Opening up a muffler? Is it even a good idea?

[DennisCA]

I've been thinking about opening up this muffler on my brush cutter to get a little more oomph. But not sure how to go about it. This one has a long pipe inside the main opening and then the exhaust does a 90 degree bend again before leaving. So I guess I could remove the last forced bend and it'd flow a bit more freely, or I could remove the tube inside. Both maybe?

I'm not even sure it's a good idea. I do like this thing, it has a decent amount of power for it's small size, it's old too, maybe late 80s early 90s. If it could have a little more grunt I'd like that.

I'm not sure how opening up the muffler would change the characteristics of the machine, does it give more power across the whole register, does it make it weaker at lower rpms and stronger at higher rpms, or does it just allow it to rev faster? I'm just not sure it's a good idea or not.

 

[scottr]

You have a easy muffler to make a front outlet plate .
You could try a formula of engine displacement (cubic inch) to muffler outlet (square inch) of 10 to 1 .
It works on pre epa saws .
Scott

 

[DennisCA]

Right so this is a 34.4cc machine so it should have a 3.4 opening (square cm in this case). That means a 21mm diameter hole. A little less. Perhaps 20mm is worth trying then, on a new cover.

 

[scottr]

Right so this is a 34.4cc machine so it should have a 3.4 opening (square cm in this case). That means a 21mm diameter hole. A little less. Perhaps 20mm is worth trying then, on a new cover.

You need the displacement in cubic inches so 34.4cc divided by 16.38 .
Then use the 10 : 1 ratio to get the square inch area for the opening .

 

[DennisCA]

Righty then I get 13 or so millimeters and I'd say that's pretty close to what's already on there, going by eyeball only now.

 

[scottr]

When I do the math I get 2.1 cubic inch displacement . 10 : 1 gives a 0.21 square inch outlet which would be about a 1/2" diameter hole .
Looking at the view of the back of your muffler front plate it looks like the pipe outside diameter is about 3/8" diameter .

 

[DennisCA]

I'll have to measure I don't trust my eyeball measurement I always fetch the wrong wrench... 13mm is about half an inch, hair above.

 

[scottr]

Yes .
There's a flat area around that pipe where it mounts to the back of the front plate that looks to be about 5/8" wide .
Does this muffler have a spark arrestor screen ?

 

[fields_mj]

So, the OP asked multiple GOOD questions that nobody has really addressed. Before he goes and messes up his brush cutter, he wants to know what the effect SHOULD be. My non-technical opinion is that if the exhaust is too restrictive, you loose power, mostly at the higher end of the RPM range because at the lower end, because you're not dumping as many CCs of exhaust per second at low RPMs . At the higher RPS, when the engine is under a load, a restrictive exhaust keeps the exhaust gasses from exiting the combustion chamber. This means that some of your CCs are not available to fill with fresh fuel and air which robs the engine of both torque and HP.

Here's what I did. I ordered a new hotzfforma brush cutter for the purpose of beating the crap out of it running a 9" Forster brush blade to cut invasive honeysuckle. When I installed the 9" blade, it was noticeably sluggish. With a string head, it would have been fine, and I expect the heavy 9" brush blade to take a little time to get up to speed, but I was really wanting better throttle response in the woods. Looking at the exhaust, it basically had the same setup you have as far as a single 10mm pipe through the cover. I poked another 3/8 hole in it and its not NEARLY as sluggish as it was before. It might be just as sluggish as it originally was when you first hit the trigger, but it does get up to speed a LOT faster. I never actually used the blade on anything before I modified the exhaust, but I expect that it also maintains the RPM a lot better when its in use. If your using your machine to run a heavy blade, and it seems like it struggles to get up to speed more than it should, I'd say that opening up the exhaust is the first step. If you mess up, you can always cover the hole with a piece of sheet metal and a screw or steel pop-rivet.

The other thing I'd look at is the little cover on the outside to make sure that it only redirects the exhaust, and doesn't restrict it any.

Oh, and I wouldn't put anymore RTV no the cover when you bolt it back on again

 

[DennisCA]

Oh the RTV was a previous owner, I guess anyway. I've just left it there. I actually run mostly a strimmer on this since the original wasn't even semi automatic, but it's a size bigger strimmer than it was intended for. Still works though.

 

[Sidecarflip]

If you really want a power boost, have an expansion chamber made for it.

 

[DennisCA]

Googled that and it looks a bit ridiculous.

 

[scottr]

I'll have to measure I don't trust my eyeball measurement I always fetch the wrong wrench... 13mm is about half an inch, hair above.

DennisCA , did you measure the inside diameter of the pipe ?

 

[DennisCA]

Not yet, a bit busy IRL with other things ATM

 

[Sidecarflip]

Googled that and it looks a bit ridiculous.

Not at all. Competition sawyers all have expansion chambers on their competition saws, your issue is, one you lack the mathematical knowledge to extrapolate the dimensions required to fabricate one and secondly, you lack the fabrication skills to produce one anyway and that is obvious by your comment. You won't find what is required on Goggle, or any common search engine anyway. Stick with a stock muffler as an expansion chamber is beyond your realm of making.

 

[Sidecarflip]

I've been thinking about opening up this muffler on my brush cutter to get a little more oomph. But not sure how to go about it. This one has a long pipe inside the main opening and then the exhaust does a 90 degree bend again before leaving. So I guess I could remove the last forced bend and it'd flow a bit more freely, or I could remove the tube inside. Both maybe?

I'm not even sure it's a good idea. I do like this thing, it has a decent amount of power for it's small size, it's old too, maybe late 80s early 90s. If it could have a little more grunt I'd like that.

I'm not sure how opening up the muffler would change the characteristics of the machine, does it give more power across the whole register, does it make it weaker at lower rpms and stronger at higher rpms, or does it just allow it to rev faster? I'm just not sure it's a good idea or not.

Looks mysteriously like my Stihl FS66 brush cutter I bought decades ago and still use regularly. They are made for Stihl in Japan and were bought in quantity by the USFS back then. Mine makes plenty of suds completely stock and it's the terminal RPM of the business end that is important, not the power it produces, unlike a chainsaw where pulling the chain in a FL cut is important.

I suggest you buy a Stihl Tri-Cut poly blade head for it, it's a wicked trimmer with the Tr-Cut head (if you have one and they are still available btw).

Mine gets a steady diet of canned fuel, in my case Echo Red Armor 50-1 and you need to keep the air cleaner sponge clean at all times as well.

I like your bent 'scrench' as well. I always like photos of equipment and what the camera captures that the picture taker don't realize is in frame. Why I like YT video's. Not for the content (most I find inane) but for what the camera takes in that the poster don't realize is in the picture.

 

[Hermio]

I've been thinking about opening up this muffler on my brush cutter to get a little more oomph. But not sure how to go about it. This one has a long pipe inside the main opening and then the exhaust does a 90 degree bend again before leaving. So I guess I could remove the last forced bend and it'd flow a bit more freely, or I could remove the tube inside. Both maybe?

I'm not even sure it's a good idea. I do like this thing, it has a decent amount of power for it's small size, it's old too, maybe late 80s early 90s. If it could have a little more grunt I'd like that.

I'm not sure how opening up the muffler would change the characteristics of the machine, does it give more power across the whole register, does it make it weaker at lower rpms and stronger at higher rpms, or does it just allow it to rev faster? I'm just not sure it's a good idea or not.

Do you value your hearing? Most muffler mods increase the sound level significantly. Be careful with that. Over-the-ear muffs are best, but they make your head hot on summer days.

 

[Sidecarflip]

I use cheap HF ear plugs myself most times. You can buy a hundred, individually packaged in pairs for under 10 bucks.

 

[Hermio]

I use cheap HF ear plugs myself most times. You can buy a hundred, individually packaged in pairs for under 10 bucks.

I use them on my tractor. But they are not enough when running a chainsaw. Earplugs realistically reduce noise by 10-15 dB. Most over-the-ear muffs reduce it by about 30 dB.

 

[fields_mj]

I've never found a set of over the ear muffs that are as effective as pretty much any ear plug. More comfortable, yes. More effective at reducing sound, no I have several types of muffs and plugs, and use them regularly. One thing that I've been frustrated by so far as that nobody seems to be making any good behind the head electronic muffs with bluetooth. There are loads of the normal muffs, but any time I need ear pro, I'm almost always wearing a hat or helmet of some kind. I've tried a few electronic ear plugs with blue tooth, but none of them stay in very well. The bluetooth feature is really handy for not missing messages from my wife, so they pay for themselves pretty quickly in terms of reduced headaches.... Just wish I could find a behind the head muff in that configuration.