getting some revs out of the 041

[mcb]

pulled the gasket back out which is how i intend to run the saw, in addition to milling the base about .020" and bringing down the compression height margin on the piston. this will minimize quench pad distance at tdc, allow adequate clearance for the peripheral squish band and retard transfer port opening as much as possible to extend blowdown and get some breathing done.

so i rezero'd my wheel by ensuring that the crank was at top dead center and the piston was at its highest position in the bore.. (when the "minute hand" sweeps to and from a single number, reversing direction, thats TDC. i set my gauge to "0" for this to make it easy)

bent the pointer right ontop of zero and rechecked my measurements.

ive been using a .004 feeler to mark when the port is open or closed for consistency.. looks like this:

turns out i had my transfer port opening off about 3* .. they actually open at 120ATDC, giving me 11 degree of blowdown timing, which you can count here as the difference between exhaust open and transfer open.

im still convinced the 041 is a slow revver because the blowdown period is too short because of the late (retarded) exhaust valve opening time at 109*ATDC when it should, IMO, be closer to 90*ATDC. i did a poor job of explaining why these spit fuel back a few posts up. it is indeed because of inadequate time frame to exhaust itself but i said that it was a simple case of intake charge contamination. its more than that. because the blowdown period is inadequate, the pressure above the piston is still very high when the transfers open. the exhaust gas is blowing into the crankcase through the transfer port, and then right out the intake/carb. my prediction is more blowdown will do this saw a world of good.

ive read 30* of blowdown is customary for high performance application, but im thinking too much rpm will kill bearings and rattle the saw to death, so i feel like 25* is more reasonable and still more than double what i have now, plus id like to keep some torque and you cant have torque without compression. you dont build compression if you have the valves open all the time.

 

[mcb]

so we all know blowdown is how long the exhaust is open (first event of the cycle) before the transfer port opens (second event of the cycle.) clearly i need to extend the space (time) between these two events. i can lower the transfers only by epoxy or base turning which are both limited, and would reduce flow through the transfers from the crankcase. bandaid solution.

so my other choice is make the exhaust open sooner, or advance its timing, by making it occur closer to TDC, hence, "raising the roof" of the exhaust port. okay, how much?

so i put my degree wheel back to the current exhaust open point at 109*ATDC, then set my dial indicator to an arbitrary and easily readable setting of .200" (its touching the piston at my start point which is essentially zero.)

i rotated the crank until the pointer landed at 95*ATDC which will give me my 25 degrees of blowdown

and measured the stroke of the dial, which was .180"

unfortunately .180" is no small amount to raise this exhaust roof.

clearly it will take the flange right off and render the current muffler useless, so ill have to fab another.

as for the port = 70% of bore rule, i doubt my skirts will allow it, but heres what that would look like.

so it looks like ive got some thinking to do before i bust out the grinder and epoxy. have to figure out if too much duration in the exhaust could be harmful to any other cycle if the blowdown and compression are right on target.

 

[Brmorgan]

Well I'm watching this thread intently but unfortunately have nothing productive to add to the discussion anymore! You're talking way above my pay grade now... Thanks for the great pics though. This thread is getting saved page-by-page on my computer for future reference. Are you really considering raising the exhaust by what looks to be ~1/8" though? That will kill your compression if nothing else, I'm thinking, unless you're thinking of turning the base down somewhat to compensate.

 

[Terry Syd]

Blowdown

MCB, I have just gone through some tuning with a new Husky 450. It came with 12 degrees of blowdown.

After reworking the intake skirt and muffler, I was ready to find out what the engine needed on blowdown to bring the cutting speed up. I took 2 degrees off the front of the piston crown next to the exhaust port. That took the exhaust duration from 150 to 154 degrees and moved the blowdown from 12 degrees to 14.

That helped considerably with the cutting speed. I then took another 2 degrees off the front of the piston. At 158 degrees of duration and 16 degrees of blowdown the cutting speed was again higher, but I had lost some of the user friendly low-end. Overall, I preferred the 154 degrees of duration.

My new piston will be trimmed for 154 degrees. Of course, I could now raise the exhaust port 2 degrees and simply use the new piston, but I want to keep the factory chrome on the top of the port.

If you have an old piston for that engine, you could sacrifice it and trim the piston a bit until you found the duration you wanted.

 

[jimbo1490]

Check out the start of my rebuild here:http://http://arboristsite.com/showthread.php?t=121397&highlight=041+rebuild

I was lucky enough to find both seals and both bearings at my dealer. I could not comment on where to find them anywhere else.

What's so special about the seals and bearings that you can't just go to any generic bearing dealer and find them?

Jimbo

 

[ironman_gq]

I wouldn't take the whole exhaust flange off on top to get your blowdown I would just blend back to the flange from the cylinder wall and make it easier for you and maybe shoot for 20* blowdown to keep compression up while still getting a significant gain

 

[blsnelling]

20° would have been a plenty, especially if you wanted to retain any of that legendary torque. Before I remove nearly the entire flange, I'd have taken some from the bottom and less from the top. No need to make it so that you can't use the original muffler.

 

[Brmorgan]

Some saws you can, some you can't. OEMs sometimes decide to be *******s and use proprietary stuff so we have to buy replacement parts from them.

 

[jimbo1490]

Some saws you can, some you can't. OEMs sometimes decide to be *******s and use proprietary stuff so we have to buy replacement parts from them.

I've used these guys a couple of times for those 'impossible' items. Their prices were reasonable and quality outstanding. If they ain't got it in stock, they will MAKE it.

Jimbo

 

[mcb]

20° would have been a plenty, especially if you wanted to retain any of that legendary torque. Before I remove nearly the entire flange, I'd have taken some from the bottom and less from the top. No need to make it so that you can't use the original muffler.

just to be clear, i havent altered anything on this saw yet, still gathering my thoughts.

Are you really considering raising the exhaust by what looks to be ~1/8" though? That will kill your compression if nothing else, I'm thinking, unless you're thinking of turning the base down somewhat to compensate.

more like 3/16" :greenchainsaw: no harm in considering it right?

i'll be bringing the squish from .057" or so down to around .020" which is considerable .. this will retard the transfer opening a degree or two and give me as much additional blowdown from the jug as i can get, and raise static compression. i doubt it would run on pump gas WITHOUT raising the exhaust quite a bit after that much squish clearance coming out. think about the exhaust port closing point. most saws close at around 90* before TDC and then begin compressing. with the current port roof my 041 is closing at 109* before TDC, so its compressing for nearly 10 degrees longer. i dont think i have a choice but to raise the roof if this thing is gonna stay below 200psi cranking compression. even if i were to grind the roof .180" upward it would still start compressing at 95* BTDC which is on the slightly early side. i guess im illustrating how misplaced the 041's exhaust port is in comparison to modern performance timing. yeah, if it was a modern 90* BTDC exhaust close and i went up 3/16.. the saw would be junk.

just have to decide how much blowdown to go for.

MCB, I have just gone through some tuning with a new Husky 450. It came with 12 degrees of blowdown.

.... I took 2 degrees off the front of the piston crown next to the exhaust port. That took the exhaust duration from 150 to 154 degrees and moved the blowdown from 12 degrees to 14.

That helped considerably with the cutting speed. I then took another 2 degrees off the front of the piston. At 158 degrees of duration and 16 degrees of blowdown the cutting speed was again higher, but I had lost some of the user friendly low-end. Overall, I preferred the 154 degrees of duration....

excellent info terry. did you change compression at all?

 

[mcb]

i think im gonna cut the base and set squish soon, then probably toss a plug and starter back on it to see where my compression lands and re-degree.

then i can start tailering the roof of the exhaust port in increments to land wherever a 93 octane work saw should. say 160psi on fresh hone/piston/rings?

 

[ironman_gq]

175-180 you should be fine and have a lot of torque left

 

[Terry Syd]

Clamshell

It's a clamshell design, so I can't bump compression by dropping the jug. If I did do a compression check with a guage it wouldn't show much difference in compression from stock as the postitions of the ring and the top of the exhaust port remain the same as stock.

 

[Brmorgan]

I run my 181SE at 180 PSI on 91 octane and it burns just fine. I don't know the compression on my 041 in the video I linked to on the first page of this thread, but it's up there pretty high. I can hold it up by the starter handle for an awful long time without it dropping even one revolution.

If this works out for you, I have a spare 041 topend that I could send your way... I lack the knowledge and machinery to do it myself.

 

[mcb]

thanks guys, lets see how this one turns out first.

 

[SpaayDawg]

mcb, aren't you worried that raising the roof on the exhaust port that much with effectively decrease the compressive stroke? Wouldn't that lower the compression, or is the thought to have more exhaust out with more fresh charge in with just a little more compression from no base gasket?

Have you given any thought toward customizing the crank to complement the port work? Will the crank need to be as heavy if the compression stroke is shortened by raising the exhaust port? If the exhaust is raised that much and the compression stroke is "shortened" will the piston end up traveling at different speeds in the cylinder during the burn and compression strokes?
Am I thinking way too far into this?

 

[SpaayDawg]

What's so special about the seals and bearings that you can't just go to any generic bearing dealer and find them?

Jimbo

I haven't ever used "aftermarket" bearings or seals, I've always been able to find OEM. One reason is that I have no idea where I would look for them, maybe some other people could lend some input as to where to find some.

 

[ironman_gq]

find a local bearing supplier. Chances are that if you are in an area with any kind of industrial or commercial manufacturing you will have a bearing house nearby. Up where I am we have a huge mining industry and several bearing suppliers. Basically you bring in your bearing and they measure it up and find one in the catalog that will fit. They can usually get them in different grades and sealed or non sealed. I would stick with either Timken or SKF brand bearings for anything critical. Both are very high quality.

 

[Terry Syd]

gas dynamics

MCB, the reason I mentioned cutting the piston crown to increase blowdown is that blowdown is related to the dynamics of the gas flow. Once you have enough for the engine speed, it's enough. You would be able to find that point much easier trimming the piston and running the engine, than relying upon a linear measurment such as compression which isn't related to gas dynamics.

You could easily go past the point of sufficient blowdown (for your engine) if you are aiming at a certain compression figure. In that case, you would end up with a 'peaker' engine and with less compression than you could have had.

Of course, if you don't have a piston to sacrifice, then slowly raising the exhaust is your only option.

 

[mcb]

mcb, aren't you worried that raising the roof on the exhaust port that much with effectively decrease the compressive stroke? Wouldn't that lower the compression, or is the thought to have more exhaust out with more fresh charge in with just a little more compression from no base gasket?

Have you given any thought toward customizing the crank to complement the port work? Will the crank need to be as heavy if the compression stroke is shortened by raising the exhaust port? If the exhaust is raised that much and the compression stroke is "shortened" will the piston end up traveling at different speeds in the cylinder during the burn and compression strokes?
Am I thinking way too far into this?

In response to your first paragraph, piston ported two strokes are forced to strike a balance between valve open events and valve close events because they are mirror images in terms of valve timing. notice people just say 93* for exhaust timing? not "before" or "after" top dead center? thats because its exactly the same on each side and doesnt need to be spoken. Yes, raising the exhaust roof will delay (retard) the exhaust closing point and lower the compression ratio. this would be of issue were the compression ratio not being altered to compensate as in my case.

so lets take a look at the other side of raising the roof, exhaust valve opening point. a 4 stroke engine would be opening the exhaust MUCH sooner than a 2 stroke (because cams allow independant open and close that port windows dont) to take advantage of the much higher cylinder pressure occuring around 55* to 65* ATDC. (consider how late my saw's 109* is in terms of exhaust timing) the blast of high pressure evacuates the cylinder quickly and effectively, and tends to create a scavenging effect in its wake. if you want high RPM performance you open the valve sooner, making use of the higher pressure to better evacuate the cylinder at the expense of wasting some of the energy your fuel released during combustion. if you want torque peak down low or require max economy, you let the valve stay shut and hold the expanding gasses for as long as you can to allow each fuel particle to have done the most work. the point where maximum advantage against the crankshaft occurs is 90* atdc because the rod journal is at a right angle to the crank axis.

as for my saw and the amount im considering grinding out.. dont think of it in terms of how much im going to blindly hog out the port, think of it as the amount of material required (on this particular saw) to make the exhaust ports open and close at times similar to what could be expected out of modern two strokes. even with .180" out of the roof, the exhaust close point will be 95* BTDC, which isnt uncommon. the balance to strike with the exhaust roof is how completely do you evacuate on the exhaust stroke, vs how much gas do you trap on the compression stroke. i feel the 041 is biased heavily toward trapping compression stroke at the expense of exhaust evacuation, so much that its a detriment to the saw. i never, ever use this saw, because i cant stand how slow it is, torquey or not. thats what im trying to "fix" you could say.

your second paragraph is largely overthinking it. look at the bigger picture, the piston comes to a complete stop and reverses directions. travelling at different speeds is the nature of its life. as for crank weight, if they were balanced to begin with we wouldnt need AV mounts.

hope this helped jeff