Muffler Modding Stihl 390.

[SawJunkie]

Supposed to rain here tomorrow and figured it will be a good day to do a muff mod on my 390. It's the first saw I bought and has been good to me. I put a new 20" ES bar on it this afternoon as I have wore the original E bar out. My question is how much should I open up the muffler? Do I just pull the front plate off and drill one big hole or a series of smaller ones? How many RPM's can I safely turn after doing the mod? I was thinking around 13,500 but maybe thats too lean? I do have a tach. Info/advice/tips will be greatly appreciated!

 

[timberwolf]

First thing get full adjustment on the carb needles start with a new spark plug too, then get 100% comfortable tuning the stock saw by ear, put it out of tune half a dozen times and retune by ear and then check with the tach to see how close you get each time to the same WOT RPM.

Then you are ready to open the muffler up, the best place IMHO is in the depressed area under the front deflector as it bypasses the junk inside. Drill a hole there about 5/8 of an inch in diamiter and trim back the outlet on the deflector to give about the same exit area.

Tune it by ear, make some cuts and check the plug color. When you are happy with how it sounds and runs check the tach for reference.

Always fun too to video some before and after cuts or have someone runn a stop watch just so you know how much you gained.

 

[SawJunkie]

First thing get full adjustment on the carb needles start with a new spark plug too, then get 100% comfortable tuning the stock saw by ear, put it out of tune half a dozen times and retune by ear and then check with the tach to see how close you get each time to the same WOT RPM.

Then you are ready to open the muffler up, the best place IMHO is in the depressed area under the front deflector as it bypasses the junk inside. Drill a hole there about 5/8 of an inch in diamiter and trim back the outlet on the deflector to give about the same exit area.

Tune it by ear, make some cuts and check the plug color. When you are happy with how it sounds and runs check the tach for reference.

Always fun too to video some before and after cuts or have someone runn a stop watch just so you know how much you gained.

The limiters are long gone off the carbs. I am fine with tuning it by ear. I got a tach last year for giggles and to help me really know whats going on. Are there any pictures floating around here on the mod? I searched but couldn't find any. I need to order a rim sprocket as well. The stock spur sprocket has seen better days. Not sure what rim I need though going to have to some homework on it.

 

[timberwolf]

You are well on your way then. Try looking up 290 muffler mods too, same muffler more popular saw.

It is not hard to figure out, soon as you pull that front deflector you will see the rectangular depressed area that ends up on the inside of the internal basket, look in the inlet of the muffler and you will get the idea of what I am saying. That is the best area to drill because it bypasses internal restrictions.

 

[SawJunkie]

Spend most of the evening reading and looking at mods done here. Seems a fair amount of them use one large hole followed by 3-4 smaller ones in the rectangular area. Some are using a 3/8 and 3/16combo while others use a 1/2 and 1/4 inch. Some completely open the indented area. I may start with the 3/8th and 3/16. I am leaning more towards the larger 1/2 and 1/4 but I can always open smaller holes up later if I want.

 

[Rescue1]

Some pics would be awesome as I have a 310 that I've been wanting to do the mod too and give it more umph. Let us know how it works out for ya..

 

[SawJunkie]

Went ahead and did the muffler mod on my saw this afternoon. I drilled a 1/2 inch hole in the center of the indented area and 4 1/4 inch holes, one in each corner. I also opened the deflector a bit. While I had the muffler off I looked at my piston and everything was great. I'm running a castor based oil. Still machine marks on the piston and this saw has been run hard. I tuned the saw and checked it with the tach and I was turning 14,500!! I know that's a bit fast I will more than likely richen it some more to be safe, but it really likes being set there. I'll have more time to fool around with the saw next week. I may even just go ahead and remove the rest of the indented area. I don't think it would hurt..

 

[timberwolf]

Good Stuff

14,500 is not a surprising RPM given a shorter bar and chain.

Never found there to be significant differences between one big hole in the muffler and several smaller holes.

Each 1/4 in hole is 0.049 in2, 1/2 inch hole is 0.20 in2
total 4x1/4 + 1 x 1/5 = 0.39 in2. Should work out to about 72% of the area at the exhaust port which is going to give most of gains with minimal side efffects on low speed and idle.

One 11/16 hole would give about the same ~0.37 in2 with just one hole to drill and layout.

 

[SawJunkie]

Good Stuff

14,500 is not a surprising RPM given a shorter bar and chain.

Never found there to be significant differences between one big hole in the muffler and several smaller holes.

Each 1/4 in hole is 0.049 in2, 1/2 inch hole is 0.20 in2
total 4x1/4 + 1 x 1/5 = 0.39 in2. Should work out to about 72% of the area at the exhaust port which is going to give most of gains with minimal side efffects on low speed and idle.

One 11/16 hole would give about the same ~0.37 in2 with just one hole to drill and layout.

Thanks for that little math problem there. Math was never my strong point. So basically I should just let it alone with what I did? I used the drill press and cross slide vise for drilling. Worked pretty slick. I didn't bother to lay it out. I drilled the 4 holes first and then the center one last. I got it off center just a bit. Maybe my head was crooked when I was looking at it!! At any rate, I don't think the exhaust gases will ever notice! I was thinking of just removing the rest of the indented area but maybe thats to much? I made a couple cuts in an old stump with the bar buried/new chain, and it seemed to me it was maybe bogging just a tad. Then again I have been running the 440 lately and my judgement is probably skewed by its performance. I need to run it more for an accurate observation.

 

[timberwolf]

Yes, set up on a cross slide would make easy work of it. Going bigger might not gain much if the bottle neck is the outlet size at the deflector.

Cutting with a big saw then switching to a smaller saw can throw the preception for sure, that where making before and after timed cuts helps.

If it feels boggy then give it a fraction of a turn richer on the HS and see if it feels better down at 14,000 or there abouts.

 

[SawJunkie]

Yes, set up on a cross slide would make easy work of it. Going bigger might not gain much if the bottle neck is the outlet size at the deflector.

Cutting with a big saw then switching to a smaller saw can throw the preception for sure, that where making before and after timed cuts helps.

If it feels boggy then give it a fraction of a turn richer on the HS and see if it feels better down at 14,000 or there abouts.

I'll let you know my findings next week. Have to go to my real job so I can afford this CAD stuff. If anything I'll enlarge the center hole just a hair with a carbide burr in a die grinder and take a little more off the deflector, richen to 14,000 and see what happens.