Chainsaw Porting Theory

[Al Smith]

I might add with the above cut and paste it was too long to do in it's entirety .However it's interesting if somebody wants to Google it .Cline in his writings defines pre ignition as being before top dead center for varied reasons .He defines detonation as happening from the unburned gasses of combustion happening after 14 degrees past top dead center .Similar in nature but not one in the same.,

 

[drf256]

So I've come to learn that "4 stroking" occurs when the mix is too rich to fire. So that combustion cycle goes unburned, and then it mixes with some exhaust days and ignites on the next cycle. If it takes 2 cycles, then it's "6 stroking".

I'm having a hard time understanding why an engine goes from 4 stroking with no load to 2 stroking under load.

You lay the chain into wood, so now you have resistance.

Does the motor suddenly have more chamber heat to burn the mix better?

What's actually happening?

 

[Brush Ape]

I'm gonna post freely here. If I'm wrong, tell me why. That way I can learn.

Well just one minor observation so far......Giant Member -eh??? We'll have to get some cookies lined up on the tailgate and see who can rake more off.

 

[drf256]

Having a serious spider problem in my house


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[Brush Ape]

House? It looks like you live in the projects.....

 

[Moparmyway]

So I've come to learn that "4 stroking" occurs when the mix is too rich to fire. So that combustion cycle goes unburned, and then it mixes with some exhaust days and ignites on the next cycle. If it takes 2 cycles, then it's "6 stroking".

I'm having a hard time understanding why an engine goes from 4 stroking with no load to 2 stroking under load.

You lay the chain into wood, so now you have resistance.

Does the motor suddenly have more chamber heat to burn the mix better?

What's actually happening?

The load on a WOT chain is minimal and does little work, when you put the chain in wood, more work is done, so more power is needed.

At no load, the tune is slightly rich for the load and we see the typical 2 stroke studder or 4 stroke. That same tune cleans up with more load, and as the load increases, more fuel is needed to cool the machine and have the necessary power to do the increased work

 

[drf256]

Another question.

Has anyone here experimented with a tight small combustion chamber and a dished piston?

I was considering trying this on a 262. Swap in a 261 piston.

 

[drf256]

The load on a WOT chain is minimal and does little work, when you put the chain in wood, more work is done, so more power is needed.

At no load, the tune is slightly rich for the load and we see the typical 2 stroke studder or 4 stroke. That same tune cleans up with more load, and as the load increases, more fuel is needed to cool the machine and have the necessary power to do the increased work

Agreed. But what's physically happening in the chamber that makes a richer mix burn cleaner under load?

The A/F ratio in the case is unchanged. So should be what's in the chamber. The throttle plate is wide open and the ignition timing is the same.

The richer you make the mix, the more bar pressure it takes to eliminate 4 stroking. Up to the point that it can't be eliminated at all.

The RPMs at WOT certainly drop under load.

But why is the chamber burning the mix cleaner under load?

 

[drf256]

House? It looks like you live in the projects.....

Times are tough. No toilet seat till I sell off some saws.

I'm thinking of going with a shag rug cover from the 70's.

 

[Sty57]

Having a serious spider problem in my houseView attachment 12175


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WTF
I would throw a stick of Dynamite in there and run!

I hate Spiders and Clowns.

 

[drf256]

I stand corrected, possibly, about transfer backflow.

Blair doesn't describe rpm changes with this flow.




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[Sty57]

Has anyone put a EGT on a saw to watch how much the Temps change from 4 stroking too 2 stroking?

 

[Magic_Man]

Agreed. But what's physically happening in the chamber that makes a richer mix burn cleaner under load?

The A/F ratio in the case is unchanged. So should be what's in the chamber. The throttle plate is wide open and the ignition timing is the same.

The richer you make the mix, the more bar pressure it takes to eliminate 4 stroking. Up to the point that it can't be eliminated at all.

The RPMs at WOT certainly drop under load.

But why is the chamber burning the mix cleaner under load?

I understand what your saying but don't know that I can explain why. I guess the way I see it is that when the saw is 4 stroking without a load it's just free revving. There is little to no resistance or work to be done. So your tuning the carb at a high rpm until it's rich. Now, when you put that same engine under a load rpm's drop, more heat is created, and the combustion process slows down allowing the fuel to completely burn.

Maybe I'm way off key here, that's just how I see it.

 

[drf256]

I understand what your saying but don't know that I can explain why. I guess the way I see it is that when the saw is 4 stroking without a load it's just free revving. There is little to no resistance or work to be done. So your tuning the carb at a high rpm until it's rich. Now, when you put that same engine under a load rpm's drop, more heat is created, and the combustion process slows down allowing the fuel to completely burn.

Maybe I'm way off key here, that's just how I see it.

That's what I'm getting at. The "why".

 

[Magic_Man]

That's what I'm getting at. The "why".

More time to burn the mixture.

 

[drf256]

The more work you do, the more energy has to be expended, so more fuel has to be burned. No doubt.

I still can't wrap my head around what's changing at the molecular level under load.

It has to be heat.

You get a saw with really high compression warmed up good, it needs a richer mix.

When you load the engine, it has to make more heat for power output. Probably what it is.

 

[Magic_Man]

The more work you do, the more energy has to be expended, so more fuel has to be burned. No doubt.

I still can't wrap my head around what's changing at the molecular level under load.

It has to be heat.

You get a saw with really high compression warmed up good, it needs a richer mix.

When you load the engine, it has to make more heat for power output. Probably what it is.

Yes, that's what I was getting at, it's a combination of the higher heat and longer burn time.

But then again, we may be completely wrong lol

 

[Al Smith]

"4 stroking" really is just a slang term and is the basic and conventional method of tuning a chainsaw engine by sound .Some might think a tachometer is a must but believe it or not ,it is the method a majority of race saws builders use .It's worked well for me and I'm not a race saw builder .

 

[paragonbuilder]

I think you are both right. Don't forget you've got @250 cycles per second. How many cycles does it actually take to drop revs and add heat? 50 cycles is 1/5 of a second.
Also on the more time to burn theory, at wot it four strokes, back off the throttle a whisker and it cleans up just like loading the saw.



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[drf256]

Fascinating Sh!t for sure.