Need help on mods

[Bruiser]

Inspired by Stihlman036's woods job he did on a Husky 345 (see: http://www.arboristsite.com/showthread.php?s=&threadid=6324 ), I've become inspired to do something similar to my Husky 350. This will be my firewood and clearing saw. I realize that the 350 is a full-on consumer saw, but I think some mods will really wake it up. They sure helped Chris's 345. I also realize that several guys here make a living modifying saws and I don't wanna take anything away from them. If this works out as planned, I'll surely send my next, more pro-model saw off for the next stage of modification. I can handle opening up the muffler. I have a 346xp rim sprocket on order. I removed the screen in the intake boot. I pulled the cylinder and polished and chamfered the intake port. I opened up and polished the exhaust port, beveling it to match the muffler gasket. I also polished the transfers and combustion chamber, although they are hardly mirror finish... the castings here were pretty rough. I'd like to avoid going into the bottom end if possible, but want to raise the compression a little. I'm thinking the best way is to mill a little off the bottom of the cylinder. I realize that with these little cylinders a little milling can raise the compression a lot, so I need some help figuring how much to mill off. Is 0.020" in the ballpark? Also, I assume that the windows on the piston sides will need to be raised at least an equal amount to compensate for the lower cylinder. Should I open them any higher, increasing the exposure time (there's not a lot of room to remove much there)? Anything else I need to do? Carb work?
Thanks for any input.
Dave

 

[glens]

I'm certainly no expert on 2-stroke engine modification but I have an opinion nonetheless.

I've been somewhat alarmed by some of the things I've seen regarding the stated order of the work being done.&nbsp; If you modify port timings and <i>then</i> work on the piston skirts (if that's even a good idea, especially if you don't then work on the crank as well) the associated port timing work was pretty much pissing in the wind unless you knew ahead of time exactly what you were going to do with the skirt.

Likewise, if you time the ports and <i>then</i> lower the cylinder to increase compression, you'll be altering the timing after having worked on it.&nbsp; Again, you hopefully knew ahead of time exactly how much you were going to lower the cylinder and took that into consideration.

In either of those cases, and in any event, I should think it would be much easier to set the timing <i>last</i> "to begin with".

I've never said anything because I assume the stated order does not match that of the actual modifications.

Regarding polished passageway surfaces, I should hope you would not get them to become too smooth.&nbsp; Very smooth surfaces can be somewhat "sticky" in terms of "aerodynamics", as it were.&nbsp; You wouldn't want to do anything which would tend to cause the liquid payload to separate out from the mixture.&nbsp; Look to shark skin (natural "phenomenon") and golf balls (result of experimentation and later confirmed scientifically) for some examples why slightly rough surfaces are better when in intimate contact with flowing "fluids".

That's really all I have to say.&nbsp; Things like passageway shapes and such are other, more "delicate", matters.&nbsp; I only wished to speak to the more "ham-fisted" stuff, so to speak.

 

[Crofter]

I think you are at risk of doing permanent damage to your cylinder. The questions you ask suggest you should do some more paperwork before you do any metal work. Enlarging passages if that is not required can actually hurt performance.

Frank

 

[bwalker]

Polishing is mostly eye candy. The true gains from a saw are made with adjustments to port timeing and muffler derestriction. Bulk flow is seldom a issue with a two stroke and like crofter said you are likely doing mor harm than good. Transfer and intake ports do not like to be smooth.

 

[Bruiser]

I realize now that raising the piston skirt is working in the opposite direction of lowering the cylinder. The only other 2-stroke I've modded was a kart engine with a real head, much easier to adjust compression and with a much larger bore to ft my ham fists into. Any suggestions? Enlarging the passages that I did shouldn't harm performance. This saw needs to move more volume. It could actually use a lot of hogging out, but I'm not looking to reshape ports.
Dave

 

[bwalker]

Enlarging the passages that I did shouldn't harm performance. This saw needs to move more volume. It could actually use a lot of hogging out, but I'm not looking to reshape ports.

YES YOU CAN! I will repeat. BULK FLOW IN A TWO STROKE IS SELDOM A ISSUE. Its the timeing and interaction of the ports that makes more power. Simply enlarging a transfer will likely flow less at the rpm band you want it to because your are killing velocity.
If you want more power send it to a reputable tuner and have them do it right.

 

[Bruiser]

shhhhh, no need to holler... I didn't mention anything about enlarging transfers. We'll just agree to disagree here. I know enough about the theory to know that flow is an issue. Thanks for the input, though, even if a little overstated.
Dave

 

[bwalker]

I know enough about the theory to know

Not nearly enough or else you wouldnt ask the questions you are asking. BTW cutting the piston skirt will increase your intake time area.

 

[Bruiser]

Actually, I just meant the windows on the side of the skirt, where the tranfer ports are exposed, not affecting intake time. This would work in the opposite direction of which way I need to go though, so no need. No need to turn this into an argument Ben, I'm just looking to get some input. And again yours is appreciated. I still stand by my statement that flow is an issue here. IMO it's the main issue. These these are only air pumps after all.
Dave

 

[bwalker]

I still stand by my statement that flow is an issue here. IMO it's the main issue. These these are only air pumps after all.

Read any basic two stroke tunning book and you will relise this is not that case. Timeing is everything.

 

[buddyballs79]

Hey good luck, I bought the 345 from Chris and I really like it. I have since done the muffler on my other saw and it seemed to help. I have looked up old posts here with the search and found some good pictures and info so you might want to try that, Chris is a super nice guy so try asking him. Again, good luck and tell how it went.

 

[Crofter]

Bruiser;

You are right in saying that flow is an issue but you seem to be missing the concept of velocity of the gasses and their direction into the cylinder and how this acts to sweep out the exhaust with a minimum of mixing with the incoming charge. The shape of the end of the port has a lot more importance than you think. Increasing horsepower while also retaining flexibility AND idle and accelleration is not as simple as you think. What seems obvious is not always so!

Frank

 

[bwalker]

Two stroke porting is seldom intuitive. Enlarging ports can be very damaging to power because it kills port velocity and shifts peak flow to a rpm your saw wouldnt run at. I know tuners that actually epoxy transfer ports in bike cylinders to widen the power curve.
Polishing is somewhat useful on the exhaust port and head because it may help lessan carbon buildup(so will good oil and correct carb settings). Its far from neccessarry though and will provide no gains in power. Polishing the transfer ducts and intake port is a bad idea. Many guys will actually bead blast the intak ports to help with vaporisation which is important for a two cycle. Also keep in mind that when flowing air through any pipe most of the flow is occring in the middle of the pipe. The area along the walls has hardly any charge motion at all.

 

[woodsjunkie]

bwalker,
You seem to be very knowledgeable with two stroke engine.
So why don't you put you money where your mouth is and build a saw.
Most of us on this site are from Missouri The show us state.
I'm sure Ken , Dennis and Dan would like the competition.

 

[bwalker]

woodsjunkie, Me and a porting tool is a disaster. I just dont have the touch required to port a cylinder. I have tried it on practice slugs with bad results Not to mention the fact that there is a lot more to tuning than the theory I know. There is a lot of trial and error although this is lessened now days with the advent of sim software. If I honestly though I could build a saw that would spank Kens I would do it, but I know I cant.

 

[Bill G]

Bruiser,

I will not argue on either side of the modifications you have mentioned. They may or may not increase performance. My question is WHY ? The mods you mention could easily destroy a good saw. I would never risk an expensive saw just to gain a moderate performance gain. Buy a stock 346XP or 357XP and you will have more power without mods. If you want to play with modifying a saw why do it with an expensive one. You could buy three cheap Poulans and modify them to the point of exploding the jug. To me the point of modifications is not the end game but the learning process of getting there. Personally I would get my experience at a cheaper price tag. I would bet the pro saw builders have exploded many saws. That is how they got to be pros

Just my thoughts

Bill

 

[Tzed250]

Get it all together

On a two-stroke, as any IC engine, everything is a system. The carb, intake, transfers and exhaust all have to work with each other or less power will result. Don't kid yourself into thinking that the engineers in Sweden left metal in the transfers just so someone would have something to grind out of there. You might make the transfers "flow" more by removing material, but does the carb now determine max flow, or is it the exhaust? This is why flowbenches were created. So that it is not a guessing game. Flowbenches exist to derive more flow, while keeping the system balanced. If you remove metal from the transfers you run the risk of having the plating peel if you do not do it just right. The exhaust is probably the most restictive element. You might want to start with small mods there. As others have said, polishing the interior surfaces of the transfers can hurt "mixture" flow as well as power. The fuel droplets tend to get hung up in the boundry layer next to the wall, then impede flow as the boundry layer gets thicker. A rougher surface (about 125 microinches) will help the fuel to stay in suspension. Get a couple of two-stroke tuning handbooks and they will help to "turn the light on" for you. Good luck


John

 

[timberwolf]

Win some loose some

Bare in mind that chainsaw modifications as all things are cause and effect / win and loose. You state that this is a fire wood saw, as a consumer fire wood saw it is designed with traits that lend itself to best use as just that "A fire wood saw".

Considder these win loose senerios remember for a fire wood saw, a race saw would have a different value system for determining acceptable losses.


Improved chain sharpening technique

Gain: improved cutting speed, reduced wear, increased saw life, increased fuel efficiency, improved usability.

Loss: time required to learn and maintain skills, chain may require more maintainence time to keep it up.

Are losses acceptable in relation to gains?: YES


Muffler modification, matching & polishing exaust port.

Gain: Improved cutting speed, reduced heat, increased saw life, improved usability.

Loss: Time to make modification, Increased sound levels, compramise EPA and Waranty, increased fuel consumption.

Are losses acceptable in relation to gains?: IMO YES, I wear hearing protection, and don't mind burning a bit more gas if the saw works well and lasts longer.


Port modification with incured changes to timing, squish ect.

Gain: potential gain in peek preformance charicteristics, increased cutting speed.

loss: reduced usability, void warranty, reduced power band width, reduced fuel efficiency, risk of engine dammage and reduced saw life, time and expence, non stock saw replacement parts problem.

Are losses acceptable in relation to gains?: For cutting fire wood NO, as a race saw or science fair experiment YES


Removing choke plate and installing primer

Gains: Reduced intake restrictions can improve cutting speed and engine charicteristics, primer is more efective than choke for cold starts as long as you don't flood it.

Loss: time and expence, reduced usability, waranty and non stock parts problem, may have starting problems in real cold weather.

Are losses acceptable in relation to gains?: (for fire wood) Distant Maybe, for a race saw Yes.


Use thin kerf race chain

Gains: increased cutting speed, reduced load on saw.

Loss: Bar would pinch and get stuck in cuts, kick back potential, chains can break if they contact a nail or barb wire, short chain life, expensive and time consuming to maintain.

Are losses acceptable for fire wood? No for a race? Yes


There are lots of other modifications that can be done, they all have wins and losses. The idea is to determine what the objective is and hedge all decisions to give the wins where required for that aplication, making sure the losses are acceptable.

If you find a win-win solution your likely to loose you money getting it.

Timberwolf

 

[Jacob J.]

<p>Hi Folks<p/>

<p>The local saw builders I've talked and worked with here in Southern Oregon ( Hunt's saw shop, in Roseburg, Or. for one ) say the same things. Determine the compression and piston mods you're going to do for one before attacking the timing. Glens was correct about the sequencing. Cranks are also made at a certain weight. If you lighten the piston you may have to shave weight off the crank. One local saw build chucks the crank-rod-piston assembly in his lathe and balances the entire set-up as it would be in the saw.<p/>

<p> If you're rebuilding a saw and using the old cylinder, one good way to clean it up is to stuff ragsn tightly in it and sand blast the outside. This way you can sand blast the exhaust port and not have to spend time cleaning all that carbon out by hand. You can also sand blast the intake port for a good, even rough finish.<p/>

 

[bwalker]

Jocob, Piston weight and crank ballance dont go hand in hand as you suggest. A lighter piston will vibrate less with out any crank work. Cranks can be rebalanced, but the facvtory does a good job of this or you would see failures galore. Changing the balnce factor of the crank is something altogether differant. I would imagine the crank has aballance factor such that the saw vibrates the most at low rpm as the saw is rarely used in that area and would have smooth high rpm power.