Part 8: The Stratified Chainsaw

[huskyboy]

I've tried everything at one time or another. So helped, some not so much. I've finally realized that when you combine the strato windows, and the intake width, you end up with over 100% of the bore diameter in area/width. That's a big deal. Yeah.....the stratos are used to purge, but there is still volume that contributes to the overall picture. With that in mind, I just began to look for gains in other places.

You have a good way of simplifying strato explanation in a simple way to understand. At least for me. I think TM Scott also had a simple explanation somewhere too in another thread. I think it was the 462 one. I guess at some point the question should be asked “is it good enough for a worksaw”. A guy can go crazy trying to find every gain in a saw.

 

[Mastermind]

You have a good way of simplifying strato explanation in a simple way to understand. At least for me. I think TM Scott also had a simple explanation somewhere too in another thread. I think it was the 462 one. I guess at some point the question should be asked “is it good enough for a worksaw”. A guy can go crazy trying to find every gain in a saw.

The most important question I ask when contemplating modifications.......

Will it last?

 

[Stump Shot]

I've tried everything at one time or another. So helped, some not so much. I've finally realized that when you combine the strato windows, and the intake width, you end up with over 100% of the bore diameter in area/width. That's a big deal. Yeah.....the stratos are used to purge, but there is still volume that contributes to the overall picture. With that in mind, I just began to look for gains in other places.

What do you think @huskihl

 

[Stump Shot]

The most important question I ask when contemplating modifications.......

Will it last?

+1

Ruined saw cuts no wood.

 

[huskihl]

What do you think @huskihl

Kind of the same I guess. I only shine them up a little and make sure they’re beveled. They have to make a 180° turn in the piston. Kinda hard to make anything efficient out of that

 

[Ketchup]

How much blowdown are Delayed Scavenge saws running by comparison to an earlier Strato?

 

[Nutball]

In theory the strato saws have the potential to be more powerful than non strato because they are more efficient, or does the strato design actually restrict things when it comes to porting?

 

[Terry Syd]

Go back to what Randy just stated. When the intake and strato ports open, they can open with more than 100% of the diameter of the bore, and further they open immediately. There is no having to put a curve on the port to keep it from snagging the piston skirt.

That combination makes it a very efficient piston port design. They open with a 'bang', like NOW, no delay.

 

[Nutball]

That's what I was thinking. Thanks

 

[Ketchup]

Go back to what Randy just stated. When the intake and strato ports open, they can open with more than 100% of the diameter of the bore, and further they open immediately. There is no having to put a curve on the port to keep it from snagging the piston skirt.

That combination makes it a very efficient piston port design. They open with a 'bang', like NOW, no delay.

This is what originally got me fired up about stratos. Unfortunately I’ve come to believe the transfer uppers limit how much charge makes it’s way into the combustion chamber. It’s possible that enlarging the transfer tunnels and advancing the mixed charge intake opening could deliver more into the combustion chamber, though my experiments in that direction haven’t indicated much. Maybe in combination with longer transfer duration and a low, wide exhaust you could have something.

There is still the longer, heavier piston to overcome as well.

So many experiments...

 

[Flip]

So many experiments...

I'm fascinated by them. Better fuel economy with the potential for great power. Add on top of that a simple computer that will keep it tuned better than I ever will has made me lose interest in most older saws. Lots to learn about these new engines.

 

[Mastermind]

Maybe in combination with longer transfer duration and a low, wide exhaust you could have something.

This.

 

[Lightning Performance]

They open with a 'bang', like NOW, no delay.

I went this way on my last 200T build. No staggered or angled anything. All in all done all at once on the transfers, intake and exhaust. You can make a very flat port with 40mm holes. The key was having everything perfectly even and not going crazy widening the ports. I did three jugs with completely different transfer, staggers and they all ran about the same. So that was abandoned this time. Flat even transfers had the widest power band. It gained more down low and mid range. Might have even lost a bit up top. It had the most case pressure also out if all the ported combos before that. Longer ain't always better. Other changes upstream and down stream benefited the overall package. The ported carb and mesh filter was a must. Stock carb left it lacking and that never happened before on any of those builds in the past. One other small change in the muffler boosted the mid range even farther. Very happy with it and so is the new owner.

It took some hand sanding to get the rings to have no drag passing all the ports. How I check that is my little secret. Having those motors completely drag free before assembly goes a long way in how long they last and heat cycles. Blow by is a death nail in top handle saws.

Just disrupting your strato thread a bit

 

[Maintenance Chief]

So I'm no saw porter, but I will clean castings up .
The ms441s I mess with always seem to have a casting malformation in one of the transfers. I usually take Emory cloth and fold it multiple times and tediously smooth out all the bottom feed ports.
In the aftermarket BB kit the cylinder is expanded but encroaches into the ports so I used a 3/16 file to shape the into actual ports .
I might use a flat file on the outside of the cylinder skirt below the transfer port.

 

[Terry Syd]

I might use a flat file on the outside of the cylinder skirt below the transfer port.

Speaking of the outside of the cylinder, I clean up any flashing that would impede the air flow. Sometimes I will drill out something to allow the air to flow between the fins. I will also remove the flat edge of the fin and bevel the edges of the fins where the air impacts the edge of the fin.

Basically, I am trying to ensure good air flow through the cylinder fins. A cooler engine will run stronger and last longer.

 

[Nutball]

Speaking of the outside of the cylinder, I clean up any flashing that would impede the air flow. Sometimes I will drill out something to allow the air to flow between the fins. I will also remove the flat edge of the fin and bevel the edges of the fins where the air impacts the edge of the fin.

Basically, I am trying to ensure good air flow through the cylinder fins. A cooler engine will run stronger and last longer.

I do that a lot too. I have a favourite burr for increasing the gap between the fins on the front and back where the casting seam can make the fins extra thick. Also drill the block on top if it has one.

Not sure what kind of burr this is if ever I wanted to look it up and buy another. I paid $2 for it from a guy who hot a bunch probably from China.

 

[srcarr52]

I do that a lot too. I have a favourite burr for increasing the gap between the fins on the front and back where the casting seam can make the fins extra thick. Also drill the block on top if it has one.

Not sure what kind of burr this is if ever I wanted to look it up and buy another. I paid $2 for it from a guy who hot a bunch probably from China.

View attachment 297334 View attachment 297335

Some AM cylinders are almost closed off with casting flash and misalignment. I've used a 4-1/2" cut wheel in an angle grinder to open them up.

 

[Maintenance Chief]

Some AM cylinders are almost closed off with casting flash and misalignment. I've used a 4-1/2" cut wheel in an angle grinder to open them up.

Haha funny, but probably true.

 

[Ketchup]

How much difference do you guys think there is between charge temperature and air temperature? One goes through the case, one across the piston.

Seems like they would be be relatively similar but the air would be cooler.

Sort of related, when looking at stratos that mix slightly before dividing is the intention lubrication of the piston or less of a barrier between charge and air in the transfer tunnels?
That style seems mostly to be delayed scavenge Stihls. Maybe an important part of the recipe.

Final thought. Does fresh air promote combustion when the transfer uppers crack open? Presumably the exhaust gasses have used up their oxygen supply until that fresh air sweeps into the chamber. Can combustion continue during that decreasing pressure stage? Is it making power or just burning waste?

Just blathering…carry on with practical porting.

 

[Ketchup]

Delayed Scavenging: Why?