Chainsaw Repair

How To Basics - Carb Fixes + Mods - IPL and Service Manuals => How To Basics and Fixes => Topic started by: JOE.G on December 25, 2012, 04:04:47 pm

Title: Muffler mods
Post by: JOE.G on December 25, 2012, 04:04:47 pm
I hear of people removing the Spark screens, does it make a difference? In Power? In sound? Pros? Cons?

My 562 XP doesn't have one but the rest of the equipment in my sig below does, Should I remove it from any of them? Should I leave it in some of them? Thanks
Title: Re: Removing Spark Screens, Does it make a difference?
Post by: 3000 FPS on December 25, 2012, 04:18:32 pm
I have a bunch of Mufflers that I have modded and I always leave a spark screen in.  The reason is because if you cut on National Forest land or BLM land it is required by law that they be in there to prevent fires.   If you are looking for all out performance then removing it does help but I would be willing to bet if the spark screen you have is good and clean that without doing a complete muffler mod you may not even notice the difference.  What I like to do is open up the baffles inside the muffler and open up the outlet for the muffler but leave a spark screen in.  Opening up the muffler though will definitly make it louder.  But to answer your question removing the spark screen does help improve performance.


(http://chainsawrepair.createaforum.com/index.php?action=dlattach;topic=1419.0;attach=2005;image)
(http://chainsawrepair.createaforum.com/index.php?action=dlattach;topic=1419.0;attach=2007;image)
Title: Re: Removing Spark Screens, Does it make a difference?
Post by: 3000 FPS on December 25, 2012, 04:27:08 pm
On this one if you look real close into the pipe you can see the screen in there.

Also I forgot to mention that anytime you change the flow of exhaust through a muffler you need to change the fuel mixture setting also because it will affect it.

(http://chainsawrepair.createaforum.com/index.php?action=dlattach;topic=1419.0;attach=2009;image)
Title: Re: Removing Spark Screens, Does it make a difference?
Post by: 3000 FPS on January 12, 2013, 11:58:02 pm
Thought  would show another MM.  This is on a J-red 670 and once again there is a spark arrestor screen inside.  I used one from a Poulan muffler that wraps around the baffle.

(http://chainsawrepair.createaforum.com/index.php?action=dlattach;topic=1419.0;attach=2133;image)
Title: Re: Removing Spark Screens, Does it make a difference?
Post by: 3000 FPS on January 13, 2013, 12:04:08 am
This Muffler Mod is on a Poulan 335.  You can see I cut a slit in the side of the muffler and then open it up with a screw driver and pliers.   If you look real close you can see the screen inside.

(http://chainsawrepair.createaforum.com/index.php?action=dlattach;topic=1419.0;attach=2135;image)
Title: Re: Removing Spark Screens, Does it make a difference?
Post by: JOE.G on January 13, 2013, 07:02:39 am
 I see you like to keep your screens in, Some people swear they restrict others say they don't.
Title: Re: Removing Spark Screens, Does it make a difference?
Post by: Al Smith on January 13, 2013, 07:04:42 am
TW AKA Brian W. of Ont where ever  he went had a formula of how much to increase the size of the exit hole to equal about the same size were the screen left in a standard size hole .

As far as to screen or not to screen there are mixed opinions on that subject depending on local .In this portion of Ohio you'd be hard pressed to start a fire unless you were in the middle of a wheat field in July .If you were I'd well imagine you'd have a very upset farmer to deal with .Probabley not a good plan .
Title: Re: Removing Spark Screens, Does it make a difference?
Post by: TORX on January 13, 2013, 09:08:23 am
For my personal use. I remove the spark screen. But if i'm muff moding someone elses saw, I'll put a screen in the extra port. Just because....
Title: Re: Removing Spark Screens, Does it make a difference?
Post by: 3000 FPS on January 13, 2013, 12:05:20 pm
I see you like to keep your screens in, Some people swear they restrict others say they don't.

Well Joe I do keep them in because these saws will be used on Public land and it is required by law.   But I do believe that if the baffle is opened up and the exit hole is opened up to, if not larger than the size of the exhaust port on the cylinder, that I will still see decent gains in performance.   That is one reason I try to use screens that wrap around the baffle 360 degrees because it gives alot of area for the exhaust to flow through.
Title: Re: Removing Spark Screens, Does it make a difference?
Post by: 3000 FPS on January 13, 2013, 12:12:57 pm
Ok I decided to take a picture of the type of screens that I like using the most.

(http://chainsawrepair.createaforum.com/index.php?action=dlattach;topic=1419.0;attach=2144;image)

Title: Re: Removing Spark Screens, Does it make a difference?
Post by: Cut4fun on January 13, 2013, 12:45:47 pm
I just noticed this was about spark screens. If there is a muffler mod thread let me know and I'll move Brian's info if needed. Might be some screen stuff thrown in later.  :-\

Brian did some testing on a 026 back in 2007.

Quote from: timberwolf;605833
I did a little testing on opening up a muffler on an 026, this was posted about the web, but did not generate too much interest, though maybe it could be helpful.

Basicaly what I did was weld up a blank muffler cover for an 026 with a front plate that could be easily modified with different opening sizes.

Each time I tried a new opening the saw was tuned by ear to the edge of 2 stroking and then I made several cuts in a 10 inch poplar taking the best time.

Saw is bone stock except for a improved 260 air filter.

What I found was from a stock opening to 120% of the exhaust port size there was about a 20% improvement in cut times. Going further up to 400% of the port area continues to give small gains totaling another 5%.

I did also find that maintaining a good idle became problematic over 190% of the port area.

Here are a few pics and graphs







Title: Re: Removing Spark Screens, Does it make a difference?
Post by: Cut4fun on January 13, 2013, 12:48:19 pm
Brian was into graphs etc to back up his testing.

Quote from: timberwolf;605837
Here is the data visualized

Title: Re: Removing Spark Screens, Does it make a difference?
Post by: Cut4fun on January 13, 2013, 12:53:36 pm
More info

Quote from: timberwolf;605887
torque stayed good.

Goin in I though there would be a point where it might start to miss in the cut, but didnt see that.

One surprise was, as I started to open the muffler up I needed to do more increase on the LS and actually needed to lean the HS a little. I guess the LS was over contributing. As the increased demand at HS was not nearly what the increased fuel demand was at LS, however the fuel trrough the LS can make up a good 1/4 to 1/3 of the total fuel flow at full RPM.

The major issue was having a decent idle above 190 % of the exhaust port the idle started getting a little touchy and I needed to bring the idle RPM up to keep the saw from either loading up at idle and stumbling on spool up or being too lean at low speed and reving on after the throttle had been dropped.

Quote from: timberwolf;605925
In general maybe, but each saw has it's own muffler and port dimentions, port timing and RPM, so I would expect to see variation from one saw to the next.

Some saws are much more choked down to start with, so would show beter gains, some of the older saws were more open and less would be had by opening up the muffler.

I opened up a Cat muffler on a 359 and the gain was closer to 35-40% on the cut times.

Chain can play a role, if the chain is dull and your pushing for all your worth a muffler mod and some extra HP won't make any difference.

Andy works at a stihl dealer in Wash. Have dealt with him in the past and very knowable person.

Quote from: Lakeside53;606098
Great work!  It seems to validate  that 125% of exhaust port area is a good rule of thumb for decent results.

Quote from: timberwolf;606112
Yes it does validate the 125% of port opening rule, not even bad for the 85% of flange rule. And in line with the 80 20 rule 80% of the gains made with the first 20 of the work.

On the flip side there is some to be gained to go beyond that point, I want to look a little more at the down side, the LS jet was getting opened up pretty good to keep it happy, I wonder how much that hurts fuel efficiency? Engine temperature also the extra fuel might be adding a little cooling to the mix?

Going to try to repeate it with a modded 026 later in the week.

Quote from: timberwolf;606401
Thanks Lake, I will check the carb, it is possible it is a 194 too.


My thought on the tubes is also that they can offer more in the way of sound attenuation, I have some exhaust design software that has sound level prediction. From what it shows me the longer the tubes the greater the possible sound attenuation.

From Practicle experiience, if it is like the stinger on a pipe, making it longer holds more heat in the pipe and raises the pipes RPM, so I think it may be a greater restriction than just a plain hole. On the other hand, looking at fluid dynamics theory, flow through a hole in a plate has greater friction than flow out a nozzel shaped opening. From practical experience I found I could go to bigger opening with tubes and still keep the low end, but I want to test this in a more controlled manner.

But at this point it is easy to try the ported saws and see how they do.

Saw number one is a very strong woods saw built for torque with high compression in the 200 psi range, low exhaust numbers and a short blowdown. It is interesting, this saw picks up nearly nothing on pipe. My thought is that this saw will be hurt by the smaller exhaust port opening more than a stock saw, the reason being that there exhaust pressure BMEP at exhaust port opening is higher than stock and blowdown shorter, so if there is too much restriction to getting the exhaust out cylinder purity will take a dive and then filling from the transfers will be delayed. Because the exhaust port is wider and has a greater initial opening width combined with a higher BMEP the initial outrush of exhaust should be more intense than stock. I think the Modded saw should continue to gain decently past the point where the stock saw started experience diminishing returns on the increased opening. What you guys think?




Title: Re: Removing Spark Screens, Does it make a difference?
Post by: 3000 FPS on January 13, 2013, 12:53:59 pm
I have read the documentation that Brian did with the 026.  It was really well done to say the least.   

   Even though this thread is about spark screens it is hard not to include muffler modding because after all eliminating the screen is a modification, and performance I believe is what the OP was interested in.
Title: Re: Removing Spark Screens, Does it make a difference?
Post by: Cut4fun on January 13, 2013, 12:57:01 pm
ported saw testing I think.

Quote from: timberwolf;609503
Had some time and gave the test a run through with the modified saw, I used the same bar and chain, though the chain was not realy enough for the saw and I was pushing too much to do the cut times and saw justice.

This saw is not a high reving saw, exhaust port duration is acctually a little lower than stock. Built for torque, that it has, will pull 3/8 8 pin on muffler. Though in this test I was using 8 pin .325 for both saws.

Here are the numbers, again experienced the same need to richen the LS on the first couple openings and acctually lean the HS. This time the gains did not level off as much as with the stock saw.

Again up arround 180-190% of the port area the idle started getting touchy and it was nessicary to crank up a few hundred RPM.

Title: Re: Removing Spark Screens, Does it make a difference?
Post by: Cut4fun on January 13, 2013, 12:59:55 pm
Confirming what I have always said on chains too in his 026 muffler testing. If the saw has the balls to pull 3/8 use it.

Quote from: timberwolf;610389
I have tried just about every 325 and 3/8 I could, the .325 Stihl RS is good with an 8 pin sprocket, but in all cases 3/8 oregon, carleton, windsor, or Stihl RS cut faster tan .325, both out of the box and square filed. Some of the .325 chain has smaller cutters and does not work worth a crap, the Stihl RS is the only one I found to work well in .325.

I run only 16 inch on the 026, have tried 18" and 20", but don't bother with that, just switch to a bigger saw to cut with a 20"

Title: Re: Removing Spark Screens, Does it make a difference?
Post by: Cut4fun on January 13, 2013, 01:02:28 pm
Quote from: timberwolf;610658
Yes it confirms it, but for me it clairifies some things too.

If factory RPM max is to be respected 85%

Stock Saw up to 125% if RPM is increased above factory spec

Modified gains can be found over 150% if speed is your objective

I want to do some testing on heat and fuel consumption next, bought an IR temp gun yeasterday.

Like the spagetti method, KISS

As far as any ruels of thumb off the size of the exhaust port flange, I would dump that idea esp if the saw has been modified.

Quote from: timberwolf;611900
For the fuel test, I rigged up a 2 oz (60ml) bottle with 3 lines, one for fill, one breather, and one to the carb. For each test I filled it to the top of the lines, so I should have been within 1 or 2 tenths of a ml each time. After each test I refilled the bottle with a graduated syringe. Using the modified saw and square chain.

For the test I put on a muffler opening and tuned the saw by ear, double checked the RPM on the tach, shut it down and refilled. Then cold started and made 5 cuts in 10.5 inch poplar, shutting the saw down as the last cookie fell.

Here is the fuel consumption numbers ml for 5 cuts

17.0 Opening 0.09
16.4 Opening 0.18
15.8 Opening 0.27
15.0 Opening 0.36
14.8 Opening 0.45
14.6 Opening 0.54
14.9 Opening 0.63
16.2 Opening 0.73
17.2 Opening 0.79
17.4 Opening 1.50

Fuel Efficiency acctually improved as the muffler was opened up, but dropped off about the same opening as improvements started to decline on cut times, only with very large openings was fuel consumption worse than stock. Don't know if this would be the same with a stock saw?

By time the saw uses more fuel as the muffler is opened, so the tank gets dry sooner, but the woodpile gets bigger faster. Too a point.

But I think this is just part of the story, It seams there is a lot of LS ajustment made, and a saw spends a lot of time at idle, so need to check that too.

Title: Re: Removing Spark Screens, Does it make a difference?
Post by: Cut4fun on January 13, 2013, 01:05:57 pm
Quote from: timberwolf;1564342
1.5 x .5 is plenty esp for a stock work saw.

Never found much difference related to saw size, port openings seem to be fairly porportional to displacement. Bore to stroke ratio plays a little with it as well as RPM though which affects bleed down period as well. Muffler volume factors in also. A large volume muffler is softer in terms of building back pressure right after exhaust port opens and will work with a smaller outlet than would be ideal for a small volume muffler.

Quote from: timberwolf;2473507
But ..... TW did not use a spark screen.

I'm guessing a spark screen restricts flow considerably ?

It will but depends on how the screen is positioned, if it is tight to the opening then the screen should cut the flow in about half. Something I have been thinking of doing some flow bench work on.

But if the screen is held off the actuall opening so that there is a larger area of screen availible (at leat twice the area) it likely makes little difference.

I was hoping he tested screens, but last post confirms he didnt.
Title: Re: Removing Spark Screens, Does it make a difference?
Post by: 3000 FPS on January 13, 2013, 01:23:26 pm
Very good information Kevin thanks for posting it.  I will now always know where to find it.
Title: Re: Removing Spark Screens, Does it make a difference?
Post by: Al Smith on January 14, 2013, 07:55:40 am
There's always the option of using a coarser screen mesh too with wider openings .Now how effective that would be in catching a spark is anybodies guess .

Now that cutting a giant hole might work if you have a giant carb .Problem is with most stock sized carbs you can crank out enough low speed they idle and blow black smoke at the same time .Then  if you goose it to accelerate up to run speed the pressure drop is so fast the saw stumbles badly most of the time .You have to kind of like "pump" the trigger to keep from stalling it which is a pain in the buttocks .If you can get them wound up though they will scoot .Not a grand plan on a work saw .
Title: Muffler mods
Post by: doreadeal on September 22, 2013, 08:34:01 pm
Nice work on those mufflers! With stock mufflers (even modded) I can see the screen not changing much. The discharge port is almost always much bigger then the baffle port. Now if you were to make a muffler in witch the reflection pulse was done without the baffle, but rather on the outer can of the muffler, I could see a screen being harmful to output. My educated guess is that it would have a less then good effect on the reflected pulse and require more restriction of the discharge port. Just a guess...