Ms461 saw design

[Chris-PA]

I don't want to come off as a jerk, but I have to say that I don't think these analogies to turochargers or superchargers, or tuned ram intakes on V8s are helpful. The operation of these 2-stokes are very different as well as the intentions and the mechanisms of strato systems. They just don't have any analogs with 4-strokes, because 4-strokes don't have the whole scavenging loss issue.

 

[J rod]

You have that backwards chief...The shorter/larger the intake, the slower the charge will be, but it is capable of flowing more resulting in higher theoretical rpm/power. A smaller/longer intake results in higher charge velocity creating better low rpm torque with a sacrafice in total volume of flow, resulting in a theoretically lower working rpm. As always, if other changes are not made to accommodate a larger/smaller intake/carb, actual results may vary.

Yea i was not thinking about runner size, I just know any of our race cars we always used a higher intake but probably the plenum was more open to match our cam numbers for more upper rpms,I know truck intakes are way shorter than on performance cars,

 

[blsnelling]

No one has suggested that the 461 is not more efficient. I think we can all agree on that. However, I've still seen no technical evidence that explains how the 461 is a strato engine. Generally speaking, the intake, transfers, and exhaust timing are no different than that of most any other traditional 2-stroke saw. With that being the case, then fuel bearing air should be at the same locations in the 461 as any other saw. Regardless of how long the transfer ducts are, the entire duct is fill with fuel charged air, and opens at the same time as any other saw. I see no way the long transfer ducts could delay the fuel. The muffler is the same, so there's no more back pressure either.

 

[old-cat]

Study Semantics. Wordfact = perceived reality. HAHAHAHA

 

[MustangMike]

Chris, point taken. The analogy I was trying to make is that the the "more efficient" solution in each case is based on the exhaust, and then heat becomes problematic. Other than that, you are absolutely correct. However, even 4 cycles spit out unburnt fuel, which is why they have Cats.

Brad, thanks, very good, clear, easy to follow explanation. What impact do the longer transfer ducts have, or do you think it just has more to do with the angle it enters the combustion chamber? Will a certain length transfer duct set up a rhythmic action that will be in sync with the saw at a certain RMP range?

 

[one.man.band]

siding with treemonkey on this one. windthrown had an accurate description of the result the process back in post #6.

the long runners and added case volume delay the pressure buildup time before fresh fuel/air can overpower the backwash to enter the chamber.... irregardless to what port timing.

the opposite is true when case volume/running length is reduced, speeding up the process, for quicker chamber fill.

if stratified, just means layered... the top portion of trans could just be the burnt fuel layered over the fresh fuel/air charge. agree with brad that every piston ported 2ST does this. but maybe stihl decided to amplify the benefit of the effect?

don't know if the case volume of the 361 was increased over the 360. IMO, the 461 has 361 tech written all over it, but with a new catch phrase "delayed scavenging stratified design" for the masses.

-omb

 

[Termite]

What are the similarities and differences between the 461 design and the Dolmar 6100? Aren't they doing something tricky with the exhaust to meet EPA regulations?
I bet Dolmar hasn't spent the $ Stihl and Husky has.

 

[Mastermind]

The 6100 uses a reed valve on the strato ports.

 

[KG441c]

So Scott, are you saying that your take on the 461 is that by design, it is simply a more efficient engine? And since it is more efficient, it has less unburned fuel in the exhaust?[/QU
If that was true

The 6100 uses a reed valve on the strato ports.

Are thoso the same concept as the Boyeson reeds? Boyesons design was a 2 stage cutout overlapping carbon fiber reeds

 

[Chris-PA]

Are thoso the same concept as the Boyeson reeds? Boyesons design was a 2 stage cutout overlapping carbon fiber reeds

It is just a small reed valve fresh air strato system added on to an otherwise typical engine that has small case volume, quad transfers and conservative timing. Probably adds just enough air to delay the mixture enough to meet emissions.

 

[MustangMike]

OMB, excellent observation regarding the impact of larger case volume. In conjunction with that longer runners may help to ensure exhaust gas never enters the crankcase.

 

[Mastermind]

The funny thing is.......I've yet to take an MS461 apart to find the transfers discolored from exhaust gasses flowing into the transfer ports.

 

[one.man.band]

OMB, excellent observation regarding the impact of larger case volume. In conjunction with that longer runners may help to ensure exhaust gas never enters the crankcase.

believe treemonkey was the one who spelled all this out previously.


edit: spelling

 

[one.man.band]

The funny thing is.......I've yet to take an MS461 apart to find the transfers discolored from exhaust gasses flowing into the transfer ports.

unless we are missing something..........

"delayed scavenging stratified design" = "blah blah blah blah"

-omb

 

[Mastermind]

Blah blah blah is about right, at least from what I see inside this engine.

I wish I understood all this stuff a little better........I have no doubt that I'm missing something.

 

[KG441c]

Blah blah blah is about right, at least from what I see inside this engine.

I wish I understood all this stuff a little better........I have no doubt that I'm missing something.

A little talk with a Stihl Engineer would be nice if they would tell you the truth

 

[MustangMike]

(I tried to post this a few hours ago, but my internet went down, very frustrating.)

A lot of excellent points being presented, on both sides of this issue.


Randy, excellent information (that most of us would never know) but I have a few questions if you don't mind:

1) Are there other saws that operate with this method where the transfers are obviously exhaust fouled?

2) Is it possible that when the flow reverses almost immediately that it washes this out?

Sorry about the dumb questions, and hope you don't mind I'm trying to pick your brain, but there is no way I would have the experience to understand these things on my own.

Another observation (unrelated) that I have trouble understanding. When I pulled the mufflers apart on my 362 C-M and my 044, the 362 (with about 1 month of operation) had more soot in it than the 044 (after more than 20 years). I would have expected the opposite due to the Tech improvements. Can anyone explain why this happened?

 

[Mastermind]

I think the 461 is suppose to delay transfer flow, not reverse flow.

No I don't see any saws with exhaust foulings in the transfers Mike.

I did just pull apart a 346XP that had a bunch of blowby fouling on the piston and in the crankcase. (The thread about one or two rings came to mind)

I would imagine the 362 runs leaner and burn at a higher temperature......hence the soot.

 

[MustangMike]

Thanks Randy, I would have never figured the reason for the soot. And thanks for the other information also, there is no substitute for experience.

I'm trying to wrap my brain around how "delay transfer" works, I still don't have it. Is it just a momentary equal pressure moment (in the cylinder and crankcase) that delays the flow from the transfers?

 

[Chris-PA]

Thanks Randy, I would have never figured the reason for the soot. And thanks for the other information also, there is no substitute for experience.

I'm trying to wrap my brain around how "delay transfer" works, I still don't have it. Is it just a momentary equal pressure moment (in the cylinder and crankcase) that delays the flow from the transfers?

It takes time to push the mix up the transfers and into the cylinder. If some other gas has just been pushed down the transfers then there will be a further delay until the flow is reversed, that gas is pushed out and the mix reaches the cylinder. That's the delay.

The intent is just to waste time while the exhaust port is open, but still get the fresh mix up there before the transfers close.