Flow of Air in a Chainsaw and Porting

[Farmer_Nate]

I am new at wrenching on saws. I am researching a lot about building and of course about modding.

Porting:

A lot of porting posts talk about "helping flow" or "smoothing flow" or "flowing more air". I wonder if flow is supersonic or subsonic in the engine? I'd expect exhaust at least to be supersonic. If that is the case, the rules kinda change, don't they? I don't think most people consider that.

 

[thompsoncustom]

I've never considered this before tho I'm not sure how it would effect air flow.

To be supersonic that air would have to be moving at over 768mph and I assume like a gun shot you would get a supersonic crack which you don't in a chainsaw or any engine.

 

[Farmer_Nate]

Good point about the sonic wave. I need to read more at the library.

I wonder what makes all of that exhaust noise? High pressure going to low pressure abruptly, I guess causes a wave? "Uncorking" the exhaust, like popping the cork on a bottle of bubbly.

Nate

 

[Lightning Performance]

Good point about the sonic wave. I need to read more at the library.

I wonder what makes all of that exhaust noise? High pressure going to low pressure abruptly, I guess causes a wave? "Uncorking" the exhaust, like popping the cork on a bottle of bubbly.

Nate

The saws are not supersonic except a very small portion of the exhaust port area, nothing more. If your actually going to study flow make sure you study wet flow vs dry flow. The only dry portion is the exhaust cycle nothing else. You'll never find any information here about it becuase most builders don't study airflow or wet flow. Most of the older flow charts are skewed or just flat out wrong. Applying old school tech will not help your program at all. In fact you will get the wrong information so nothing is what it seems. Most saws like other simple tools are limited by the constraints of the casting, heat (or too much of it) and the piston. Next is the carb and then your air filter and mufflers. The easiest thing to do is get more air out so you can get more air in. Sounds quite simple but it's not. Direction changes are the real power killer most times not overall volume once you go inside the engine.

GL

 

[Farmer_Nate]

Yeah, I recently started working on resurrecting and rebuilding an old saw. I tried to eliminate abrupt direction changes of flow, considering the entire path through the saw. I filed out sharp edges on the plastic intake runners, filed smooth the inside of the plastic intake to match the intake on the saw to eliminate steps in the passages, and slightly ...very slightly, "ported" the saw cylinder. I am eager to find results. I won't know till all the parts come in and I get her back together.

Of course I have gutted a few mufflers in my time, that seems to help a bit.

 

[Tomos770]

https://www.2strokeengine.net/gordonjennings/twostroketunershandbook.php
"
Happily, in this instance it is possible to arrive at a satisfactory solution to the problem by determining wave speed -by starting with the answer and working back. In short, you can measure a lot of existing expansion chambers known to be effective, and by comparing their lengths, exhaust port timings and the speeds at which the engines develop their power, eventually come up with a figure for wave speed representing a workable average for a whole range of high-output engines. My own research, conducted along the pragmatic lines just described, was begun in about 1960 and I arrived at a conclusion in 1964 that has required only slight modification over the succeeding eight years. That conclusion was, and is, that one may use a wave speed figure of 1700 ft/sec in combination with the anticipated engine speed at maximum power to arrive at a system length (measured between the exhaust port window and the point of mean reflection in the cone that constitutes the closed end of virtually all expansion chambers). That figure provides an excellent starting point for the system, as it represents a high average and any error will merely result in a lower-than-projected power peak. Actually, the addition of more examples to my charts in recent years make me inclined to think that something like 1670 ft,/sec is more accurate, but I still use the 1700 ft/sec figure as a starting point, and subsequently shorten the system slightly, perhaps an inch, if tests indicate that the power peak obtained with the chamber is too low.

Using that high-average figure for wave speed (or indeed any figure your fancy dictates, if your findings contradict my own) you may establish the exhaust system's tuned length by means of the following formula"