Stihl 1127 muffler variations 029 to MS390

[fearofpavement]

Thought I would post up a pic of various "opening" sizes on Stihl mufflers. For those contemplating upgrading an 029 to an 039 with the 49mm cylinder, a muffler modification is absolutely essential. I have had at least one engine that would barely run until I put the modded muffler on since it just couldn't expel exhaust air.
Three of these mufflers are Stihl OEM, the "new" one is a Farmertec aftermarket which has essentially the same openings as the 3rd one. The top left has two tiny slots, the one to the right of it has only two slots but one is slightly larger, the lower left has four openings.

 

[fearofpavement]

How I mod these mufflers is I use a 3/8" mill cutter and mill the top and bottom slots left and right a bit. I leave a bridge between the holes for strength. these slots are all outside the baffled area. The depressed area is inside the baffle and if you drill that, it's a straight shot into the cylinder from there. I put two 3/8" round holes in the depression. There is adequate flow through the baffle, but just not enough area to open the muffler without weakening. I don't recommend leaving out the spark screen on this model. It's large and won't negatively impede flow.

 

[fearofpavement]

I will post up a pic of my version of muffler mod when I have a chance. I sometimes sell these on eBay but it's not proprietary information. I've tried various things and settled on a known pattern which is effective but not stupid loud.

 

[fearofpavement]

Here's what my mods look like. You can see part of the original slot in the pic so can see how much material I've taken out.

It is also very important to cut away a portion of the cover too. The new aftermarket mufflers now have a redesigned cover and I'm still experimenting with that to see how best to open it as the airflow is different.

 

[fearofpavement]

The baffle spacer goes from the front to the back of the muffler. If my holes were any closer to the center depressed area, they would be eating into that baffle. If it gets cut loose, the muffler is junk.

 

[fearofpavement]

When I do one of these mods, milling the muffler only takes a few minutes if I have the mill set up for it. (like maybe 3 minutes or so)
However grinding the cover takes longer and I spend more time cleaning the muffler than I do milling it. It is extremely important to ensure all the chips are removed because if they get in the cylinder...not good.