Muffler Mod...Did I go too far? Pics....

Jimbo45 · October 30, 2013, 02:32:10 PM

*Note, for the short version, read the last two paragraphs...that's where the question is. Sorry for the long winded post!

Hey fellers, another refugee from AS here. Looks like you guys have a real nice forum here. Thought I would make my first post, in the form of requested advice.

I have been wanting to MM my 290 for a while now. I like the saw, but it just doesn't have the zing as my old 026 or 028S. Looking at those two tiny slits under the deflector, I can see where it needed some help. I think I read every post on muffler mods on the interent in the past week. So, I did it.

I saw where some were saying to cut holes in the center of the baffle cap, some say not to, and there is a lot of opinion of how much to cut. I read on many posts that the "80% of the exhaust port" rule is a good safe guideline, and that the holes can be measured using spaghetti sticks. Cool, neat idea.

I removed the muffler, and noticed that it is fold-seamed together. Argh. I know some say to leave the muffler together, and just wash out the shavings. I don't like that idea, because I just know it all won't be washed out, and could end up in the port, and in the bore. So, I painstakingly ground off the backside of the folded crimp, and separated the halves.

I needed to get a baseline measurement for exhaust port. So, looking at the muffler gasket, it matched very well to the exhaust port, so I used it as my measuring subject....160 spaghetti sticks.

So, 80% of that is 128 spaghetti sticks. OK, I need to cut holes in the front muffler cover, to accomodate 128 sticks. The existing slots held like 11 sticks each, lol. I wanted to make the openings outside of the baffle square in the center, for noise concerns. If I can keep all the flow through the baffle, it might not be terribly loud, but still flow well. Keeping cuts outside the baffle, and still inside the deflector cover, that would hold 128 spaghetti sticks, was going to be a challenge.

I started by just cutting out as much as it looked like I could, on the bottom, where the stock slots were. After cutting it, that slot held 56 sticks. Cool, progress!

Then I cut out the top section as much as I could, and it held 61 sticks. So, adding my two slots together gave me 117 sticks, which figured to be 73% of the exhaust opening. Pretty close!

Calculating the holes in the baffle, they were over 100 sticks in volume, but I added a 1/4" hole on each side, just to make sure this wouldn't be the bottle neck.

Then looking at the deflector, I cut out the front portion of the spout as I have seen others doing. After that, I determined that it was only about half the volume of the openings I made in the muffler. So, I calculated that I would need 6 1/4" holes to make the deflector cover flow the same as the newly modded muffler.

I had already removed the limiter tabs on the jet caps, so I backed out the L and H about 1 1/8 turns to start. Fired right up, warmed it up, and did a quick tune. Not a good test, but I only had some big white oak quarters on hand that I could cross cut. It did seem to have more power for sure. It only bogged once when I really leaned on it. I won't know for sure how much it improved until I can buck some wood with it. It was a little louder, but I wouldn't say it is onoxious. I did notice that my front hand got warmer than it used to, when close to the log, since the exhaust is now blowing directly at the log. Overall, I am happy with the mod, and will have to do some more tuning and testing.

BUT, I have a simple question..... when putting the muffler back on the saw, and checking to see that the exhaust port (cylinder) was clean, I noticed that the port from the cylinder is funnel shaped. I had based my 80% calculation, on the face of the port, not the actual opening of the port at the cylinder! Uh oh! The actual opening at the cylinder looks to be about 70% the size (maybe even less) that the outside of the flange where the muffler mounts (which I used as my basis for calculation). Crap!

So, when you guys are talking about 80% of the exhaust port, are you referring to the flange, or the opening at the cylinder. If the latter, than I made my muffler holes way to big! I am probably close to 100% - 115% of the cylinder opening, all things considered, and the fact I did re use the screen. Did I screw up? I have read a test where one guy did tests, in increments, and said that there were gains to be had, up to like 200% larger muffler openings, but sacrificed idle quality over 120% or so! Mine seems to idle strong after the tuning, so maybe I'm OK? Am I going to harm this saw by running too little back pressure?

TIA

Here are the parts with the holes cut, before painting with high temp exhaust paint....

And here is it all painted and installed.....

sharkey · October 30, 2013, 04:00:41 PM

You got the spaghetti trick from Mac Dizzy didnt you? Pi x radius squared equal the area of a circle. There are other formulas for elliptical holes, but the spaghetti trick is hard to argue with.

What you need to do now is measure the thickness of the wire in the mesh and determine how much vent area your loosing to screen.

Nice job!