What's The Story On Modified Mufflers On Stock Chainsaws?

[StihlRockin']

Ok, I'd love to learn some stuff on using modified mufflers on stock saws. I see an ad on another site about a dual ported muffler that would work for my Stihl 044.

Here's what I think I know about ported mufflers... They are louder. They allow more pressure out therefore giving the saw a little more power. They allow the chainsaw to run cooler.

If I did get one, I heard something about changing the setting for the gas/oil mixture. Do I lean or enrich the settings for it's use? And why?

Are dual port mufflers an advantage? If so,why? Are there drawbacks?

If I got one, what other things do or should change on the saw?

Thanks,

StihlRockin'

 

[old-cat]

If you have the tools and have good manual dexterity, you can spend zero and have a really good performing 044. Take the muffler off , take the screen out and open the outlet as far as it will go but don't go toward the front

 

[sachsmo]

Some saws were so choked up with the factory muffler, a free flowing one is about the best bang for the buck modification.

 

[lambs]

I've not modded one, but have read about it here on AS before the search got difficult.

Opening up the muffler seems to allow the piston to more easily move air through the saw, as it is basically a pump to begin with. When you allow the air to more freely flow, you get more air in the cylinder, upsetting the fuel mixture (making it lean). So you then need to set the carb so it provides a richer mixture to get it back to the proper ratio for that particular saw.

More fuel, more power.

But you can overkill this by opening the muffler too much, and then you actually lose power because some back pressure from the muffler is desireable.

Rule of thumb is you want to open up the muffler to no more than 80% or so of the exhaust port area on the cylinder head. Which you can measure by removing the muffler and inserting raw sphagetti into the port until full, then counting how many pieces it took to fill it in. Make sure the muffler openings do not allow more than 80 - 85% of that amount, then retune the carb to get the mixture right. There are entire threads on how to tell when it is right. Basically you want it 4 stroking at wide open throttle when out of the cut, and running smoothly when fully in the cut.

If you fail to retune the carb, running the saw too lean will definitely burn it up from lack of proper lubrication (not enough fuel mixture).

Finally, if you have an older saw, it may be opened up fairly well by design (fewer EPA standards back then) so you may not gain a lot. And some newer models are using computerized fuel mixtures that may automagically compensate for the free flowing muffler mod.

That's what I think I have learned about it from studying this site. I'm sure others will chime in with their comments as well.

 

[sachsmo]

I've not modded one, but have read about it here on AS before the search got difficult.

Opening up the muffler seems to allow the piston to more easily move air through the saw, as it is basically a pump to begin with. When you allow the air to more freely flow, you get more air in the cylinder, upsetting the fuel mixture (making it lean). So you then need to set the carb so it provides a richer mixture to get it back to the proper ratio for that particular saw.

More fuel, more power.

But you can overkill this by opening the muffler too much, and then you actually lose power because some back pressure from the muffler is desireable.

Rule of thumb is you want to open up the muffler to no more than 80% or so of the exhaust port area on the cylinder head. Which you can measure by removing the muffler and inserting raw sphagetti into the port until full, then counting how many pieces it took to fill it in. Make sure the muffler openings do not allow more than 80 - 85% of that amount, then retune the carb to get the mixture right. There are entire threads on how to tell when it is right. Basically you want it 4 stroking at wide open throttle when out of the cut, and running smoothly when fully in the cut.

If you fail to retune the carb, running the saw too lean will definitely burn it up from lack of proper lubrication (not enough fuel mixture).

Finally, if you have an older saw, it may be opened up fairly well by design (fewer EPA standards back then) so you may not gain a lot. And some newer models are using computerized fuel mixtures that may automagically compensate for the free flowing muffler mod.

That's what I think I have learned about it from studying this site. I'm sure others will chime in with their comments as well.

I have been here a few years and have seen the 80% figure many, many times. Is there science behind this thinking? Is there cold hard proof you need "some back pressure"?

I realize a tuned pipe actually pulses some of the wasted charge back into the combustion chamber, that is controlled back pressure (actually a charge boost)and has been studied and is proven science.

Guess my way of thinking is back pressure from restriction is NOT a good thing no matter how you roll it.

Yes I have done several and really haven't gone too far yet? If you have a carburetor that can be tuned, I fail to see the downside.

 

[OH_Varmntr]

I have been here a few years and have seen the 80% figure many, many times. Is there science behind this thinking? Is there cold hard proof you need "some back pressure"?

I realize a tuned pipe actually pulses some of the wasted charge back into the combustion chamber, that is controlled back pressure (actually a charge boost)and has been studied and is proven science.

Guess my way of thinking is back pressure from restriction is NOT a good thing no matter how you roll it.

Yes I have done several and really haven't gone too far yet? If you have a carburetor that can be tuned, I fail to see the downside.

If I remember all of my training correctly...

The exhaust port is the first port to open and the last port to close. Because of this, tuned pipes are desireable because the back-pressure pulse is tuned to bounce scavenged fresh air/fuel mixtures back into the engine prior to the exhaust port closing. It helps with fresh charge volumetric efficiency (VE). The higher the VE, the more air is introduced into the process. More air means more fuel since the air and fuel is mixed prior to entering the cylinder.

In non-stratocharged saws there's always going to be some scavenging going on, but you want it to be as little as possible so the back-pressure pulse pushes the fresh mixture lost during scavenging back into the cylinder before it is sealed up and compressed.

Stock mufflers do the same but not as efficiently. Too muck back-pressure (can you say most stock mufflers?) can lower fresh charge VE because it restricts the flow of exhaust being pushed from the cylinder by the incoming fresh charge. This decreases emissions (which is what they want) because some of the exhaust gasses may be trapped in the cylinder for another go around, and also limits power. Kind of like an EGR circuit on modern automobiles that introduce exhaust gasses back into the intake of the engine to be re-burned.

Too little back-pressure and it allows fresh charges to be blown out of the exhaust and not be forced back in by the back-pressure. That's the point where you've gone too far with opening up your muffler. Take your muffler off (please don't) and see how your saw runs. That proves what too little back-pressure will do to performance.

ETA: This all applies to non-stratocharged saws. I haven't been around strato saws enough to care to learn about them.

Here's a good view of it. Tuning Two-Stroke Engines : Two Stroke Theory

 

[sachsmo]

Resistance for the sake of back pressure is not a good thing.

Never seen a stock 'can' muffler that could be tuned for charge boost. a 'pipe' is proven science and if made properly will boost 2 stroke performance greatly.

Cans just bounce pressure waves and cause turbulence. Show me a 'tuned' can muffler, then you would have my attention.

This is the closest thing I could find for a "tuned can"


http://www.arboristsite.com/attachment.php?attachmentid=317396&stc=1&d=1381593842

 

[OH_Varmntr]

Ok so I guess I shouldn't have say they do the same thing. The idea I was trying to relay is that "resistance" back-pressure and the back-pressure pulse are different but their end results are they both attempt to keep fresh charge from being expelled without being combusted first.

Stock mufflers over-do it because they are also reducing both air and noise pollution. Pipes don't really care about either.

ETA: Link didn't work.

 

[lambs]

I think OH got it about right, and well put at that. I would not pretend to have experience doing this, that's why I was qualifying my response as being based on what I have read.

I think perhaps the goal of a muffler mod is to take the low hanging fruit in terms of increasing power. I'm sure a tuned pipe is probably a better answer if the goal is to maximize the performance of a 2 cycle engine, but for most of us it is not feasible or economical.

 

[sachsmo]

I disagree,

pipes are tuned to a specific rpm (usually under load in the cut) if the timing of the pressure wave is off, even a pipe is pretty useless.

any pressure wave 'bounce back' in a can would be a very specific, very low rpms. Perhaps at idle. That does you no real favors at the operating rpm of a chainsaw.

I remember 2 stroke racing as a kid, everyone would talk of the "powerband" My Dad always called it "getting on the pipe" Yup he was the best engine builder I have met in my life, and he's been gone 20+ years.

 

[lambs]

I disagree,

pipes are tuned to a specific rpm (usually under load in the cut) if the timing of the pressure wave is off, even a pipe is pretty useless.

any pressure wave 'bounce back' in a can would be a very specific, very low rpms. Perhaps at idle. That does you no real favors at the operating rpm of a chainsaw.

I remember 2 stroke racing as a kid, everyone would talk of the "powerband" My Dad always called it "getting on the pipe" Yup he was the best engine builder I have met in my life, and he's been gone 20+ years.

So would you contend that a muff mod improves performance?

 

[sachsmo]

Depends,

If you can tune for the mod, it is usually a win, especially on a choked up muffler.

Remember it is designed as a silencer. ANSI and EPA have a lot to say about what is best for us eh?

 

[exSW]

It's not always about performance.I pulled the muffler on my Dolmar 420 and all I did was cut out the Cat and adjust the carb.Made the saw run noticably cooler(as in A LOT)and everyone knows heat kills motors.

 

[Big_Wood]

I mod the mufflers not just for a power gain. I do it to move the port to a different location then stock. This is mostly for the husky's that have their exhaust port on top of the can right under the cylinder cover. It just bugs me having it there so on most i weld on dual ports and weld the factory port closed. Saws with ports on the side of the can are acceptable to me but i have seen to many of them husky's get wood chips in between the can and the cylinder cover. it sometimes results in a fire and melting plastic if not noticed in time. As of now i have not seen any saw that didn't benefit from a simple muffler mod.

 

[tolman_paul]

Fundamentally an engine is an air pump and your power is limited by how much air you can pump through it. An overly restrictive muffler has essentially the same affect as a too small carburetor, it restricts the amount of air that the engine can pump and hence it's power. It also has the additional negative attribute of making the engine run hotter

 

[outdoortype]

What makes everyone think that the saw runs cooler with a muffler mod? Have any of you guys ever measured the engine temp on stock vs modified muffler? I would think that the increased air flow and increased rpm would increase friction and heat in the cylinder thus negating any gains from otherwise. Most of the cooling is done by the evaporation of fuel and the cylinder fins providing a heat sink.

 

[StihlRockin']

Woah, woah woah! Back up the truck Chuck. Someone help please! My onboard 'puter thingee blew it's main flux capicitor that supports the major components of the Interpretation modules on my laptop.

So in essence...

What?

StihlRockin'

 

[exSW]

It's not always about performance.I pulled the muffler on my Dolmar 420 and all I did was cut out the Cat and adjust the carb.Made the saw run NOTICABLY cooler(as in A LOT)and everyone knows heat kills motors.

What makes everyone think that the saw runs cooler with a muffler mod? Have any of you guys ever measured the engine temp on stock vs modified muffler? I would think that the increased air flow and increased rpm would increase friction and heat in the cylinder thus negating any gains from otherwise. Most of the cooling is done by the evaporation of fuel and the cylinder fins providing a heat sink.

Any more questions?

 

[z71mike]

Woah, woah woah! Back up the truck Chuck. Someone help please! My onboard 'puter thingee blew it's main flux capicitor that supports the major components of the Interpretation modules on my laptop.

So in essence...

What?

StihlRockin'

Just cut your muffler open a little bigger and retune. You'll like the result.

 

[shawn.m]

Just cut it

Did muffler mod on my 372xp years ago, tuned it up with more fuel, unlimited coil.
Skip chain and this thing still rocks!
Less back pressure equates lower pumping loss, more fuel equals more power when tuned.
Did muffler mod on 372xp, rancher 55 and cheapo Poulen .. All responded to muffler mod with more power and quicker to rev.