The theory and physics of muffler mods, and their practical effects
- Thread starter ap1
- Start date
Help Support Arborist Forum:
ap1
ArboristSite Lurker
I've read every post and found some very informative explanations.
Thanks.
Working on the premise that some (how much is left to the boldness of the saw owner, and the thickness of his wallet) increase in engine speed is desireable, enrichening the air/fuel mixture will limit the amount of rpm increase, as well as providing extra fuel for added power, and to help cool the faster running engine.
According to one of the sticky threads on this forum, the exhaust outet should be no more than 80 to 85% of the area of the exhaust manifold outlet. Using my MS390, I took some measurements and ran calculations based on those measurements.
I measured the muffler inlet and used it as the manifold outlet measurement. The dimensions were approximately 27mm x 20mm for an area of 540sq. mm.
I couldn't measure the diameter of the holes inside the muffler so I gaged their size against the 5mm holes which the muffler studs run through. The holes inside the muffler are either 4mm or 5mm, which, given that the area of a circle is a funtion of the square of the radius, results in significantly different areas. I then counted the number of holes (22 on the 290,310,390).
Assuming 4mm diameter holes, the total area through the muffler is 276 sq. mm. 276/540= .51. I.E., the area through the muffler is 51% of the area of the exhaust manifold outlet.
Assuming 5mm holes, the result is 81%.
I then made the same calculations for the the two exhaust slots on the muffler outlet and for the actual opening where the exhaust finally exits the whole muffler unit. The results: The cover plate opening was a pathetic 80 sq. mm, and the two slots were a nearly equally pathetic 90mm.
As for the spark arrester screen, I had to approximate a little. As best I could measure, the mesh of the screen was around .5 mm. The surface of the screen exposed to flow through I measured as 30mm x 30mm, with 12 holes/cm for a total of 36 holes per row x 36 rows, for a total of 1,296 holes. With mesh size of .5mm, the area of each hole is calculated to be .25 sq. mm. .25 x 1,296= 324 sq. mm of actual flow through area.
If the mufflers inside holes are 5mm, the calculations appear to be in line with a fairly well designed muffler system, the exceptions being the coverplate hole area and the two slots on the muffler outlet.
However, even if the mufflers inside holes are only 4 mm, there's still plenty of power to be gained over the stock setting.
I noted earlier that the exhaust manifold outlet area was around 540 sq. mm, and that the area of the little square hole where the exhaust actually exits out the cover plate is 80 sq. mm. This is a mere 15% of the manifold area, and we're looking to approach 80%.
I need to mention that when speaking of back pressure in an exhaust system, that back pressure should come as close to the end of the whole exhaust system as possible. This maximizes the volume of exhaust gas subject to pressurization and enhances engine performance.
What does all this mean?
Well, at least for us 290, 310, 390 owners it means that we need to look at drilling out holes in the cover plate and in the muffler outlet. It probably isn;t possible to reach an 80% ratio using a stock muffler, but it is possible to reach a 50% ration, which is still a helluva lot better than the stock 15%.
More generally, I hope anyone who's thinking of doing a mod can use my methodology to systematically and at least fairly accurately calculate their own stock system when determining how much and where modding is needed.
I hope this adds something worthwhile to the discussion.
One additional thing about power and torque.
For purposes of this discussion I believe it's appropriate to think of power as the ability to develope speed, whereas torque is the ability maintain speed under load (i.e. in actual cutting situations).
Stihl's fastest running saw is the 361, with a top engine speed of 14,000 Rs. The 4xx series saws and up all run at 13,500. The 390 at 13,000 Rs.
Now, it's important to remember that an increase in HPs will result in an increase in torque, thus my generalization of power as ability to develope speed is not entirely comprehensive, but for now it will do. Unfortunately, Stihl doesn't have torque specs readily available, so I'm left to generalize.
That said, given two saws, one with a larger bore but of equal stroke to the other, AND given equal carburation, AND running the saws at the same RPMs, the saw with the bigger bore will have more torque. That is it will not bog as easily as the smaller saw.
In reality, the 361 has a larger carb and undoubtedly consumes more gas in order to develope it's slightly greater HP rating than the 390.
I suspect that a 390 with a slight muffler mod would easily raise the HPs to that of the 361, giving it greater torque than the 361. Although the 361's speed would still be greater, the 390 would maintain what speed it does have better than a stock 361.
Which one would ultimately cut faster would depend on whether greater torque would be enough to overcome greater speed.
They're both awsome saws.
Thanks.
Working on the premise that some (how much is left to the boldness of the saw owner, and the thickness of his wallet) increase in engine speed is desireable, enrichening the air/fuel mixture will limit the amount of rpm increase, as well as providing extra fuel for added power, and to help cool the faster running engine.
According to one of the sticky threads on this forum, the exhaust outet should be no more than 80 to 85% of the area of the exhaust manifold outlet. Using my MS390, I took some measurements and ran calculations based on those measurements.
I measured the muffler inlet and used it as the manifold outlet measurement. The dimensions were approximately 27mm x 20mm for an area of 540sq. mm.
I couldn't measure the diameter of the holes inside the muffler so I gaged their size against the 5mm holes which the muffler studs run through. The holes inside the muffler are either 4mm or 5mm, which, given that the area of a circle is a funtion of the square of the radius, results in significantly different areas. I then counted the number of holes (22 on the 290,310,390).
Assuming 4mm diameter holes, the total area through the muffler is 276 sq. mm. 276/540= .51. I.E., the area through the muffler is 51% of the area of the exhaust manifold outlet.
Assuming 5mm holes, the result is 81%.
I then made the same calculations for the the two exhaust slots on the muffler outlet and for the actual opening where the exhaust finally exits the whole muffler unit. The results: The cover plate opening was a pathetic 80 sq. mm, and the two slots were a nearly equally pathetic 90mm.
As for the spark arrester screen, I had to approximate a little. As best I could measure, the mesh of the screen was around .5 mm. The surface of the screen exposed to flow through I measured as 30mm x 30mm, with 12 holes/cm for a total of 36 holes per row x 36 rows, for a total of 1,296 holes. With mesh size of .5mm, the area of each hole is calculated to be .25 sq. mm. .25 x 1,296= 324 sq. mm of actual flow through area.
If the mufflers inside holes are 5mm, the calculations appear to be in line with a fairly well designed muffler system, the exceptions being the coverplate hole area and the two slots on the muffler outlet.
However, even if the mufflers inside holes are only 4 mm, there's still plenty of power to be gained over the stock setting.
I noted earlier that the exhaust manifold outlet area was around 540 sq. mm, and that the area of the little square hole where the exhaust actually exits out the cover plate is 80 sq. mm. This is a mere 15% of the manifold area, and we're looking to approach 80%.
I need to mention that when speaking of back pressure in an exhaust system, that back pressure should come as close to the end of the whole exhaust system as possible. This maximizes the volume of exhaust gas subject to pressurization and enhances engine performance.
What does all this mean?
Well, at least for us 290, 310, 390 owners it means that we need to look at drilling out holes in the cover plate and in the muffler outlet. It probably isn;t possible to reach an 80% ratio using a stock muffler, but it is possible to reach a 50% ration, which is still a helluva lot better than the stock 15%.
More generally, I hope anyone who's thinking of doing a mod can use my methodology to systematically and at least fairly accurately calculate their own stock system when determining how much and where modding is needed.
I hope this adds something worthwhile to the discussion.
One additional thing about power and torque.
For purposes of this discussion I believe it's appropriate to think of power as the ability to develope speed, whereas torque is the ability maintain speed under load (i.e. in actual cutting situations).
Stihl's fastest running saw is the 361, with a top engine speed of 14,000 Rs. The 4xx series saws and up all run at 13,500. The 390 at 13,000 Rs.
Now, it's important to remember that an increase in HPs will result in an increase in torque, thus my generalization of power as ability to develope speed is not entirely comprehensive, but for now it will do. Unfortunately, Stihl doesn't have torque specs readily available, so I'm left to generalize.
That said, given two saws, one with a larger bore but of equal stroke to the other, AND given equal carburation, AND running the saws at the same RPMs, the saw with the bigger bore will have more torque. That is it will not bog as easily as the smaller saw.
In reality, the 361 has a larger carb and undoubtedly consumes more gas in order to develope it's slightly greater HP rating than the 390.
I suspect that a 390 with a slight muffler mod would easily raise the HPs to that of the 361, giving it greater torque than the 361. Although the 361's speed would still be greater, the 390 would maintain what speed it does have better than a stock 361.
Which one would ultimately cut faster would depend on whether greater torque would be enough to overcome greater speed.
They're both awsome saws.
Last edited:
ap1
ArboristSite Lurker
I forgot something. I'm not a professional engine tech/mechanic. However, I grew up working on cars and motorcycles. In fact I still work on them. I also have some education in and understanding of physics. I probably should have majored in mechanical engineering instead of Economics, because I've had a long standing fascination with the internal combustion engine. As such, when I bought property which need clearing, I developed a fascination with chainsaws and two stroke engines in general.
This is the best site for all that neat stuff.
This is the best site for all that neat stuff.
StihlyinEly
Addicted to ArboristSite
And International? I had a '68 International Travelall with a posi rear end, if memory serves. Took me places in 1st gear that 4x4s feared to go.
Tzed250
Addicted to ArboristSite
.
Huge assumptions have been made...
.
Huge assumptions have been made...
.
Then we get to GM... Leave it to GM to make it confusing for all brands under their mark...
Chevrolet = Positraction
Buick = Positive Traction
Pontiac = Saf-T-Track
I don't want to de-rail the thread again, as I am very interested in the OP's question.
However, I would be negligent if I didn't point out that this knowledge was central to the cause of justice in "My Cousin Vinny"!.
Philbert
ap1
ArboristSite Lurker
Edisto:
Yeah yeah, I know that more holes equals more friction and all that stuff. But at least I took the time to do some calculations. All you're doing is lazily sitting back and being a nattering naysayer. Have you bothered to determine the coefficient of friction through the holes?
I thought not.
Again, Edisto:
No, you're missing the point. As I so clearly stated but you so clearly ignored, I was speaking in general terms. What you missed is that HP need not increase for torque to increase. Go to bed.
Edisto, it's easy to be a nattering nabob of negativism. It's a whole different thing to actually expend the effort and time trying to figure out a problem.
When you're ready to crunch numbers and kick my ass with math, let me know. I'm more than willing to concede the title of manliness to you when you show me something mathematically sound.
To be sure, I fully understand the principle of which you speak. The problem is that instead of adding a useful factor to the equation, you've simply mentioned the obvious fact that there is an additional factor. And moreover, you haven't addressed the possibility/likelihood that ALL chainsaw mufflers have the multiple hole design, and that such a fact would render your analysis meaningless, because such a situation would make the whole extra friction thing a moot point.
Yeah yeah, I know that more holes equals more friction and all that stuff. But at least I took the time to do some calculations. All you're doing is lazily sitting back and being a nattering naysayer. Have you bothered to determine the coefficient of friction through the holes?
I thought not.
Again, Edisto:
No, you're missing the point. As I so clearly stated but you so clearly ignored, I was speaking in general terms. What you missed is that HP need not increase for torque to increase. Go to bed.
Edisto, it's easy to be a nattering nabob of negativism. It's a whole different thing to actually expend the effort and time trying to figure out a problem.
When you're ready to crunch numbers and kick my ass with math, let me know. I'm more than willing to concede the title of manliness to you when you show me something mathematically sound.
To be sure, I fully understand the principle of which you speak. The problem is that instead of adding a useful factor to the equation, you've simply mentioned the obvious fact that there is an additional factor. And moreover, you haven't addressed the possibility/likelihood that ALL chainsaw mufflers have the multiple hole design, and that such a fact would render your analysis meaningless, because such a situation would make the whole extra friction thing a moot point.
A. It's not my saw.
B. In spite of A, I was trying to help. If you are shooting for 80%, you will get a lot less adding up a bunch of smaller holes.
Maybe I should go to bed because I'm still confused. HP is the product of torque and rpm divided by a constant. If you increase torque, you increase HP at a given rpm...
And it's a waste of effort if you are headed in the wrong direction. Like I said, I was just trying to help.
It didn't seem obvious from your post.
I haven't addressed it because it isn't a possibility.
When you're finished getting your back up, here is a link you might find useful.
It also had a GM distributor, I bet. IH used parts from a number of mfrs., if I recall correctly. My '62 Binder had the old GM adjustable dwell dist. a great convenience. What a cool truck. 0˚ TDC spark timing, that's engine longevity.
I imagine advancing the timing will heat up an engine as much as if not more than a muffler mod. Also, I just discovered today that installing a shorter bar will effectively lean out the engine. Less resistance, more chain speed, a saw that burbled with a 16" bar, needed an extra 1/8 turn with a 12" bar. Probably needs more H opening up still.
A very interesting thread.
captain
ArboristSite Lurker
One piece that I see get overlooked when calculating muffler outlet size is the effects of the spark screen. I believe (correct me if I am wrong) that the spark screen generally covers approximately 40% of the outlet and can have a significant effect on performance. I recently did some timed cuts with my 2153 and the times were substantially faster without the muffler screen in place.
thomasjf
ArboristSite Operative
Because of where I cut, I always use spark screens. Therefore, when doing my MM I take that into consideration when calculating my opening size. Given a near 50% screen restriction, my exit is approximately double the "area" that it would be if not using a screen.
that's what i was thinking. just look at any modern dirt bike and you can see that a 2stroke expansion chamber on a chainsaw is not practical. having said this, i have no doubt that if someone were to design a smaller version for chainsaws, people would buy them despite the unwieldy inconvenience.
thomasjf
ArboristSite Operative
The older 930 super Jred used a sparkscreen that was bubbled inward to give it near double the surface area as the opening. The idea of increasing the screen area to compensate for its restriction has been around for many years. Surely wasn't my original idea.
Last edited:
RavensRoost
ArboristSite Operative
Memory tells me it was called Traction-Lock or Traction-Lok, International pickups from 60's.
neutral4x4
ArboristSite Operative
You guys are making too many assumptions.
Muffler design is far from the only factor in 2 stroke performance. Each engine is a system that can't work without each individual part.
The power that comes out of an engine at any given RPM range is dependant on a ton of different factors.
1) Air filter restriction
2) Carb diameter
3) Intake port area
4) Intake timing
5) Bore diameter
6) Transfer port area
7) Transfer port angle
8) Blowdown
9) Compression numbers (depends on your crank case volume, your squish band, your rings, Your combustion chamber shape, and a number of other things)
10) Exhaust port area
11) Exhaust port timing
12) Spark plug heat range
13) Jetting throughout your rpm range
14) Muffler restriction (only one of the many factors that determines how well your saw runs, and I'm sure I left quite a few out)
All of these factors can be altered to make your saw run better for you if you know how to do it and have the tools. The reason muffler modifying is most often considered is that it isn't a very precision operation, and it requires the least knowhow to make it work.
And yes, any time you modify a major part of your engine, you should rejet to compensate for the additional heat output of your engine. It isn't a really big deal on a saw because all you have to do is turn a screw. When I work on bikes, I usually have to drill out jets (Those that know about it, please don't flame me, I have reasons to drill over buying new jets).
You always want your head temp under 400 degrees Fahrenheit. 400 degrees is right about where you can expect your piston to seize in the bore. Thats why you upjet the carb. If you up the air flow through an engine, you need to up the amount of premix that goes through it to keep the correct operating temp. I highly doubt that there a lot of people running head temp gauges on their saws, but if you're pushing the edge, it isn't really a bad idea.
On the note about multiple holes not flowing as well as a larger hole with the same area: The air in a thin layer around the edge of the hole IS NOT MOVING! The maximum air flow is always in the middle of the hole. In extreme cases (such as inside of your engine), air is actually really thick. Sure, at low pressures, air is thin and not really a big problem, but when air is moving over about 12mph, the viscosity changes. Have you ever looked at race motorcycles? Theres a reason that they all have those large fairings on them. Drag isn't negligible, so there isn't any reason it should be discounted in arguments. That just nullified your whole calculation. Bringing up half of the info won't increase the effectiveness of your argument.
Honestly, unless you're running a race saw, is any of this going to help you? Probably not. If it works, why not do it? If you rejet, keep the saw under the same load, and stay at the same rpm, the saw won't have increased wear.
While you're at it, side gap the plug. That'll give you a more complete burn and increase your overall power.
Muffler design is far from the only factor in 2 stroke performance. Each engine is a system that can't work without each individual part.
The power that comes out of an engine at any given RPM range is dependant on a ton of different factors.
1) Air filter restriction
2) Carb diameter
3) Intake port area
4) Intake timing
5) Bore diameter
6) Transfer port area
7) Transfer port angle
8) Blowdown
9) Compression numbers (depends on your crank case volume, your squish band, your rings, Your combustion chamber shape, and a number of other things)
10) Exhaust port area
11) Exhaust port timing
12) Spark plug heat range
13) Jetting throughout your rpm range
14) Muffler restriction (only one of the many factors that determines how well your saw runs, and I'm sure I left quite a few out)
All of these factors can be altered to make your saw run better for you if you know how to do it and have the tools. The reason muffler modifying is most often considered is that it isn't a very precision operation, and it requires the least knowhow to make it work.
And yes, any time you modify a major part of your engine, you should rejet to compensate for the additional heat output of your engine. It isn't a really big deal on a saw because all you have to do is turn a screw. When I work on bikes, I usually have to drill out jets (Those that know about it, please don't flame me, I have reasons to drill over buying new jets).
You always want your head temp under 400 degrees Fahrenheit. 400 degrees is right about where you can expect your piston to seize in the bore. Thats why you upjet the carb. If you up the air flow through an engine, you need to up the amount of premix that goes through it to keep the correct operating temp. I highly doubt that there a lot of people running head temp gauges on their saws, but if you're pushing the edge, it isn't really a bad idea.
On the note about multiple holes not flowing as well as a larger hole with the same area: The air in a thin layer around the edge of the hole IS NOT MOVING! The maximum air flow is always in the middle of the hole. In extreme cases (such as inside of your engine), air is actually really thick. Sure, at low pressures, air is thin and not really a big problem, but when air is moving over about 12mph, the viscosity changes. Have you ever looked at race motorcycles? Theres a reason that they all have those large fairings on them. Drag isn't negligible, so there isn't any reason it should be discounted in arguments. That just nullified your whole calculation. Bringing up half of the info won't increase the effectiveness of your argument.
Honestly, unless you're running a race saw, is any of this going to help you? Probably not. If it works, why not do it? If you rejet, keep the saw under the same load, and stay at the same rpm, the saw won't have increased wear.
While you're at it, side gap the plug. That'll give you a more complete burn and increase your overall power.
nanuk
ArboristSite Guru
neutral4x4
ArboristSite Operative
Side gapping is a simple and easy way to make your engine perform better. A side gapped plug will give your engine a more complete burn for higher hp all over the power curve. Since the spark is out in the open and only jumping from one point to another point (which takes less energy than from one flat surface to another flat surface), more fuel can be burned in each cycle. The blue lines in the diagram represent the spark path. Notice how protected the first spark is compared to the second spark.
To side gap a spark plug, simply cut the ground strap electrode as seen in the diagram. With a file or sand paper, make sure the ground strap doesn't have any sharp edges. Gap the plug between the closest two parts of the electrodes. Clean the plug thoroughly (you don't want any metal shavings in the cylinder area). Install the plug as normal and run the saw.
It isn't hard and takes a couple of seconds.
