Muffler mods

[MasterBlaster]

At the start of this, let me tell everyone that I am no kind of mechanic and have no aspirations towards becoming one. I never have been mechanically inclined, not my 'forte. Okay.

I have learned from this site that the practice I have of removing my spark arrester is not the correct thing to do. I did not know that a muffler 'sucks' as well as 'blows'. Cool.

I have also been told that if I keep the screen clean I should have no problems.

Okay. Here is my inquiry. How come when I yank the SA out of a new saw, I IMMEDIATELY get an increase in power? It can't be because the screen is dirty because it is a new saw.

also...

My present 200 is nearly three years old and has no SA. It has a lot of miles on it and still runs nearly as good as new. Have I just been lucky?

 

[Stumper]

Butch, I'm not really a mechanic either. Maybe some of those who are will weigh in. You are right that removing even clean Spark arrestor screens can enhance performance. The reason is simple. -The screens are restrictive. Much of the screen's surface area is just that-surface area. A screen impedes airflow(gasflow). On some designs it doesn't matter much because the total screen area is huge. On others, removing the screen ammounts to making something like a 70% larger port in the muffler. I run some of my saws with sceens and others without. On public land regulations require screens so I comply but most of my work is residential. It is true that the exhaust "sucks as well as blows". That makes for a very good reason to leave mufflers in place but I don't see it for screens. The exhaust port may suck a little exhaust gas back out of the muffler but it doesn't suck pebbles off the ground! The nastiest thing it is likely to pull in through the cylinder exhaust port is a chunk of carbon that has built up there. Dissassembling mufflers to clean screens makes for a good opportunity to scape the port but .....I haven't had much build up in saws without restrictive mufflers!

As far as spark arrestors themselves are concerned.... that is what they are -not muffler backflow filters. IMO they are just plastic banana, feel good, who cares if it makes a difference, legislation. Ever run a saw without a screen in the muffler late into the twilight? I've never seen sparks flying out of the muffler but they do fly off the chain. I'm not talking about "rockin' out" either. "Clean" dead hardwoods throw a surprising number of sparks. I still have never started a fire with a chainsaw. They are real handy for cutting fuel for a fire though!

 

[MasterBlaster]

Thanks, Stumper. I was scolded on an earlier thread about doing that and I was puzzled. I'ld allways done that, even way back to my Poulan days. When I was told I could damage my saw, I got a little concerned. Iwould hate to have to put the screen back in.

I've never done that to a big saw. Would that be part of what is called 'opening' up a muffler?

 

[Stumper]

A lot of guys running opened mufflers are loggers cutting in the national forests. They are required to have spark arrestors so I think the standard muffler opening prescription is to add more muffler ports and screen them all. Pulling the screen works wonders on the little Stihls and Echos.

 

[Kneejerk Bombas]

The safe and easy thing to do is to increase the size of the hole the screen covers. The screen is always larger than the hole.
Leaving the hole open to the cylender is asking for trouble. The possiblilties are unlimited.
The metal the mufflers are made of is thin. Holes can be enlarged with a file or drill quite simply.

 

[Kneejerk Bombas]

Here's an animation Huskyman linked earlier:

http://www.maximumsled.com/mountain/garage/2strokeprinciples/two-stroke_animation.gif

Notice how the back presure actually moves gases back into the cylinder. Imagine sawdust, sand, dirt from a mud wasp, or other foreign material swirling around inside your exaust getting sucked into the motor.

 

[Stumper]

Very nice , but inaccurate animation. If all the exaust gases plus outside air were sucked back in on the intake stroke the saw could not possibly run. Some scavenging of exaust takes place but it is small. Mostly intake air and fuel is sucked in. Overly restrictive mufflers are a problem but in order to mufffle at all there has to be an expansion chamber. Typically we maintain a slightly pressurized condition in the muffler for best performance. Even if you have a completely non restrictive pipe the expansion chamber alone means that the scavengable gases in closest proximity to the exaust port are the gases that came out of the engine on the previous stroke(s). You say spark plugs don't go bad. I say that 2-cycles don't suck gravel through their exausts.

 

[bwalker]

Mike, That animation is a good example of how a tuned pipe sytem works, but isnt really applicable to a saw. Saw's use untuned sytems that do not use sonic waves to scavenge the cylinder like a sled engine with a tuned pipe does.

 

[Jacob J.]

Stumper, That animation is not quite as inaccurate as one would think- a tuned pipe can double the power of your 2-stroke engine merely on the basis that it forces some of the unburned fuel/air mix BACK into your engine on the next stroke, making the engine burn those gases a second time. I know local racers with 090's were squeezing right around 26 horse out of them with the porting, carburetion, and tuned pipe work. This included pop-up pistons and the like but not a remanufactured combustion chamber.

 

[bwalker]

Stumper, Keep in mind the duration of the "suck" is no where near as long as the animation suggests. A properly tuned pipe will actually pull exhaust out of the cylinder and ramanyfresh mixture back in.

 

[Crofter]

Stumper; I agree stones are not going to get sucked up off the ground and back into the cylinder, but mud and sand blasted off the bark by the exhaust could get into the muffler and migrate to the port area. Its not going to demolish your saw instantly but it might be a small insurance to keep the screen in.

Frank

 

[Stumper]

Jacob and Ben, Right! The system recirculates unburned exaust back through another cycle. It does NOT suck fresh air all the way back through the exaust and into the cylinder. The old saws didn't have screens. They didn't die because of dirt sucking through the exaust from outside.(And many of those saws had horizontal cylinder with right side exausts that raised dust storms during low stump cuts!) Granted, newer saws turn faster and are worlds better but they still don't suck fresh air through the exausts.- Do they? I think that the advice to leave screens in because of outside dirt getting sucked in is a red herring. If outside air could get back in then the screen is an inadequate filter-it'll keep gravel out- even big sand- but the volcanic ash is coming through. Hold a strip of paper in front of the muffler ports of a running saw. Does ANYONE have a saw that will actually suck the paper against the muffler port? I'd wager that everyone's saws push the paper away and leave it flipping around AWAY from the port in the exaust stream. If I'm wrong present the evidence and I'll repent and put the screens back in all my saws. As it is about half are happily running without the screen-and have been for quite a while.:angel:

 

[MasterBlaster]

My screens been out of my ms200t for over 2 years now, no problemo.
Seems like proof enough to me!

 

[bwalker]

Its really all moot since a saw doesnt have atuned p[ipe(stock of course) and there is no sucking, positive or negative going on with a untuned sytem. The main value I see in leaving the screens on is it keeps those pesky mud wasp's out of you engine.

 

[Kneejerk Bombas]

With a restricted stock exaust you would get more back presure which could cause exaust to move back into the cylinder, I'd think.
Holding a piece of paper over an exaust is a bad example, try holding a handfull of flour, putting your face up to it and blowing. The majority of it will move away, but some will come back in your eyes.
The only foreign matter that might go in your saw motor is stuff that found its way there when the saw was not running, but if you can just make the hole bigger and keep the screen on, why not?
I have a couple 066s that get carried around in the truck, they might not get used for a week or more. They each cost over a thousand dollars, all set up. You think I'm leaving the screen off one of those? Ummm...no.