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The myth of high compression in 2-strokes

Dennbb

Dennbb

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It is not the pumping losses that limit power. Second, increasing compression does not reduce power...within the bounds of keeping the ignition advance due to detonation. Some of y'all are rolling fuel octane, and durability into this, and a few have even mentioned heat transfer out of the piston...
cheers,
Douglas
Increasing compression in a 2 stroke limits RPM.
 
Slotracer577

Slotracer577

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Many things interrelate, generally more compression is more hp, but it comes at the cost of heat and increased likelihood of detonation. It also tends to decrease power above peak rpm, ie it signs off harder. Your fuel quality will be one of the major limits. Also squish velocity will be a limit as well. Squish velocity is a function of squish band width and height, if too high you can make the squish band narrower. This was a common change to 70’s and 80’s motocross engines which tended to have wide squish bands which lead to bad detonation. Fix is to make it narrower.
squish height tends to get lumped in with increasing compression. IMHO squish height is more important than the actual compression. It should be set so that the piston has just .001-.005” clearance when running. This pushes any mixture into the center of the combustion chamber and increases turbulence in the combustion chamber. This increases flame speed allowing less timing to be used and gives a more complete burn both increasing efficiency. really a gain with no down side. I think the only reason it’s not done from the factory is it takes too much time on assembly to get it right. Test after test show this helps any engine.
 
Termite52

Termite52

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There are systems to prevent the problems you describe. For chain saw use I mix about two half gallon clear bottles for an average day. almost all the time I will use one bottle up in two days. If the second bottle sits around more than a few days then I just dump it. Its the easiest cheapest thing to do. If I have a heavy week of cutting coming I will mix several bottles of mixed gas to start out the day. Clear bottles tell you if there is contaminates in the fuel or if moisture is present. For other equipment the situation is different. Here three out of every household here has a generator, at least half are programmed to start up every few days and run for fifteen minutes. For all others a petcock is installed with a drain hose. About once every week or two the owner starts up generator and runs it for fifteen minutes. The owner never lets the fuel go below a half a tank. After the fuel is three months old fuel is drained with new fuel added. Here we have fifty outages a year with the longest being about three weeks at a time. Then we will go five years with only one or two outages knowing the loss of power will happen. Thanks
If you use a top grade gas like Sunoco gtx 260 you don’t have to drain the tank every three months. It comes from the factory with a preservative in it for storage. Same for small engines.
 
weimedog

weimedog

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Many things interrelate, .... This was a common change to 70’s and 80’s motocross engines which tended to have wide squish bands which lead to bad detonation. Fix is to make it narrower.
squish height tends to get lumped in with increasing compression. IMHO squish height is more important than the actual compression. It should be set so that the piston has just .001-.005” clearance when running. This pushes any mixture into the center of the combustion chamber and increases turbulence in the combustion chamber. This increases flame speed allowing less timing to be used and gives a more complete burn both increasing efficiency. really a gain with no down side. I think the only reason it’s not done from the factory is it takes too much time on assembly to get it right. Test after test show this helps any engine.
Yup, an important concept in this post. I hate generalizations as like all things, they often are miss leading and focus on trying to make some point somewhat not related. As with all the physical characteristics of a motor, there is a blend, a combination of "sweet spots' based on a design & desired characteristic and as a result there is a concept of "diminishing returns" on all those things as well. Squish bands and moving the mix to the combustion chamber is not a trivial thing and is often the goal of reducing the squish number. TO Compression, it too can be described with math and effects efficiency, but there is a point of diminishing returns based on a whole host of other issues. Conversely if compression didn't matter...50psi would be fine in your hot saw. ALL the factors of these motors working in concert define the power characteristics. From primary compression to port timing, from air box to muffler. X-sectional area of the ports. All of it. Not many are old enough to have experienced the curse of the old "open" class bikes from the 70's and 80's. Especially the air cooled ones.. Just got rid of my last , a 1979 KTM 420 MC80 :)
 
Ted Jenkins

Ted Jenkins

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Yup, an important concept in this post. I hate generalizations as like all things, they often are miss leading and focus on trying to make some point somewhat not related. As with all the physical characteristics of a motor, there is a blend, a combination of "sweet spots' based on a design & desired characteristic and as a result there is a concept of "diminishing returns" on all those things as well. Squish bands and moving the mix to the combustion chamber is not a trivial thing and is often the goal of reducing the squish number. TO Compression, it too can be described with math and effects efficiency, but there is a point of diminishing returns based on a whole host of other issues. Conversely if compression didn't matter...50psi would be fine in your hot saw. ALL the factors of these motors working in concert define the power characteristics. From primary compression to port timing, from air box to muffler. X-sectional area of the ports. All of it. Not many are old enough to have experienced the curse of the old "open" class bikes from the 70's and 80's. Especially the air cooled ones.. Just got rid of my last , a 1979 KTM 420 MC80 :)
Chain saws as we know them today are in their own class and maybe only compared to a leaf blower. They are low tech low performance low cost and contain little technology. Because of these issues raising the compression on many chain saw motors yield an improvement. On two stroke race motors such as dirt bike motors compression can harm or explode literally. The reasons for most part are that race motors with sophisticated tuning can not tolerate very high compression. As a typical two stroke race motor increases RPM several things start to change like the expansion chamber will push air back into the cylinder more and more causing higher compression in the combustion chamber. If the compression becomes too high detonation will be the first sign of problems then maybe burn the piston and so on. I think that some one with a sense of engineering could improve the exhaust systems on many chain saws, but why. Simple has worked well for many decades. Yes am quite familiar with open class dirt bikes as I started playing with them in the sixties and still have a few, but do not ride them with the same enthusiasm that I once did. Sailing at 10 to 15 knots seems quite fast enough. Thanks
 
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