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transfer port question

Discussion in 'Chainsaw' started by Brent Nowell, May 11, 2019.

  1. Brent Nowell

    Brent Nowell ArboristSite Operative

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    Modifying transfer ports on a 372 or any 2 stroke for that matter.

    I see a lot of pics of people raising the port wall on the cylinder up to the bottom of the flange. I’ve searched quite a bit and maybe the answer was there and I missed it, but what are the negatives to doing this?
    Weimedog on his videos calls this the “spike special” in which it was supposed to be an easy mod with a Dremel that does not require porting work skill. However this is a large change from stock, and you are enlarging the holes on each side. Does this change the velocity of the charge? Does it slow it down because the bottom of the piston is no longer pushing through a small hole but now a large one?
    I am more inclined to smooth the ports out rather than enlarge them. One video a guy said that any transition over 7 degrees causes unwanted turbulence.

    What do you guys think?
     
  2. cuinrearview

    cuinrearview Red saw lover

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    So I'm gonna take a stab at this. Now, I'm a total newb at the game of cylinder modification, so my thoughts are worth what you're paying for them.

    On most saws more material in the crankcase=more case compression=better velocity. For some reason the effect is negligible on some Huskys. The larger opening negates the loss of case compression, or it's so high to begin with it doesn't matter. I've seen more than one builder do lots of work to the lowers and barely touch the uppers. One thing to remember too is the lower transfer work is just one part of the total cylinder mod. Two guys might do the lowers the same, but because they advance the timing to different degrees, or one raises the exhaust three degrees more than the other, the saws can run differently. I think that Dr Al once said that the first 90% of gains from a port job are easy to obtain. It's that last 10% that amounts to the work that's paid for.
     
  3. grizz55chev

    grizz55chev Tree Freak

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    That same math applies to trolling for trout, 90 percent of the fish are in 10 percent of the lake!
     
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  4. rmh3481

    rmh3481 Addicted to ArboristSite

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    The drawback is fuel falls out of suspension. Liquid or tiny droplets cannot be compressed.
     
  5. NSEric

    NSEric ArboristSite Operative

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    If you want high rpm power open the lower transfers up, if you want mid range grunt leave them alone. The stock set up is better at low rpm but most want more rpm and peak power so they open them up.
    Weimedog built a bunch of different 372's on youtube, one was a 376 stock cylinder milled down for more compression, total grunt saw. Lots of others have built 372's to scream, they have opened up lower transfers plus other porting, different mods for different setups.
     
  6. drf255

    drf255 BAD CAD

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    2 Stroke porting is like all else, sometimes something that defies theory runs better.

    90% of the gains are from 10% of the work, and the last 10% take 90% of the effort. That’s the old saying.

    I’ve never done a 372, but your thinking is exactly why quad transfers became the norm in modern saws (just like multi valved 4 strokes). You get velocity AND volume.

    One rule rarely applies to all.

    I believe keeping the upper transfers tight is better in general. Even on the same model and oem, some manufacturers of OEM jugs have different geometry in their jugs.

    On the Stihl 034S/036, the KS 034S jugs have the tightest squarest uppers and run the best stock. The Gillardoni MS360 jugs have huge tunnels and uppers and don’t run quite as well stock.

    Tight uppers with more velocity can tolerate less Blowdown because they have more pressure. Blowdown is the time in degrees between exhaust and transfer opening. You can get away with more in a quad because they flow more. In a dual port, they tend to be larger lazier flowing ports, so less BD is needed of they won’t have time to purge and scavenge as well. There is a reason one rarely sees a modern saw with only 2 upper transfers. Some have 6 now (Dolmar 421?).

    I’d go the the forbidden site (O-P-E) and ask for advice specific to your model. There is also a thread on the Chainsaw forum with links to porting theory threads. I think you’ll find them interesting. IMHO, your thought process is correct.
     
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  7. Brent Nowell

    Brent Nowell ArboristSite Operative

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    Thank you for the kind words! I love this saw and I am always striving to make things run better. I practiced on a poulan and it worked out pretty good.

    I think really all I want to do is maybe smooth the transfer lips, make a good angle instead of a round one. I will probably buy an aftermarket cylinder to practice. I really don’t want to change the torque curve just want air/fuel to flow smoothly. Why? OCD I guess haha, again I like to fix what isn’t broken. This is what I’ve done my whole life with the things I own.

    Great info here, keep em coming.
     

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