Muffler Mod, how big is too big

[timberwolf]

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.

 

[Lakeside53]

Do check your carb if you have the time. I've found the WT194 to be the best, and the stock 426 to (some saws) run out of adjustible flow...

 

[Turbocaster]

Timberwolf, Thanks for providing the data on your 026 muffler modifications. I had just completed a modification of adding two 5/16 inch holes to both of my 026's. While the power increase was noticeable, I just didn't know if it paid to go further. You certainly answered that question.

I am totally impressed and appreciative that you took the time to do the test and that you let the rest of us know the results. :>)

 

[sawinredneck]

AWESOME TW, just AWESOME!!!! I am blown away at you're thoroughness on this!! Great information, puts a lot of answers out there for the rest of us!
I tried to rep, but you know..........................

 

[RaisedByWolves]

How the hell did the page get so big?








TW (Dad?) I was following this elsewheres and I think that the addition of a exit tube will change the results some, but Im not sure which way.


The transition into the tube (Think radiused edges) may help also.


Im thinking that a tube will be more efficient........Er.......Somewhere.


Maby more flow? Torque? Same results with less noise?

 

[timberwolf]

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?

 

[oldsaw]

Okay, TW, enough blabbing, off to the next series of tests!!! We on AS have to know.



Mark

 

[timberwolf]

I know, but I am trying to wrap up a $1,000,000 fire investigation and deal with lifes little quirks. Hope to do some more saw work later in the week.

 

[Kneejerk Bombas]

I did a similar test, with a small echo, incrementally opening the exaust and doing timed cuts. Ended up getting my best time without a muffler at all. It sounded the best that way too.

 

[timberwolf]

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.

 

[timberwolf]

Here are some graphs, I wonder if I had opening 0.63 and 0.73 mixed up?

 

[timberwolf]

Tuning by ear vs tuning to 14k each time

I did another test today, this time as Ben Sugested before, I tuned to 14k each time.

It is clear that the gains drop off much quicker and virtually nothing is gained by a muffler opening greater than about 90% of the exhaust port area. As the muffler opening is increased, it did require richening the saw to keep the RPM within the factory 14k max, it was louder, would be burning more fuel, but did not appear to be producing more power.

So if someone wants to do a muffler mod and intends to keep the saw within factory RPM, the 125% rule likely is just making noise and spewing unburnt fuel for nothing. By this it looks like somewhere about 80-90% of the exhaust port area would be the sweet spot if RPM is to be kept factory.

One that has me thinking now is chain, seems some chains can take advantage of the larger opening and higher RPM beter than others, Need to look at this some more, but from what I saw today trying some chain a lighter chain can take advantage of the higher RPM more than a heavy new out of the box work chain.

 

[Bowtie]

I did another test today, this time as Ben Sugested before, I tuned to 14k each time.

It is clear that the gains drop off much quicker and virtually nothing is gained by a muffler opening greater than about 90% of the exhaust port area. As the muffler opening is increased, it did require richening the saw to keep the RPM within the factory 14k max, it was louder, would be burning more fuel, but did not appear to be producing more power.

So if someone wants to do a muffler mod and intends to keep the saw within factory RPM, the 125% rule likely is just making noise and spewing unburnt fuel for nothing. By this it looks like somewhere about 80-90% of the exhaust port area would be the sweet spot if RPM is to be kept factory.

This is good to know, as my mod is (close) to that range of 80/90% as per Lakesides advice. 1 question Timberwolf, what chain pitch and kind are you using on this 026?

 

[timberwolf]

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"

 

[Trigger-Time]

By this it looks like somewhere about 80-90% of the exhaust port area would be the sweet spot if RPM is to be kept factory.

80-90% of the piston or the muffler side of the cylinder?

 

[timberwolf]

Exhaust port area is always (as I see it anyway) at the piston, the flange is where it meets the muffler.

I am measuring the port area in 2 dimentions height and width, in reality the actual port is three dimentional due to the curve of the cylinder wall, but close enough ehh?

 

[Trigger-Time]

Exhaust port area is always (as I see it anyway) at the piston, the flange is where it meets the muffler.

I am measuring the port area in 2 dimentions height and width, in reality the actual port is three dimentional due to the curve of the cylinder wall, but close enough ehh?

Thanks...........I've enjoyed this thread, appreciate you sharing
the info

 

[stihltech]

Hmmmmm

Makes me wonder if the muffler has a small effect on the exhaust wave like a pipe does.
Is that little hole we begin with actually keep some of the raw fuel from escaping out the exhaust?
I am sure it is still an EPA thing, but it does make one wonder if muffler design could have an effect on the power curve.

 

[046]

this is only verifying what old timers have said for the longest time.
open up your muffler to 85% of exhaust port. start smaller for it's easier to go larger than reduce in size.

I'm insulted ::hmm3grin2orange: no one has remembered my spaghetti method of determining opening size.

simply fill exhaust port full of spaghetti, then count. reduce number by 15% to come up with size you need.

 

[timberwolf]

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.