No, I'm not explaining them. I'm asking about them.
I've been researching this site for a long time and I've found some very useful information. But I still have quite a few unanswered questions. Please bear with me as I take you through my train of thought.
When someone asks: "How does a muffler mod improve performance?", the usual answer is: "It increases air flow. The more easily an engine breaths, the better it runs."
Unfortunately, that begs the question: By what mechanism does air flow increse power?
On the one hand, I've read that the power increase comes from the increases in RPMs resulting from increased air flow. On the other hand, I've read that the H screw needs to be richened in order to keep the saw from over revving.
There's also the matter of greater power output (or is it more specifically, torque?) from a modded muffler at the same RPM level, compared to a non-modded muffler.
While pondering that, let me back up and explain my specific situation.
I have an MS390 which I'm considering modding. For purposes of this discussion, I'm making a rough comparison between it and the MS361. Anyone tempted to get sidetracked with the usual "pro saw versus consumer saw" debate, don't bother. My comparison has specific relevance to this discussion.
The 390 is rated to run at 13,000 RPMs. The 361 runs at 14,000RPMs. Given this difference alone (and an identical bar and chain setup), the 361 will clearly out perform the 390. However, engine speed doesn't tell the whole story. The other part is how well an engine maintains what speed it can muster under load. That's where non-speed related power/torque comes in. The 390 and the 361 have identical strokes, but the 390 has a slightly larger bore, so theoretically it would probably have a little more torque, thus what it lacks in top end speed, it partially makes up for in ability to motor through without bogging at a given speed. By comparison, the 361 generates at least a fair portion of its power through higher RPMs.
It appears that muffler modification can have two effects. One is that it increases power/torque across the RPM range. The other is that it increases actual engine speed, which results in faster cutting. It also appears that the most pronounced performance increase will be realized from a faster running engine, although that runs the risk of overheating and ruining the engine if you don't back out the "H" screw to allow more fuel through.
So here's where I need answers.
Is the purpose of richening the mixture after modding to bring the RPM level back down to spec, does it just provide more fuel to cool down a faster running engine, or does it do both?
Does muffler modding a saw necessarily mean sacrificing engine lifespan?
How much, if any of an RPM increase is "safe"?
Finally, other than the plastic crankcase, how are the actual engine materials between the 390 and 361 any different? I know the rod and bearings are the same. What about the cylinder and crank. Given the identical stroke between the two saws, I'm tempted to initially conclude that at least the cranks are identical.
Again, I don't want to debate the merits of one saw versus the other. I really just want to know how a proper muffler mod should be approched in principle. My comparison was to illustrate what I perceive to be two slightly variant approches to developing horse power.
If anyone has answers, I'll be most appreciative.
