couldn't believe what I found.
I self taught on saws and was in heaven when I found AS and all the information,I've read every build thread by every builder many times.

To me the build threads on these forums are better information than a book about bike motors,I learned to port on bike motors at ftz performance when I was 18 and the only thing that I could transfer to saws was I knew how to grind.

 

[drf256]

I think I remember u and I discussing that drop in pressure in the loop creating a venturi effect just ahead of the main charge to purge the loop and draw the main charge in?

Don't think it was me, I'm not that smart.

Definitely lots of stuff going on there.

My own personal assumption is that the charge emanates from the uppers the way that a birds wing comes off its body. The way the wing would look at TDC as the bird starts to push down. As the feathers flair.

I'm trying to make that "ribbon" of charge fill more of the cylinder. If I change little things, I should be able to change the thickness and direction of that ribbon.

 

[Keith Gandy]

Don't think it was me, I'm not that smart.

Definitely lots of stuff going on there.

My own personal assumption is that the charge emanates from the uppers the way that a birds wing comes off its body. The way the wing would look at TDC as the bird starts to push down. As the feathers flair.

I'm trying to make that "ribbon" of charge fill more of the cylinder. If I change little things, I should be able to change the thickness and direction of that ribbon.

I think we r about saying the same thing?

 

[Dub11]

So after all of this talk, does anyone think it's not a bargain to pay someone to do this?

It costs around $25-50 in consumables to port a jug.

Around $4k invested in tooling and machines.

Then porter will remove your piston, sometimes modify it as well, add rings and reinstall.

Then modify and weld your muffler. Clean it up, sometimes match the opening to the exhaust port. Match the exhaust gasket and heat shield to the port. Then paint it.

Then pull flywheel, advance timing by filing the key.

Then modify the carb sometimes and sometimes the air filter and the air filter cover.

Then ship the saw back for $20-50.

So does anyone think that paying $300 to have it done isn't cost effective?

Lol...

You are right there the hard part for me is finding one to send out thats not clapped out lol .

 

[drf256]

As we are starting to get into some theory now, I have a question.

I hear it commonly debated that carbon deposits are found in the uppers and tunnels from "backstuffing" of them by exhaust gasses at the point where they open.

Anyone ever think about what's happening at 1* ABDC?

There will be negative pressure being generated in the case then. The uppers will cease to have pressure, but there will still be charge inertia.

I postulate that some exhaust gets sucked in then, and if we can change what's sucked back then, we may find a little extra.

This may be why carbon is found in the transfers.

An exhaust finger in the right place would make that backfill be less diluted.

 

[Keith Gandy]

As we are starting to get into some theory now, I have a question.

I hear it commonly debated that carbon deposits are found in the uppers and tunnels from "backstuffing" of them by exhaust gasses at the point where they open.

Anyone ever think about what's happening at 1* ABDC?

There will be negative pressure being generated in the case then. The uppers will cease to have pressure, but there will still be charge inertia.

I postulate that some exhaust gets sucked in then, and if we can change what's sucked back then, we may find a little extra.

This may be why carbon is found in the transfers.

An exhaust finger in the right place would make that backfill be less diluted.

Like this? This is a factory stihl fs85 that the exhaust port was 50% restricted with carbon

 

[Dub11]

Like this?View attachment 61336View attachment 61337

Holy carbon batman

 

[paragonbuilder]

As we are starting to get into some theory now, I have a question.

I hear it commonly debated that carbon deposits are found in the uppers and tunnels from "backstuffing" of them by exhaust gasses at the point where they open.

Anyone ever think about what's happening at 1* ABDC?

There will be negative pressure being generated in the case then. The uppers will cease to have pressure, but there will still be charge inertia.

I postulate that some exhaust gets sucked in then, and if we can change what's sucked back then, we may find a little extra.

This may be why carbon is found in the transfers.

An exhaust finger in the right place would make that backfill be less diluted.

At 1* ABDC most of the exhaust has been purged and the transfers have completed filling the chamber, other than inertia. So where is the exhaust gasses coming from at this point?


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