How much does spark screen restrict flow ?

[mtngun]

16x16 on left, Stihl 28x28 on right. Same 0.018" diameter wire on both.

50% open area vs. 25% open area.

Obviously, the 16x16 is not going to be very effective at containing sparks. But it'll keep bugs and snow out of the engine, and in the unlikely event that my saws are examined by a forester, I'll honestly be able to say that it has a screen.

 

[RandyMac]

16x16 on left, Stihl 28x28 on right. Same 0.018" diameter wire on both.

50% open area vs. 25% open area.

Obviously, the 16x16 is not going to be very effective at containing sparks. But it'll keep bugs and snow out of the engine, and in the unlikely event that my saws are examined by a forester, I'll honestly be able to say that it has a screen.

Every time I had my saws checked for screens, they used a wire gauge to check for the correct mesh. It was poked through the exit, if the gauge didn't go through the mesh, you were good. I often had to trade out the perforated baffle on McCullochs for a screen, didn't go with both.

 

[tlandrum]

ive never hed the forester check mine with a guage but i have had them look my saw over really good after hearing it run, they checked to see that all outlets had screens and said i was good to go.

 

[ttyR2]

Does the equation have a variable that accounts for the turbulent nature of gas flow through the screen, vs. laminar flow? Fluid dynamics can make my head hurt at times.

 

[54stude]

If you take a look at a Husqvarna 2100 type screw in spark screen you can see they probably got it right. The screen screws into the muffler, and has maybe 4 times the surface area of the outlet and exhaust port. One of the reasons why the 2100 and 2101 saws were good runners.

 

[Eccentric]

Every time I had my saws checked for screens, they used a wire gauge to check for the correct mesh. It was poked through the exit, if the gauge didn't go through the mesh, you were good. I often had to trade out the perforated baffle on McCullochs for a screen, didn't go with both.

In the McCulloch SP125 General Service Info bulletin, McCulloch stated that they added an accessory .023" fire screen to meet fire regulations.

 

[Rounder]

Every time I had my saws checked for screens, they used a wire gauge to check for the correct mesh. It was poked through the exit, if the gauge didn't go through the mesh, you were good. I often had to trade out the perforated baffle on McCullochs for a screen, didn't go with both.

Whenever I'm asked to show my screen for inspection, I point at the tip of my bar and say "It's right there". Kind of scary how often it works...

 

[mtngun]

Every time I had my saws checked for screens, they used a wire gauge to check for the correct mesh. It was poked through the exit, if the gauge didn't go through the mesh, you were good.

That's good to know, RandyMac.

The only reason I'd ever be inspected would be if I were accused of starting a wildfire. They take that seriously up here, and will sue and/or prosecute the person who started the fire, if they can prove it.

I don't think I'd ever be stupid enough to start a fire and then walk away from it, but I like the idea of having screens just in case.

 

[RandyMac]

The old saws that exited to the ground could be more dangerous than the front exit, maybe that is why I got tagged so often. That and I tend to bring the anal attitude out of the officials.

 

[Rounder]

The old saws that exited to the ground could be more dangerous than the front exit, maybe that is why I got tagged so often. That and I tend to bring the anal attitude out of the officials.

Those federal LEO's seem to be rather lacking in the humor department, don't they?

To get back on topic, most modern saws I have run stock seem to run much better without the screen. Very fine mesh. My cutting pard's 441 was a pos until he took the screen out........still a pos, but a good bit better, lol - Sam

 

[Justsaws]

Order the proper screen in large sheet.

Remove front cover.

Trace cover on top of screen sheet.

Cut screen a bit larger than tracing so the screen can be fitted between muffler and front cover.

Make needed changes to front cover.

 

[Team FAST]

Check either my String trimmer mod thread

http://www.arboristsite.com/chainsaw/189994.htm

Or my Echo CS 400 Muffler Mod thread.

http://www.arboristsite.com/chainsaw/129928.htm

I flowbench the mufflers with and with arrestor screens and you can see exactly what the measured flow restriction due to the screen is in each application.

 

[nmurph]

Why don't you give us a thumbnail of your findings? Save me the reading on Echo strimmers.:hmm3grin2orange:

 

[Chris-PA]

I don't understand why some saws use that small screen perched over the outlet. With my Poulans the screen is wrapped loosely around the internal baffles - sound like some Stihls may the that way too - giving access to much more screen area. Subjectively I have not noticed any difference having them on, but there probably is some.

 

[Chris-PA]

I was going out to do a lot of cutting on the oaks that fell in October and I had the clutch cover off the McCulloch 3420 (basically a PM605) to clean it out and sharpen the chain. That meant I was looking at the screen on the exhaust - it's a small domed screen on the outlet, bigger in area than a lot I've seen on newer saws, but still much less area than my Poulans.

This is a 55cc saw and I run a 24" bar with 72LGX chisel - it cuts very well but can use all the power it can get, so I decided to try it without the screen. It was a noticeable difference, both in sound and in power. Now this was a very subjective test, and the chain was freshly sharpened and the wood was clean, but it really cut fast and held the rpms well.

Until the ignition module took a dump, that is. Oh well, I don't think I can blame that on the screen!