Muffler mods, spark arrestors etc.

[henry r]

I want to fit a spark arrestor when i muffler mod my husqvarna 359.

What i want is a muffler that flows like a modded one, arrests sparks at least as well as a stock one, is reasonable to maintain and does not add a heap of weight. (Plus not cost hundreds of $ in custom machine work etc)

I am aiming for a restrictive opening of 85% of the exhaust port. (Keeping in the middle of others recommendations of 80 to 90% for optimum back pressure)

Obviously the mesh needs to flow considerably more than the 85% to allow for clogging and to not be the constriction point.

Question 1. Where do i find out what 30 mesh flows at as a percentage of an open port of the same size?
It is listed as having an open area of 45% but i imagine the airflow at muffler pressures would have to be a far smaller %.

Question 2. With 2 stroke chainsaw mufflers do you want the point of max constriction to be as the exhaust exits the muffler or at some point through the muffler with expansion chambers on either side of it?

Question 3. Will there be any adverse effects on flow/back pressure or spark arresting performance if the screen is well before or well after the point of greatest constriction?

Question 4. Has anyone tested how quickly screens clog up?
I would like to be able to run enough screen area that it will last a days work without needing to be cleaned but have no idea how much extra screen will be needed. (I haven't run the saw with the stock screen)


Thanks.

 

[Justin Taylor]

I want to fit a spark arrestor when i muffler mod my husqvarna 359.

What i want is a muffler that flows like a modded one, arrests sparks at least as well as a stock one, is reasonable to maintain and does not add a heap of weight. (Plus not cost hundreds of $ in custom machine work etc)

I am aiming for a restrictive opening of 85% of the exhaust port. (Keeping in the middle of others recommendations of 80 to 90% for optimum back pressure)

Obviously the mesh needs to flow considerably more than the 85% to allow for clogging and to not be the constriction point.

Question 1. Where do i find out what 30 mesh flows at as a percentage of an open port of the same size?
It is listed as having an open area of 45% but i imagine the airflow at muffler pressures would have to be a far smaller %.

Question 2. With 2 stroke chainsaw mufflers do you want the point of max constriction to be as the exhaust exits the muffler or at some point through the muffler with expansion chambers on either side of it?

Question 3. Will there be any adverse effects on flow/back pressure or spark arresting performance if the screen is well before or well after the point of greatest constriction?

Question 4. Has anyone tested how quickly screens clog up?
I would like to be able to run enough screen area that it will last a days work without needing to be cleaned but have no idea how much extra screen will be needed. (I haven't run the saw with the stock screen)


Thanks.

The only question I could help with is 4 with chainsaws the spark screens won’t really clogged because of high Rpms and the heat they put out and welcome to ar

 

[Colt Marlington]

Old Poulans have very large screens near the exhaust port, and many times its size. While most saws have a smaller screen equivalent to the size of the exhaust exit.

 

[James Miller]

I opened the muffler on my 590 4 years ago and the screen is still clean. Dont think you need to worry about clogged screens unless you tune your saws way to rich.

 

[henry r]

Thanks gents, that makes my decisions a LOT easier.

 

[computeruser]

Like others said, the clogging issue really isn’t. In the old days of low RPM saws using 30wt as mix oil, perhaps, but not now. This fact will simplify your design and pricing model, and will probably make the mesh math issue functionally moot.

 

[Huskybill]

My spark arrestor clogged on my old husky 240sg only because the low speed adjustment was too rich. Leaned it and never had a problem.

Caution with spark arrestors depending on where we cut I had a forest ranger ask me if I had the spark arrestors in my saws while inspecting my next job. I did a lot of state forest work in the past.

 

[Huskybill]

I purchased this mesh on eBay.

https://www.ebay.com/itm/t-304-Stai...tainless-Steel-Wire-Mesh-6-x-24-/293071525937

 

[TheTone]

I bought some #30 mesh (finer than what you have) for use in mufflers. It's the next size down from the stock mesh, but flows about twice as much air as stock. I'm not sure if it would stop a viable spark, but I'm sure it would help. I try to use it on the sides of modded mufflers so anything that gets by doesn't go directly into the sawdust. I don't cut on public lands, so there's no legal issue. That #12 mesh looks pretty wide open.

 

[Huskybill]

The spark arrestors just slow the spark down to burn up when it hits fresh air.

#30 looks small

https://www.ebay.com/itm/t-304-Stai...ainless-Steel-Wire-Mesh-12-x-12-/293192632679

 

[TheTone]

See also current post: https://www.arboristsite.com/community/threads/where-can-i-get-spark-arrestor-screen.116551/

 

[Ketchup]

The math is easiest if you use the same mesh as stock. Then you just make your outlet the percentage you want of exhaust port area and cover with the same size screen.

Because you have been running the saw without a screen it gets more complicated. By removing the screen you changed the outlet size by quite a lot.

1. I’m sure you can sort the math of it, but the basic rule is make your muffler outlet bigger if you plan on using a screen. A 1/1 exhaust port to muffler outlet (100%) will flow a lot less when you put a screen back on it. You said 30mesh is 45% open area. So even with a 100% outlet you only flow 45% of exhaust port area when you put your screen on. That doesn’t even account for pressure and flow dynamics.

2. Most saw mufflers are constricted at the exit. In most cases that is where the spark arrestor is located. Many mufflers also have a cage or baffle inside that also constricts flow. EPA mufflers frequently have catalytic converter elements which are also very resrictive to flow. In most cases you don’t want any constriction. The timing and port shapes of the engine can control fuel consumption if properly designed. Strato saws were invented for this reason.
The exception are pipes. Exhaust pipes pull raw fuel through the engine and then create back pressure that back-stuffs the compression chamber with that raw fuel before ignition. Points of constriction and their location are especially important for performance exhaust pipes.

3. Before or after the greatest point of constriction would still have an effect, but a less distinct one. Typically the screen is the point of greatest constriction. Also, the exhaust traveling as far as possible before encountering the screen will more effectively arrest sparks, will clog least, and constrict least.

4. As said above, the screen should not clog on a properly tuned saw.

Not sure why I wrote such a long answer. Helps me think?

 

[MacAttack]

So...if your saw has a bridged exhaust port with 3 smaller ports, and you are calculating the appropriate exhaust exit, do you measure the entire exhaust opening on the cylinder, or the total of the 3 bridged exhaust ports?

If my question makes sense... i will need to do this soon for my homemade replacement exhaust.

 

[Ketchup]

Seems either would work. The area between the bridges is the true port area.

 

[Stihl 041S]

It’s about the flow
Go to McMaster Carr and you can buy different gauge wire and different spacing of the wires.

The government goes by the hole size.

That is what will make it legal. Hole size.

And for forever......get the inconel screen.