The theory and physics of muffler mods, and their practical effects

[ap1]

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.

 

[mtngun]

Unfortunately, that begs the question: By what mechanism does air flow increse power?

Engines are air pumps. They work by heating air, and making the hot air do work (push the piston). To increase power, you either have to move more air through the engine, or else make the air hotter.

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?

Chainsaws are cooled and lubed by their fuel mixture. The carb is tuned to maintain decent temperature and lubrication regardless of mods.

Does muffler modding a saw necessarily mean sacrificing engine lifespan?

It may reduce the lifespan of your hearing.

How much, if any of an RPM increase is "safe"?

Depends on who you ask. I'm old school and try to stick to the factory recommended max rpm.

My only other thought is that I think chainsaw muffler mods are over rated. Yes, modern chainsaw mufflers are restrictive and inefficient, but on the other hand, if you eliminated the muffler all together, you wouldn't gain much. The best exhaust system is a tuned pipe, which isn't practical on a chainsaw.

 

[olyeller]

You choose

Perhaps the best comparison is to look at mufflers from the same make and model, newer and older versions. Staying with Stihl, in most cases the newer saws have a more restrictive exhaust outlet than previous-generation models of the same saw. Why is that? More recent and restrictive EPA regulations that require the manufacturer to sell a saw with a lower noise level. Why did the older saws have more open mufflers? For more power--in effect, they were made to run that way prior to more rigorous EPA noise restrictions. So in many cases, modding a muffler on a newer saw is simply returning it back to its original state as it was first designed to run. And therefore, modding a muffler should not reduce the life of your chainsaw seeing as how you're bringing the exhaust flow back to where it was origianlly designed to operate.

In general, any time you restrict the exhaust flow in any internal combustion engine you will reduce power output, whether from its ability to rev on the top end or simply rev more freely within the powerband. This principle applies to chainsaws, autos, truck, motorcycles or whatever. Lots of documented evidence for these latter vehicles; go do a search and you can see how a more free-flowing exhaust enhances power output on a dyno. Again, with an increase in noise output, however. The same principles apply to chainsaws.

So for all practical purposes, if you want your late-model MS 390 to pump out more power, mod the muffler and richen the fuel metering. If you want a quieter saw, leave the muffler as is. It's your saw, and the choice is yours.

 

[Tzed250]

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This ought to kick off 2010 real well...


.

 

[StihlyinEly]

ap1, I think you pose some interesting questions, and I'm one of those who will be learning more than the usual basic stuff from the answers.

I've done two muff mods, and in the case of the MS290 it really made a noticeable difference in power. But that doesn't mean I know all the fine points of this. C'mon guys, let's get the fine points!

 

[WoodChuck'r]

.


This ought to kick off 2010 real well...


.

My thoughts exactly. :deadhorse:


opcorn:

 

[StihlyinEly]

My thoughts exactly. :deadhorse:

It's only a :deadhorse: if people can't control themselves and fall into the usual pissing match. I'm sure the OP and I aren't the only ones who have wondered about the details of what's behind increased performance in a muff mod.

Maybe my search feature is broken, but I was unable to find past threads with detailed answers asked for by the OP.

So sue me.

 

[Andyshine77]

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?

It does both+ more fuel = more power.

How much, if any of an RPM increase is "safe"?

That depends on the design of each engine, and whether or not it' getting enough fuel.

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.

Totally different saws. The 390 has an engine block the sits in a plastic case. The 361 has a mag case with two halves. The crank is fitted between the two halves, and the cylinder is bolted to the top of the case.

 

[ap1]

Edit. I wrote the post before most people answered. I'll read those posts and comment.

Thanks.

 

[Saw Dr.]

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.

FYI, the 390 does NOT have a plastic crankcase. It is a metal clamshell engine, including the bottom pan, sitting in a plastic cradle. The clamshell is a horizontaly split design split throught the center of the crankshaft axis. The 361 uses a vertiaclly split crankcase, like most pro saws. The crankshafts are not identical as the engine design is very different.

Perhaps the best comparison is to look at mufflers from the same make and model, newer and older versions. Staying with Stihl, in most cases the newer saws have a more restrictive exhaust outlet than previous-generation models of the same saw. Why is that? More recent and restrictive EPA regulations that require the manufacturer to sell a saw with a lower noise level. Why did the older saws have more open mufflers? For more power--in effect, they were made to run that way prior to more rigorous EPA noise restrictions. So in many cases, modding a muffler on a newer saw is simply returning it back to its original state as it was first designed to run. And therefore, modding a muffler should not reduce the life of your chainsaw seeing as how you're bringing the exhaust flow back to where it was origianlly designed to operate.

:agree2:

I believe this is why some of the newer models grew slightly in displacement. To bring power back to previous levels while still running the restricted muffler. 026 bore is 44mm, 260 grew to 44.7mm. The 025 was 42mm bore, and the late ones and the 250 grew to 42.5mm.

 

[Andyshine77]

What I'd like to know is if one of the "goals" of a muffler mod is to achieve some measure of RPM increase.

Or is the primary purpose of a muffler mod to simply free up power by simply allowing the engine to run more freely?

Or is it some of both?

Often times both are achieved, but not always. Sometimes you just gain power without more rpm's, sometimes you will lose rpm's and power, but that is very rare.

One other thing to remember is the hot exhaust gasses are now exiting the exhaust faster, this helps keep the exhaust side of the piston and cylinder cooler.

 

[Andyshine77]

FYI, the 390 does NOT have a plastic crankcase. It is a metal clamshell engine, including the bottom pan, sitting in a plastic cradle.

Cradle/case either way I'm not a fan.

 

[eyolf]

One of the things you must consider is that it is very likely that port timing and static compression figures for the 361 and 390 are not the same. I'd hazard a guess that the 390 makes its power at 13K for that very reason. The 390 is meant as an all-round unit for the occasional user that believes he needs the power...for that, Stihl wants to give him a wide powerband.

The 361 is more attractive to certain pros and seasoned hobbyists that are happy with peak power even if it comes in a narrower range.

The EPA thing is important for noise, but also for trying to reduce the unburned HC emissions. Impede exhaust flow, and impede HC...to a point, anyway. Close it down too far and the crankcase pumping won't eficiently fill the cylinder; reversing in the intake tract causes too much mixture variation and you start to send out half-burned fuel, lots of CO, etc.

That's a big reason why most saws want a little richer fuel flow...they are so restricted that intake reversion is picking up a little fuel each way through the carb.

Anyway, open up the muffler to a point and increase performance: crankcase scavenging and pumping are more efficient. Some restriction is still important: run your saw without any muffler at all and see what happens. It will be almost as "peaky" as if it had a tuned pipe, but the peak will be lower, and not very pronounced...the main effect would be to destroy all your midrange power, and burn lots more gas as a lot flies right out the exhaust port.

 

[Andyshine77]

One of the things you must consider is that it is very likely that port timing and static compression figures for the 361 and 390 are not the same. I'd hazard a guess that the 390 makes its power at 13K for that very reason. The 390 is meant as an all-round unit for the occasional user that believes he needs the power...for that, Stihl wants to give him a wide powerband.

The 361 is more attractive to certain pros and seasoned hobbyists that are happy with peak power even if it comes in a narrower range.

The EPA thing is important for noise, but also for trying to reduce the unburned HC emissions. Impede exhaust flow, and impede HC...to a point, anyway. Close it down too far and the crankcase pumping won't eficiently fill the cylinder; reversing in the intake tract causes too much mixture variation and you start to send out half-burned fuel, lots of CO, etc.

That's a big reason why most saws want a little richer fuel flow...they are so restricted that intake reversion is picking up a little fuel each way through the carb.

Anyway, open up the muffler to a point and increase performance: crankcase scavenging and pumping are more efficient. Some restriction is still important: run your saw without any muffler at all and see what happens. It will be almost as "peaky" as if it had a tuned pipe, but the peak will be lower, and not very pronounced...the main effect would be to destroy all your midrange power, and burn lots more gas as a lot flies right out the exhaust port.

Good post.

 

[StihlyinEly]

One of the things you must consider is that it is very likely that port timing and static compression figures for the 361 and 390 are not the same. I'd hazard a guess that the 390 makes its power at 13K for that very reason. The 390 is meant as an all-round unit for the occasional user that believes he needs the power...for that, Stihl wants to give him a wide powerband.

The 361 is more attractive to certain pros and seasoned hobbyists that are happy with peak power even if it comes in a narrower range.

The EPA thing is important for noise, but also for trying to reduce the unburned HC emissions. Impede exhaust flow, and impede HC...to a point, anyway. Close it down too far and the crankcase pumping won't eficiently fill the cylinder; reversing in the intake tract causes too much mixture variation and you start to send out half-burned fuel, lots of CO, etc.

That's a big reason why most saws want a little richer fuel flow...they are so restricted that intake reversion is picking up a little fuel each way through the carb.

Anyway, open up the muffler to a point and increase performance: crankcase scavenging and pumping are more efficient. Some restriction is still important: run your saw without any muffler at all and see what happens. It will be almost as "peaky" as if it had a tuned pipe, but the peak will be lower, and not very pronounced...the main effect would be to destroy all your midrange power, and burn lots more gas as a lot flies right out the exhaust port.

Thanks for those perspectives. Learning, learning, learning all the time . . .

 

[Gypo Logger]

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.

I think rather than try to confuse things by trying to come up with absolute figures and quantitive theories, it better to work backwards and try to understand how stock mufflers retard a saws performance. It's just air in and air out according to the optimal capacity of the saw. There's other factors in power other than just the muffler, but the muffler gives the biggest gain depending on how constricted it is.
Gypo

 

[lloyyd]

"Hey, how exactly is a rainbow made? How exactly does the sun set? How exactly does the posi-trac rear end on a Plymouth work!? It just does. It just does."

 

[M.R.]

Just to toss this into the mix. as I've yet to see it mentioned.

In welding errr...rather pipe work in reguards to opening these mufflers up & just how much math one wants to apply.

A basic rule of thumb.
'Is that anytime you double the diameter you increase the volume four times.'

:greenchainsaw:

 

[Tzed250]

"Hey, how exactly is a rainbow made? How exactly does the sun set? How exactly does the posi-trac rear end on a Plymouth work!? It just does. It just does."

A Plymouth never had a posi-trac. Chrysler Corp cars had Sure-Grip third members. Posi-trac is a GM thing.


.

 

[Trigger Man]

I think the OP has some good questions that he is asking, looking forward to more replys, has anyone ever confirmed that a muffler mod will increase or decrease the life span of a chainsaw. Any pro's ever run two exact same saws with one having a muff mod and compared the life span of the two?