Biggest perpetuated Myths about Modded Saws.

[Mr.]

So here it goes......... You hear them all the time. What are the biggest myths flying around. I'll start it off.

1. You should set your squish at 20 thousandths. Really? For every saw? Regardless of combustion chamber shape or size?

2. Make you exhaust as wide as possible for the most power. Huh? What if you are also raising it? What if the exhaust comes close to the transfers? What if you can already stick a turkey leg it there?

3. If you take the air filter off it'll really wake it up. Seriously, the air filter is always robbing power? The carb can ALWAYS outflow the filter?

Post em up!

Fred

 

[04ultra]

Muffler outlet must be 80% to ??? of the exhaust port...........

 

[2000ssm6]

My saw is faster without a muffler.

 

[bigbadbob]

The "ANTI SUCK-BACK" mod does'nt do a thing!!!!:jawdrop:

 

[Mr.]

Those were a couple of good ones. Another one of my favorites is that "if you buy $1000 worth of software, then you are an expert."

Fred

 

[bookerdog]

Those were a couple of good ones. Another one of my favorites is that "if you buy $1000 worth of software, then you are an expert."

Fred

Now thats a good one.

 

[husq2100]

ok Mr. so what in your mind determinds squish on a modded saw?

ill give you an example:

husky 288xp,
must be a work saw, and i want to keep torque.

alot seem to say that .02-.025 is a good squish, so are you saying its bad or just isn't on the fine edge of performance

i would like to know as im building 2 288's and dont want to stuff them up

cheers, Serg

 

[Mr.]

For saws without a custom combustion chamber, i.e. two piece heads........

the squish does not mean much. With good eqipment it would be difficult to measure the difference in power or temperature between .014 and .025 inch.

The compression will vary a little, but that's about the size of it.

The rules change when the combustion chamber becomes a variable (changable.)

Fred

 

[timberwolf]

the squish does not mean much.

The compression will vary a little, but that's about the size of it.

This sounds a bit like a myth in its self to me.

Taking the 288 saw as an example with a 8cc combustion chamber and .025 squish height and say 160 lb of compression with a squish band 7mm wide, 10,000 RPM in the cut.

By my calculation:
Piston area is 2290 mm2, area under squish band is 1018 mm2
The squish area to bore area is 45.13%
Trapped end gas is going to be 7.15%
Maximum squish velocity (MSV) is going to be 31.89 m/s @ 12 deg BTDC

Now if squish is brought down to a tight .014 squish height here is what I see happening
Head volume will drop to 7.36 cc
Compression will go up to about 173 PSI
Trapped end gas will drop to 4.21%
Maximum squish is going to go up to 45.14 m/s and be delayed until 9 deg BTDC

Net effect is going to be about a 3% gain in power netted from combustion due to the reduction of end gas and about 2.5% gain from improved thermal efficiency from the additional compression. Also increased squish velocity should improve combustion efficiency a little also. However in the case of going to a squish this tight the MSV is coming quite late in the combustion phase and there may be some interference between outgoing gas from the squish and the expanding combustion cloud forcing more unburnt fuel into the squish where it becomes additional end gas. Overall dropping the squish height from .025 to .014 should produce a little better than a 5% gain in output and that is pretty significant and measurable with a block and stopwatch.


A MSV of 30 m/s is often referred to as a good maximum, However this number relates to bikes and sled research with a more square bore to stroke ratio in mind, with saws having a significantly over-square design both the squish gasses and expanding combustion cloud must travel further before they come into interference, so the upper limit of productive squish velocity is most likely higher than 30 M/S

PS

Made the program to do the calcs up my self, it's all pretty straight forward math, but pretty much impossible to do without a computer simply due to the number of calculations required. Did not cost me a dime, so I guess that busts the myth on needing

$1000 worth of software

 

[Mr.]

If you can find a machine that is within 3% accurate on measuring the kW of a chainsaw, then..................

If not, a theoretical 3-5% gain does not really amount too much.

BTW what are you using as the coefficent for the efficency of a heat engine. (As described by Newton)

Fred

 

[Mr.]

It has been my thought that such a small squish band, as in present in a stock 288, cannot be factored in the same manner as a relatively wide squishband.

There seems to be an inverse relationship between the two.

Fred

 

[timberwolf]

3% on reduced end gas and about 2.5% on top of that for thermal efficiency, plus another bit for improved combustion due to turbulence is going to be a gain over 5% (being conservative). As far as I am concerned there is no coefficient that can be applied to power gains in a two stroke in relation to compression. From what I have found though your looking at an average of about 2% for every 10psi, but it is a diminishing return, the first 10 psi gain is going to do more than the next 10 psi after that.

I make no claiims to have any way to know just how much increased combustion turbulence improves output, but it is certainly a pretty well established practice by those who know.

Theoretical, oh yah, theoretically the difference between an envelope with the prize ribbon and otherwise trying to make up storries about how the other guys cheated.

5% on a 5 second cut is a 1/4 second, races are won and lost by much less than that. 5% is even quite noticable just by feel. But if you don't think squish is importaint, don't worry your head about it any.

 

[Mr.]

I said that it doesn't matter much for a stock 288 head. It comes into greater play with a custom combustion chamber.

Thermal efficency of a chainsaw is ~.25 and it decreases as we modify them in the conventional method we use. That is why I asked you about a coeffiecient.

It is obvious that you do not want actually discuss your theory and math with your response.

I have never accused anyone of cheating. I think most of that talk comes from further north of here. I really don't give a damn if people do cheat. It just means that they can't win any other way.

BTW

I make no claiims to have any way to know just how much increased combustion turbulence improves output,

I think your quantified chart states otherwise.

Fred

 

[timberwolf]

The chart does not have anything to do with the effect of turbulence on output, it has to do with the squish velocity, and when it occurs.

As for thermal efficiency, chainsaws are not ~25%.

You might want to take a read of "The High Performance Two Stroke Engine" Chapter 3, pg 45 and table 3.2.2, the just of it is thermal efficiency depends on the swept volume, and can range from 10-15% for very small engines to about 50% for large industrial engines. Chainsaws would fall in about 20% or a little less at best. You would need to be into about 1000 cc in a single cylinder engine or very high compression before a 25% thermal efficiency would be expected.

Here is a little tid bit of reading also. http://www.popularhotrodding.com/enginemasters/articles/hardcore/0606em_understanding_compression_ratio/index.html

The importaint part from the article:

Thermodynamics Made Easy
It takes the barest of mental agility to appreciate that increasing the CR will raise cylinder pressure, thus causing torque output throughout the rpm range to simply follow suit. What is less obvious is that the increase in output from the higher CR comes about largely due to an increase in thermal efficiency. The thermal efficiency is a measure of how effectively the engine converts the heat-generating potential of the fuel, when burned with an appropriate amount of air, into mechanical power.

I don't know what you are doing on mods that lowers thermal efficiency, but the way I am working at it I make every effort to turn heat into motive power and in that shoot for high thermal efficency.

 

[timberwolf]

One more good read, check out page 3 Fig. 6. It shows pretty clearly how compression affects thermal efficiency, also the article shows how the diminishing return effect comes in with respect to increases in compression and the parrasitic losses that cause it.

http://www.bridgestonemotorcycle.com/documents/higher_compression6.pdf

You get where I am coming from in saying there is no single coefficient for thermal efficiency that can be applied?

 

[husq2100]

hang on Mr.

in post #8 you said "For saws without a custom combustion chamber, i.e. two piece heads........

the squish does not mean much."

then in post #13 you said "I said that it doesn't matter much for a stock 288 head. It comes into greater play with a custom combustion chamber"

now post 8 seems to contradict post 13????

what am i missing?

cheers, Serg

 

[Crofter]

Maybe some of these examples of "myths of saw modifications" are not such good examples! It is not clear whether they are the myth as posted or the reverse is the myth.

Not sure here whether the motive is really to dispel myth or whether it be something else.

 

[Jacob J.]

I know one thing- a difference of .010" in the squish of my 281 with the stock combustion chamber sure made it a lot harder to start.

 

[romeo]

hang on Mr.

in post #8 you said "For saws without a custom combustion chamber, i.e. two piece heads........

the squish does not mean much."

then in post #13 you said "I said that it doesn't matter much for a stock 288 head. It comes into greater play with a custom combustion chamber"

now post 8 seems to contradict post 13????

what am i missing?

cheers, Serg

He is saying that the squish height means little compared to the combustion chamber area. If all saws had the same dish in the top it would matter but they don't. This 372 head has .025 squish but it builds around 197 lbs of compression. I wish the pic came out cleaner, but I'm not going to pull it apart to take another one.

 

[Crofter]

Romeo, for sure when you start machining and reconfiguring the geometry of the squish band and changing the ratio of the squish area to piston area, you could well be into new territory in arriving at reasonable squish velocity. Agreed that squish velocity is not synonymous with compression ratio. I would think that a 20 thou. squish clearance would be reasonable advice for a ballpark figure for simple modification. It would likely be more useable to an entry level whittler than telling them to shoot for 100 ft per second squish velocity. Arriving at it that way sure would take a lot of number crunching.

I just dont see it as a great myth to be debunked even if it is not a universal truism. I have no idea who is supposed to have been promoting that figure but I have the feeling that some of the so called myths here might have a designated personal target.