Stihl MS 441 Mods and Results

[Wild Knight]

The ring pin is located in the exact same position on my 441. I thought the same; it is located exactly to prevent widening of the exhaust port.

Obviously, you have noticed the strato cavities on the piston. These hold extra air that is added to the transfer charge, thereby 1) scavenging the exhaust and 2) leaning out the fuel charge. I think you are on to it now, but the mixture coming in through the intake is rich. It has to be b/c fresh air is being added by the strato port. If you enlarge the intake, but leave the strato ports the same, then you will be getting more fuel into the crank while at the same time NOT leaning it from air from the stratos. You must open both the intake and strato ports proportionally. This is why you had to lean out the carb once you put everything back together.

FWIW, I ran Amsoil 100:1 for a while and will NOT be doing it anymore. When we got the saw apart, it had barely a sheen of oil inside. Additionally, the piston had gotten very hot. I think the only thing that kept it from seizing was the high amounts of anti-seizure compounds in Amsoil. 80:1 might be OK, but 100:1 just isn't enough oil. 50:1 will leave the insides blue like a little smurf village, which is what you want to see.

 

[parrishturf]

The ring pin is located in the exact same position on my 441. I thought the same; it is located exactly to prevent widening of the exhaust port.

Obviously, you have noticed the strato cavities on the piston. These hold extra air that is added to the transfer charge, thereby 1) scavenging the exhaust and 2) leaning out the fuel charge. I think you are on to it now, but the mixture coming in through the intake is rich. It has to be b/c fresh air is being added by the strato port. If you enlarge the intake, but leave the strato ports the same, then you will be getting more fuel into the crank while at the same time NOT leaning it from air from the stratos. You must open both the intake and strato ports proportionally. This is why you had to lean out the carb once you put everything back together.

FWIW, I ran Amsoil 100:1 for a while and will NOT be doing it anymore. When we got the saw apart, it had barely a sheen of oil inside. Additionally, the piston had gotten very hot. I think the only thing that kept it from seizing was the high amounts of anti-seizure compounds in Amsoil. 80:1 might be OK, but 100:1 just isn't enough oil. 50:1 will leave the insides blue like a little smurf village, which is what you want to see.

If you look closely at the fuel ports in the cylinder one is adjacent to the strato port, just above it. Today I blended all the ports into the cylinder, raised each about .025" and allowed the strato port to come very close to fuel port. The saw is making the most HP ever now. I cannot get my mind around what is happening in the notch in the piston vs the shaped cavity. I have seen the flow diagram and allot is going on. My goal today was to make the saw lean out...No luck yet. Black to dark gray plug. Saw cuts at about 12,000 with skiptooth chain 20" bar, you can load the saw all you want and cuts down to about 9,000 rpm in poplar or oak. The saw cuts best without any burble on top, the carb screws are .75l 1.25H.


So far I cannot beet the times of the stock 460 with stihl dual port muffler. We try hard to keep the chains and bars the same and even change operators. The 460 is 5 seconds faster in oak; the 441 25 seconds, and 1-2 seconds faster in poplar 441 is 7.5 sec. The oak is a block 19 wide 20 deep. Poplar is round about 18 inch dia.

Amsoil:
What evidence showed hot piston?
Stihl 441?
Were you 80:1 or 100:1?

Stihl oil bakes a molases like gel in no time especialy on 441.
I am running 1:80 in all our equipment. The br600 blowers are 3 years old, no exaust valve failure and never a stuck ring.
Both the 441 and 460 are brand new broken in and raced with 80:1.
If I ever have an oil related failure I will post it!
At 32:1 amsoil does leave a strange coating that looks like grey paint, not a normal color, a little weird, and all over plug and piston.

 

[Wild Knight]

Keep at it. We had to find a 32" round to find a 5 second difference between a muffler modded 460 and my stock 441. A ported and muffler modded 441 should be as fast or faster than a 460.

I will post the pics of the 441 and the heat problem when I post my mods to my 441 (not done yet). In all fairness, I broke the saw in with Stihl orange bottle, then switched to 100:1 Amsoil. Still, there was NO sheen of oil on the crank, case or jug when we pulled it apart. If there had been a nice coating of oil I would attribute the heat to dyno oil, although dyno oil at 50:1 though should still prevent this type of heat. I run turbo blue (110 octane), which may have saved my arse from detonation over pump gas. Who knows? I just know this saw needs more protection. So, I'll go back to 50:1 with either Amsoil, Klotz or another premium synthetic.

For now, take a look at this piston. Mine looked pretty much the same, only my carbon was thicker on the piston crown, and not quite as brown down the skirt. The underside of the piston is exactly the same. The 441 may have a problem with heat, possibly because it essentially gets run very very lean during the strato scavenge part of the cycle. The fluctuation in heat in the combustion chamber may be very radical, as it constantly is alternating between lean and rich.

http://arboristsite.com/showthread.php?t=119470

As far as the notch, you got me. I've had that piston in front of 6 guys I trust from AS, and none of us can figure out what it does. Two thoughts are: 1) it relieves a slight bit of pressure from the crank or 2) it carries a little charge up and down to lube the jug in a critical area. This is just speculation though.

I do believe that it is purposefully cut out, not just a simple weight reduction because they made the notch blended and directional.

So, you have changed the duration of the transfers and strato ports, not just the width? Did I read you correctly?

 

[parrishturf]

Keep at it. We had to find a 32" round to find a 5 second difference between a muffler modded 460 and my stock 441. A ported and muffler modded 441 should be as fast or faster than a 460.

I will post the pics of the 441 and the heat problem when I post my mods to my 441 (not done yet). In all fairness, I broke the saw in with Stihl orange bottle, then switched to 100:1 Amsoil. Still, there was NO sheen of oil on the crank, case or jug when we pulled it apart. If there had been a nice coating of oil I would attribute the heat to dyno oil, although dyno oil at 50:1 though should still prevent this type of heat. I run turbo blue (110 octane), which may have saved my arse from detonation over pump gas. Who knows? I just know this saw needs more protection. So, I'll go back to 50:1 with either Amsoil, Klotz or another premium synthetic.

For now, take a look at this piston. Mine looked pretty much the same, only my carbon was thicker on the piston crown, and not quite as brown down the skirt. The underside of the piston is exactly the same. The 441 may have a problem with heat, possibly because it essentially gets run very very lean during the strato scavenge part of the cycle. The fluctuation in heat in the combustion chamber may be very radical, as it constantly is alternating between lean and rich.

http://arboristsite.com/showthread.php?t=119470

As far as the notch, you got me. I've had that piston in front of 6 guys I trust from AS, and none of us can figure out what it does. Two thoughts are: 1) it relieves a slight bit of pressure from the crank or 2) it carries a little charge up and down to lube the jug in a critical area. This is just speculation though.

I do believe that it is purposefully cut out, not just a simple weight reduction because they made the notch blended and directional.

So, you have changed the duration of the transfers and strato ports, not just the width? Did I read you correctly?

Yes...I have changed the timing of all the transfer ports, and removed a great deal of the separation from the strato port and its adjacent fuel port. I also made a muffler with a 1/2 inch pipe replacing the factory port. I welded the pipe in the upper corner replacing the entire corner of the muffler.

Torque was back and rpm good. The torque was recognizable without the bar. The prior muffler had the factory port and a 3/8" slit 1.25" long in front.

I have now been able to beat the 460/dual port muffler. 3 or 4 seconds faster in 19" poplar making down/up double cuts.

There is more in this saw, as yet I have not opened the exhaust but about 1.5mm due to the ring pin. Saturday I will make the exhaust port asymmetrical; wider to other side away from ring ends.

The strato port to intake transition has the bifurcation inside the intake. I am considering relocating the divider into the jug, and increasing the diameter of the butterfly opening. I think the butterfly can remain the same size, as the saw is perpetually rich starting and can require full throttle after initial priming, to start.

more to come...

 

[Terry Syd]

2 cents

The notch in the front of the piston may be to relieve that area when the piston gets hotter. There are significant temperature gradients across the piston as the back of the piston is getting a blast of air on the skirt. It may be that the front tended to expand and create a clearance problem.

On my Husky, that relief has a small hole through to the inside of the piston. For a brief few degrees that relief is vented to the opening strato port and the mixture still resident in the transfer port is pulled forward along the relief and into the front of the piston. I expect this is to give the piston a more stable expansion.

On my Husky the bottom of the transfer port is the timing mechanism for the opening of the strato function. The timing can also be changed by raising the height of the cutaway in the piston.

I agree on the eventual mixing of the two charges, however increasing the bias of the intake to the strato charge has netted some gains. It may be that a more uniform cylinder mixing with the larger intake charge is not only providing more fuel mixture into the cylinder, but also a faster burn rate from a more uniform charge. In any case, the gains were there.

The position for the pin to locate the ring gap is a real pain. The idea of having an asymetrical exhaust port doesn't appeal to me as it will probably promote an asymetrical flow in the cylinder during scavanging. It will be interesting to see how it works out. After a few hours of running you may be able to get an idea by checking the carbon deposits on top of the piston to see how symetrical the flow is.

On my Husky, the strato port does not begin to open until about 1/3 throttle. The strato port therefore has no effect on the idle of the engine. Perhaps if you were to open up the cutaway on the throttle plate a tad it would provide the necessary leaning out at idle.

 

[bitzer]

I believe Terry is right about the notch. On my husky there are removable covers on the outsides of the transfers. You can watch the strato port open to the transfer and then the notch BBDC. Some of the strato air must be pushed into that notch and possibly some mix.

Terry, BTW My intake opens 15 degrees before the strato opens to the transfer. I was in the saw last week and I widened the exhaust to 72% of the bore and raised the roof 8 degrees. I have not touched the strato intakes yet and it is now running faster and stronger than the 310 I ported which had been beating the 455 on the last timing run.


I think there is strato air to spare without widening the ports.

 

[Terry Syd]

Duration?

Good gawd Bob! What are the various durations in your saw now? The exhaust was already high, and now another 8 degrees?

I want to get the dimensions of your muffler - it might be the next best thing to a pipe!

Hmm, you did have a short blowdown period, so I can see that the extra 4 degrees of blowdown would help it. I wish I had an unlimited coil so I could try nipping my piston crown for a couple more degrees of blowdown.

 

[parrishturf]

The notch in the front of the piston may be to relieve that area when the piston gets hotter. There are significant temperature gradients across the piston as the back of the piston is getting a blast of air on the skirt. It may be that the front tended to expand and create a clearance problem.

On my Husky, that relief has a small hole through to the inside of the piston. For a brief few degrees that relief is vented to the opening strato port and the mixture still resident in the transfer port is pulled forward along the relief and into the front of the piston. I expect this is to give the piston a more stable expansion.

On my Husky the bottom of the transfer port is the timing mechanism for the opening of the strato function. The timing can also be changed by raising the height of the cutaway in the piston.

I agree on the eventual mixing of the two charges, however increasing the bias of the intake to the strato charge has netted some gains. It may be that a more uniform cylinder mixing with the larger intake charge is not only providing more fuel mixture into the cylinder, but also a faster burn rate from a more uniform charge. In any case, the gains were there.

The position for the pin to locate the ring gap is a real pain. The idea of having an asymetrical exhaust port doesn't appeal to me as it will probably promote an asymetrical flow in the cylinder during scavanging. It will be interesting to see how it works out. After a few hours of running you may be able to get an idea by checking the carbon deposits on top of the piston to see how symetrical the flow is.

On my Husky, the strato port does not begin to open until about 1/3 throttle. The strato port therefore has no effect on the idle of the engine. Perhaps if you were to open up the cutaway on the throttle plate a tad it would provide the necessary leaning out at idle.

I agree the butterfly for the strato path opens late, and carb butterfly would be the best for starting, however, my goal is to identify any change in strato path that exhibits greater flow.

Has anyone ever devised a flow bench for a two stroke?

 

[Wild Knight]

Has anyone ever devised a flow bench for a two stroke?

Shoot Timberwolf a PM; I think he has experience with this.

 

[bitzer]

Good gawd Bob! What are the various durations in your saw now? The exhaust was already high, and now another 8 degrees?

I want to get the dimensions of your muffler - it might be the next best thing to a pipe!

Hmm, you did have a short blowdown period, so I can see that the extra 4 degrees of blowdown would help it. I wish I had an unlimited coil so I could try nipping my piston crown for a couple more degrees of blowdown.

I've got the numbers at the shop and I will get back to you over in the 445 thread. I don't want to hijack this thread, just wanted to point out that widening the strato ports may not be necessary. I want to do some solid timed runs on all three saws that I did before and throw my 440/460bb in there just for fun, next week some time. It is actually running smoother now and I 've got the idle pooling more managable. I will get back at it on Monday with all the figures.

 

[parrishturf]

Strato port flow:

Today focused on the strato port butterfly and the butterfly cam. The cam is plastic but can be glued to to change the opening rate and timing. There is also a set screw that can be adjusted to open the butterfly to about 98%.

My choice was to go through with resizing the strato venturi body and install a larger butterfly plate. I obtained a doner plate from an old walbro carb. and with a dremmel cutoff wheel enldrged the slit in the shaft. I then enlarged the flow path from the plastic filter housing all the way through to the devider in the rubber intake. The limit here is the thickness of the rubber and saving the flat surface of the intake to seal against the strato venturi.

This intake was already enlarged through the fuel path and was about 1mm oversize at the jugs intake port (FUEL).

Air temp was 18' today. I opened up both H & L needles 1 turn, had to crank on ether. After suficient warm up and opening the L needle another 1/2 turn (2 1/4 total). I made several cuts on the 19" poplar.

The saw is now alive...The torque is higher than ever...I am making consistant 5.5 to 6.0 second single cutts...Over 2 seconds faster than the 460/2xmuffler.

Then I tried to lean the saw out and did a plug check. I can now richen and lean the saw within the powerband and not loose all the torque. Then I got greedy and continued to lean looking for fall of of torque in wood and I made the saw diesel up to about 20K. I had pulled the plug wire and could not stop it. The choke thankfully did. A quick post mortum with a bore scope showed no henpecking and the piston skirts are normal, compression 175.

I have had this happen on motorcycles, never on a chainsaw. Have any of you had a saw run on without spark.?

Am I correct that it was lean, or does it require a mass of fuel to diesel?

 

[Terry Syd]

Pre-ignition

Sounds like you went into pre-ignition with a hot spot acting as a glow plug. The plug tip was probably the culprit. Yeah, you got it lean!

Something else occurred to me in reading your post. If the strato valve doesn't seal off the strato port, it will allow extra air past it and contribute to the idle flow. Since you opened up the L needle much further to get the right idle mixture, it appears that the strato valve may not be sealing all the way.

 

[Wild Knight]

I'm having a time following where you are right now. You have enlarged all transfers (including the strato) and changed their timing. You have altered the ports and changed their timing. Plus, you have altered the carb to make the strato port flow more. Is this correct?

Can you put a degree wheel on an tell us where everything is timed right now? Duration also?

I gather from your comments about buying another jug and piston that you were trying to find the limits that you could push the saw. Looks like you found it! Now, try to figure out where you should have stopped.