Ms461 saw design

[KG441c]

From what I understand the 461 is a delayed scavenging stratified design that uses some of the hot exhaust pushed backin the transfers and mixed with the new gases coming into the transfers? Trying to understand the design vs the standard 460 design?? Anyone? Pro vs cons?

 

[old-cat]

NO. The difference is where the transfer ports get the air/fuel from the crankcase. I'm thinking that is a high pressure location and that increased pressure increases volumetric efficiency, plus there's a "stuffer"? that pushes some of the air/fuel out of the piston. The muffler has a serious baffle that helps keep the fresh charge from escaping out the ex. port.

 

[KG441c]

NO. The difference is where the transfer ports get the air/fuel from the crankcase. I'm thinking that is a high pressure location and that increased pressure increases volumetric efficiency, plus there's a "stuffer"? that pushes some of the air/fuel out of the piston. The muffler has a serious baffle that helps keep the fresh charge from escaping out the ex. port.

I was thinking the stuffer was throwing cooler fuel mixture on the underside of piston to aid in cooling the wrist pin bearing? Also if u gutted the baffle in muffler and increased exit and installed dp muffler cover would u need to richen the tune alil or lean? Is the serious baffle u refer to to aid in holding unburned gases in longer?

 

[KG441c]

Also.would the baffle act as a stuffer to push part of the hot gases back into the transfer to be burned with the fresh charge coming in the transfers for more effiency and better comply to epa but also the baffle in the muffler will.seriously restrict exhaust effiency also?

 

[KG441c]

anyone?

 

[windthrown]

Two answers here comparing a 461 to a 460. One is that the 461 is a strato saw, whereas the 460 is not. The other is that the 461 uses the exhaust gas between the fuel charges instead of fresh air for scavaging, so that the saws run better when they are cold. They call this delayed savaging, but it is really exhaust savaging. On the earlier design Stihl starto saws there is a fresh air strato port intake above the carb intake that is used to charge the air between fuel charges. Two butterfly valves control both intakes. Both my 211 and 441 are early starto designs, and they can be pretty cold blooded when they are started cold. They both need some feathering to get them warmed up.

More details on this redesign is that the 461 strato charge comes from the exhaust from redesigned transfer ports. When the piston is going down and there is still pressure in the cylinder, the transfers open and the exhaust gasses back-fill the transfers pushing the fresh fuel charge back into the low end of the saw. This allows for exhaust gasses to act as a buffer between fuel charges so that raw fuel does not push out into the muffler like pre-EPA saws do. The piston travels further down and that forces the exhaust gases back out the transfers and into the muffler. Which is an interesting twist over the early fresh air charge used in saws like the 441. There are also fewer moving parts and no need for the second air intake above the carb or the fresh air intake porting. The problem with this design though is that the wrist pins of the pistons tend to cook from the added heat of the exhaust primed charge. The piston will fail rather quickly using this design. So they added the pusher at the base of the piston that basically pumps more cooler intake fuel mix up under the piston on the downstroke, to keep it running cooler.

Personally I am not sure if exhaust scavenging is a good idea with the need to add the pusher/piston cooler. My question is does the piston run hotter over time and will the saws last as long as cold air injected designs? The 661 uses the same exhaust scavenging design as the 461, and it also has the pusher/piston cooler. Many 661s failed in its first release. They are not saying why, but the first release 661 cylinders are being redesigned as a result of the early failures.

 

[windthrown]

And now to pose the rebuttal that Mastermind did to the above design explanation. That being that the transfers are lower on the 461 than other strato saws. His thought is that the added baffle in the muffler is what pushes the exhaust charge back into the transfer ports once they open. Though he said he ran the 461 saw with and without the baffle in the muffler and he did not notice any difference. I think they changed the lower area of the saw so that there is less pressure when the transfers first open, thus allowing expansion pressure in the combustion chamber to force the exhaust gasses into the transfers and prime the strato charge. In any event, exhaust gasses are pressed into the transfers to act as a buffer between fuel charges in the 461 and the 611 saws, and they both have the pusher/piston coolers in them. The pusher could also be acting as a baffle to reduce the transfer port pressure on the downstroke just enough to allow for the back-filling of the transfers with exhaust gasses.

 

[old-cat]

Two answers here comparing a 461 to a 460. One is that the 461 is a strato saw, whereas the 460 is not. The other is that the 461 uses the exhaust gas between the fuel charges instead of fresh air for scavaging, so that the saws run better when they are cold. They call this delayed savaging, but it is really exhaust savaging. On the earlier design Stihl starto saws there is a fresh air strato port intake above the carb intake that is used to charge the air between fuel charges. Two butterfly valves control both intakes. Both my 211 and 441 are early starto designs, and they can be pretty cold blooded when they are started cold. They both need some feathering to get them warmed up.

More details on this redesign is that the 461 strato charge comes from the exhaust from redesigned transfer ports. When the piston is going down and there is still pressure in the cylinder, the transfers open and the exhaust gasses back-fill the transfers pushing the fresh fuel charge back into the low end of the saw. This allows for exhaust gasses to act as a buffer between fuel charges so that raw fuel does not push out into the muffler like pre-EPA saws do. The piston travels further down and that forces the exhaust gases back out the transfers and into the muffler. Which is an interesting twist over the early fresh air charge used in saws like the 441. There are also fewer moving parts and no need for the second air intake above the carb or the fresh air intake porting. The problem with this design though is that the wrist pins of the pistons tend to cook from the added heat of the exhaust primed charge. The piston will fail rather quickly using this design. So they added the pusher at the base of the piston that basically pumps more cooler intake fuel mix up under the piston on the downstroke, to keep it running cooler.

Personally I am not sure if exhaust scavenging is a good idea with the need to add the pusher/piston cooler. My question is does the piston run hotter over time and will the saws last as long as cold air injected designs? The 661 uses the same exhaust scavenging design as the 461, and it also has the pusher/piston cooler. Many 661s failed in its first release. They are not saying why, but the first release 661 cylinders are being redesigned as a result of the early failures.

I'd like to read an engineering tech paper to verify this statement.

 

[KG441c]

I'd like to read an engineering tech paper to verify this statement.

Windthrowns explanation is dead on with Stihls explanation of delayed stratfied scavenging as Ive read it on the Stihl tech sheet since posting this thread. But Windthrown Im thinking Mastermind was referring to the seperator he ran the saw without besides the baffle in the muffler? Removing the baffle in the muffler would stop a large portion of the stuffing in the transfer and further reduce cooling wouldnt it? Masterminds explanation was the baffle in the muffler was more due to the stuffing of the transfers? In the Stihl tech sheet the diverter plate in the crankcase is only for throwing a fresh cool charge of fuel on the bottom of piston for cooling so removing baffle in muffler and opening muffler would only prevent this stuffing which I think is probably
epa design?

 

[old-cat]

Windthrowns explanation is dead on with Stihls explanation of delayed stratfied scavenging as Ive read it on the Stihl tech sheet since posting this thread. But Windthrown Im thinking Mastermind was referring to the seperator he ran the saw without besides the baffle in the muffler? Removing the baffle in the muffler would stop a large portion of the stuffing in the transfer and further reduce cooling wouldnt it? Masterminds explanation was the baffle in the muffler was more due to the stuffing of the transfers? In the Stihl tech sheet the diverter plate in the crankcase is only for throwing a fresh cool charge of fuel on the bottom of piston for cooling so removing baffle in muffler and opening muffler would only prevent this stuffing which I think is probably epa design?

Ok, that's fine. BUT where is that explanation connected to the MS 461? Show me a tech paper to prove it.

 

[KG441c]

Ok, that's fine. BUT where is that explanation connected to the MS 461? Show me a tech paper to prove it.

pg 5&6

 

[blsnelling]

The 461 is not stratified. It's a quad transfer port, traditional 2-stroke cylinder. The only difference is the transfers are fed from the front of the cylinder, rather than the sides. Port timings are no different than many other traditional 2-stroke saws. It has one carb venturi, one intake tract, all if it being fuel bearing air. The baffle in the muffler is no different than the 460. Check out the pictures and comments in Randy's detailed build thread he did here, http://www.arboristsite.com/community/threads/ms461-giveaway-saw-build-thread.246494/page-2.

Lots of pics here, http://www.arboristsite.com/community/threads/ms461-giveaway-saw-build-thread.246494/page-8.

 

[old-cat]

The 461 is not stratified. It's a quad transfer port, traditional 2-stroke cylinder. The only difference is the transfers are fed from the front of the cylinder, rather than the sides. Port timings are no different than many other traditional 2-stroke saws. It has one carb venturi, one intake tract, all if it being fuel bearing air. The baffle in the muffler is no different than the 460. Check out the pictures and comments in Randy's detailed build thread he did here, http://www.arboristsite.com/community/threads/ms461-giveaway-saw-build-thread.246494/page-2.

Lots of pics here, http://www.arboristsite.com/community/threads/ms461-giveaway-saw-build-thread.246494/page-8.

Please explain the cutout at the bottom of the piston, exhaust side?

 

[KG441c]

The 461 is not stratified. It's a quad transfer port, traditional 2-stroke cylinder. The only difference is the transfers are fed from the front of the cylinder, rather than the sides. Port timings are no different than many other traditional 2-stroke saws. It has one carb venturi, one intake tract, all if it being fuel bearing air. The baffle in the muffler is no different than the 460. Check out the pictures and comments in Randy's detailed build thread he did here, http://www.arboristsite.com/community/threads/ms461-giveaway-saw-build-thread.246494/page-2.

Lots of pics here, http://www.arboristsite.com/community/threads/ms461-giveaway-saw-build-thread.246494/page-8.

thanks brad

 

[blsnelling]

Please explain the cutout at the bottom of the piston, exhaust side?

That I couldn't tell you. I don't have one here in front of me to study.

 

[old-cat]

It has been said that the cut out at the bottom of the piston is to let ex. go back into the crankcase. I just took the muf cover off my MS461 to see. No such thing!
Brad is 100% correct.
I still stay with my first post above.

 

[blsnelling]

Through free porting at TDC? That is not the case.

 

[KG441c]

Wouldnt taking the baffle out of the muffler and opening existing exit hole and adding a dp cover stop some of the hot scaveged gas from.being stuffed back in transfers and let the saw exhaust more freely and run cooler?

 

[blsnelling]

I don't see how that could be happening anyway. As the piston is coming down, base compression is coming up. When the transfers open, that pressure releases, forcing the fresh charge into the combustion chamber and forcing the exhaust out.

 

[KG441c]

I don't see how that could be happening anyway. As the piston is coming down, base compression is coming up. When the transfers open, that pressure releases, forcing the fresh charge into the combustion chamber and forcing the exhaust out.

Could the large baffle in the muffler be causing enough back pressue to stuff some? ive gutted my muffler anyway