How much does spark screen restrict flow ?

[mtngun]

I bumped into this issue recently when I discovered that removing my 084's spark screen boosted power considerably, even though the screen area was 2 1/2 times the area of the exhaust port.

Turns out there's a formula to calculate % open area of a screen.

My 084 screen is made of 0.018" thick wire, and there are 28 holes per inch.

Mesh spacing = 1"/28 = 0.0357"

% open area = 25 %

So ...... the screen area needs to be 4 times as big as the target area...... and the target area is generally 85% of the exhaust port area.

084 exhaust port area = 616 smm

85% port = 523 smm (and my enlarged muffler exit was about that big )

Required screen area = 4 x 523 smm = 2093 smm

The stock screen is only 1488 smm, when it needed to be 2093 smm.

At least now I know what to aim for when I'm modding a muffler.

 

[Warped5]

OUTSTANDING POST!!!!!!!!!!!!​

A large improvement over the 'eyeball method!'

Repped!

​

 

[Eccentric]

OUTSTANDING POST!!!!!!!!!!!!​

A large improvement over the 'eyeball method!'

Repped!

​

+1. Rep sent for a good thread...

 

[nmurph]

I remember that Brian posted some results where he didn't find any measurable difference with/without firescreen. You know he usually has empirical support for his posts.

 

[RiverRat2]

OUTSTANDING POST!!!!!!!!!!!!​

A large improvement over the 'eyeball method!'

Repped!

​

Yeah,,, He's getting pretty good @ this,,,,,

Way too much time on his hands,,,,

It's beginning to make my brain hurt!!!!

I tried to rep him but it wouldnt let me!!!!

 

[paccity]

thats funny, because about half an hour ago i just got through cleaning up a 288 i just got , was doing some cut and tune and in a cut the saw leaned a little and got louder . the screws that hold the deflector and screen on came out. so ithink it makes a diff.

 

[cowboyvet]

Another great bit of info but is this geared specifically for saws based on the average rpm in order to come up with the percentage?
What about velocity of the air into the equation? I know the faster the air moves through a mesh, the more resistance builds. When air hits the wire mesh it is deflected and makes a "cushion" area. The faster the air flow, the bigger the "cushion" area gets until at some point almost no air gets through the mesh. I'm relating to a study done on the mesh tailgates on trailers but the same physics would need to apply even on the smaller scale of a saw. Easy to prove by hooking up to a trailer with a mesh tailgate and low horse power truck. You will see quickly the faster you go the harder it is to pull.

 

[CJ1]

None, because that is the first thing that falls off of my saws. CJ

 

[mtngun]

I remember that Brian posted some results where he didn't find any measurable difference with/without firescreen. You know he usually has empirical support for his posts.

That's right, TW didn't see a restriction on a flow bench with an internal Stihl screen that was bigger than the exit hole. Don't remember the relative sizes of screen vs. exit hole in his test. Would love to hear what he thinks about this.

In stock form, my 084 screen was at least 4X bigger than the tiny stock exit hole, so the screen was not a bottleneck then. After the exit hole was modded, then the screen became the bottleneck.

I have yet to measure any screens other than the 084's, so different brands and models may have different % open area. You can get screens that flow 40% - 60%, but they will have either thinner wire or bigger holes. McMaster-Carr lists the % open for the screens they sell.

 

[sefh3]

Yeah,,, He's getting pretty good @ this,,,,,

Way too much time on his hands,,,,

It's beginning to make my brain hurt!!!!

I tried to rep him but it wouldnt let me!!!!

I got him for ya!!!!

 

[CedarRock]

I filed each wire to get a thinner diameter on the inside where the air flows so each wire is actually tapered. This will allow the air to flow more smoothly and create a back-flow turbulence causing a negative atmospheric effect and therefore pull more air than is coming out and thus more power. (just kidding)

 

[mtngun]

Another great bit of info but is this geared specifically for saws based on the average rpm in order to come up with the percentage?

No. The 85% of exhaust port area rule of thumb has been around for quite a while. TW did an excellent thread on it, trying different exit hole sizes, and testing the cut speed for each. His test pretty much confirmed the 85% rule.

What about velocity of the air into the equation?

I wondered the same thing myself. TW has done some flow bench testing on mufflers that suggested only the total open area matters, but ....... I'm not sure if the air velocity on his flow bench is comparable to actual exhaust flows ?

To prove it for certain, you'd either have to do a lot of flow bench testing or else a lot of dyno testing. Speed testing in wood has a lot of variables that make it difficult to verify small changes in power.

 

[tdi-rick]

Thanks mtngun, knew there had to be a formula somewhere

 

[rattler362]

I feel they do restrict a bit that is why i take it out first thing and i dont cut on public land.On a couple of saw that i MM i put a screen back just to keep trash and critter,s out.

Mike

 

[cowboyvet]

I wondered the same thing myself. TW has done some flow bench testing on mufflers that suggested only the total open area matters, but ....... I'm not sure if the air velocity on his flow bench is comparable to actual exhaust flows ?

To prove it for certain, you'd either have to do a lot of flow bench testing or else a lot of dyno testing. Speed testing in wood has a lot of variables that make it difficult to verify small changes in power.

Just throwing thoughts out there. If you had a mm with a short pipe, you might be able to use a wind meter to measure exhaust velocity with and without a screen installed (saw hooked to tach). If your screen was big enough, the velocities should be the same both ways...
egt's might be hard on the veins of a wind meter though.

 

[Stihlman441]

So by only taking screens out we will gain flow,if you take the screen out of you 084 what in terms of opening the muff port size does this relate to.
Take screen out = what % in port size opening.

Cheers

 

[mtngun]

Take screen out = what % in port size opening.

If the screen is pressed tight against the exit hole, then taking the screen out will increase the effective exit size 4X.

As you know, Stihl mufflers use an internal screen that is much bigger than the exit hole, so there is no straightforward calculation. The exit hole should still be 85% of exhaust port, and the screen should have at least the same equivalent open area as the exit hole.

Getting back to my builds....... the 084 muffler exit needs to be 523 sq mm, about like a 23mm x 23mm rectangle. The modded single port exit is in that neighborhood. :msp_thumbup:

The screen, if it is the Stihl type that only flows 25%, would need to be 4X the exit hole or 2092 sq mm. That would take a 45mm x 45mm rectangle, and there's just no way, even if you added a second port. You'd have to make the entire front cover one giant screened exit hole ! ! !

My Oly 980 build faces the same problem -- no way to make it big enough for that darned 25% Stihl screen.

Since I cut on public lands in wildfire central, I prefer to have a screen, even if it is just a token screen to cover my behind. I'm thinking to order a coarse screen that flows 50%, like #85385T845 at McMaster-Carr. That should meet my flow goals and still give the appearance of a legal spark arrestor.

 

[nmurph]

That's right, TW didn't see a restriction on a flow bench with an internal Stihl screen that was bigger than the exit hole. Don't remember the relative sizes of screen vs. exit hole in his test. Would love to hear what he thinks about this.

In stock form, my 084 screen was at least 4X bigger than the tiny stock exit hole, so the screen was not a bottleneck then. After the exit hole was modded, then the screen became the bottleneck.

I have yet to measure any screens other than the 084's, so different brands and models may have different % open area. You can get screens that flow 40% - 60%, but they will have either thinner wire or bigger holes. McMaster-Carr lists the % open for the screens they sell.

I couldn't find the thread this afternoon; was is here or elsewhere?

 

[mtngun]

I couldn't find the thread this afternoon; was is here or elsewhere?

http://www.arboristsite.com/chainsaw/154660-12.htm

I could spend all day reading TW's old posts.

 

[tdi-rick]

So by only taking screens out we will gain flow,if you take the screen out of you 084 what in terms of opening the muff port size does this relate to.
Take screen out = what % in port size opening.

Cheers

The first thing I did when I bought the Makita years ago was ditch the screen after roughly calculating the open area. (and I'm in the Rural Fire Service)