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Woods Porting

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Al_Smith:

--- Quote from: Screaming Detroit on February 21, 2010, 05:50:30 pm ---
       In stage three you said turn down the out side of the piston, how much and is this so it inhales and exhales better?

--- End quote ---
Those so called "Stages " are just terminoligy a few saw engine builders dreamed up.

The reason for a pop up piston is several fold .If you cut a pop up it maintains the compression better ,causes the flame front to spread better and in essense raises the exhaust port without exposing a lot of piston to the exhaust which has a tendency to burn the piston .

Not everyone does it and still have really good running saws .It's just one method to enhance performance .

How much of a pop up depends on who is doing it I suppose .I have an 038 mag with a 30 thou pop up and a 200T with 12-15 thou . I've seen as high as 40 or so thou on saws done by others .There may be higher than that but none I've ever personally seen .

windthrown:

--- Quote from: bandmiller2 on February 21, 2010, 07:44:25 am ---Windthrown,you must fall in the "lot of man" catigory,I've never cut west coast.Only know northeast trees.How does porting affect the longevity of a saw in the work envourment??If you used two identicle saw one wood ported outher not which would live longer?? thanks Frank C.

--- End quote ---

Well, I do not fall trees with my willie. ;)

"Woods" porting as it is called is targeted to modifying a daily runner, so the porting is less aggressive/lighter than for a race saw. If they are done right, a woods port can enhance the life if a chainsaw. They will run cooler and breathe better and have more power. If they are done wrong, they can create a less lively saw. For these reasons I generally leave the transfers alone and leave the mechanical timing set to stock or near stock values. I do light port grinding for better engine breathing, shave the jug down just a smidge, open up the muffler to original stock design or a bit more, and tune the carb richer. Also note that in most cases a modified saw will not gain any value on the market. As stated, porting will void the warantee, and in many cases builders create some odd-ball saws that just do not have the power that would be desired from a ported saw. However, when they are done right, the ported saws have better power, better response, they run cooler, and in many cases they will outlast their stock cousins becasue they do not run as lean. Lean saws run hot, and running hot can lead to engine scoring, typically on the cylinder wall below the exhaust port where the most heat builds up. 

As for why companies do not port their own saws, it is the result of US EPA laws. In many cases they do make more open saws, but they are not for sale in the US. For example, the European Stihl 361 and 290/310/390 saws have better (more open) mufflers and richer carb settings. In many cases they originally made and sold more opened up saws in the US and they have become choked up over the years due to Federal EPA (and California state) regulations. The Stihl 026/260 is a good example of this. The early model 026 mufflers had more or larger ports in them, and the carbs were fully tunable. Now they have choked up mufflers and smaller ports, internal baffles, and they have limiter caps on the carbs to keep them from being richened up. By law no dealer can modify a chainsaw or face stiff penalties from the EPA. The EPA sent out flyers and notices to all chainsaw and small engine dealers late last year warning about doing any modifications. Owners are not restricted or limited by these laws though. ;)

windthrown:

--- Quote from: Screaming Detroit ---In stage three you said turn down the out side of the piston, how much and is this so it inhales and exhales better?

--- End quote ---

The main reason to turn down the piston crown is not for better gas flow from an intake to exhaust gas flow perspective (in some cases it actually screws it up *see below). Nor would you turn down the piston alone. That would lower the compression and make the squish larger (both bad from a performance standpoint). The main reason that pistons are turned down is for increasing engine compression when done in combination with turning down the cylinder. Higher compression will usually result in more power. To increase compression, most builders turn down the base of the jug. You can also remove the base gasket, or use a thinner gasket to get the same effect. Turning down the jug a lot or using a no gasket may casue a problem in that the outer crown of the piston will hit the top of the cylinder at TDC in the outer squish area. To prevent this from happening, the outer rim of the piston crown is turned down which domes up the piston crown center and prevents the piston from coming into contact with the cylinder head at TDC. The second reason that pistons are turned is to get a thinner squish band area. This has to do with the combustion chamber and piston crown shape. The gasses are pushed out of the squish area from the outside of the cylinder/piston just before TDC toward the dome area of the cylinder 'head'. This causes good movement and mixing of the fuel, prevents dead space and pockets of exhaust gas from forming, prevents uneven burning of the fuel, and helps to prevent pre-ignition. These are all good things.

There are several 'rules of thumb' regarding the limits to take the squish. The squish is measured at the outer edge of the piston at TDC. Typically you measure the squish by poking a thick solid solder wire bent at the tip into the spark plug port and push it in to touch the side of the cylinder wall with the piston below TDC. Then pull the piston up to TDC and back down, and then remove the solder wire and measure the thickness of the squshed end. A stock saw might have a squish of 0.035-0.04". Typically a woods ported saw would have a squish somewhere around 0.02", and a race saw might have as little as 0.01" squish. The stock and building recipies vary a lot in terms of squish. The piston turning limit is that you do not want to turn the piston down into the top ring slot, and you need to leave a gap (minimum 0.1") between the crown and the top ring slot.  

* Note that turning the piston and cylinder base down both change port/mechanical timing of the intake, exhaust and transfer ports. They also counter each other; lowering the jug directly lowers the ports, and lowering the piston crown in effect raises the ports back up. Typically squish is decreased on a ported saw, so the net effect is that the ports are lowered. Note also that you can increase the squish by turning down the cylinder without turning down the piston. But in this case, you are lowering the ports.

John Mc:

--- Quote from: bandmiller2 on February 21, 2010, 07:44:25 am ---Windthrown,you must fall in the "lot of man" catigory,I've never cut west coast.Only know northeast trees.

--- End quote ---

Remember, Windthrown mentioned he is bogging it in 3 ft DBH trees. probably not too many of them in your neck of the woods, are there, Bandmiller.

bandmiller2:
John,there are more than you think,just yesterday I was walking in my neighbors woodlot many ewp in that 36" catigory wether their solid to the core I don't know.Most of the logs I get are from a tree service and come off old estates and yards some are real lunkers but I don't like working with them,16" to 18" is what i like.Frank C.

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