Free chainsaw tach

[Badger]

Hey guys,

Just upgraded from my Poulan Pro 295 to an Echo 590. I reading this forums and others I became parinoid about it coming from the factory a lean and burning out they cylinder. Furthermore, the Poulan ran much slower so to my ear it sounds like it is over reving at WOT.

I was going to buy a tach so I can check the speed of the motor to ensure I am not wrecking my saw until I though of a different method. I took about half the necessary coursework for my Masters in Acoustical Engineering. I guessed that majority of the noise would be higher frequency and the engine frequency, which is directly proportional to engine RPM would be easily to find on an FFT plot.

Using my IPhone and the iAnalyzerlite app, I took my chainsaw out for a run. Sucess

You can see that at WOT I am measuring 182.2 Hz... Which multiplied by 60 (to get RPM's) equals roughly 10,900 RPM. Still think I want to check it against actual measurements just to be sure, but I'm 99% confident in my reading.

Better yet, I now know I'm running too rich as recommended RPM is 12,500 (I think... The manual is downstairs and I'm not going all the way down there)

 

[pen]

That's very cool. Especially as a guy who pushes science.

That said, I'd either love to run your saw myself to verify, or else listen to / watch a vid of it going

 

[D8Chumley]

105 dB, maybe I should be wearing my ear plugs when I run mine

 

[Knots]

105 dB, maybe I should be wearing my ear plugs when I run mine

Maybe? As a guy who hears "wheeeeeeeeeeeeee" all the time, I'd highly suggest it. Kinda like how they "suggest" that you pull the ripcord when you jump out of the aircraft...

 

[D8Chumley]

I should know better I'm a heavy equipment operator and even though most of our stuff has cabs I should still wear them. What's sad is I had a pair around my neck tucked under my hoodie but got busy and just didn't use them.

I have tinnitus in my left ear if that's what you are referring to with the "weeeeee". My wife refuses to believe me but I only really hear it when it's quiet around me

 

[Knots]

Yeah - I was talking about tinnitus. Between motorcycles, factories, aircraft, and loud concerts I lost quite a bit. I spend my time now trying to salvage what hearing I have.

Mine is there all the time if I think about it. When it's quiet it's impossible to ignore. In a setting where a lot of people are talking, I have difficulty understanding people close to me. From what people who have it worse than me say, it can really drive you nuts.

 

[D8Chumley]

Mine is there all the time if I think about it. When it's quiet it's impossible to ignore. In a setting where a lot of people are talking, I have difficulty understanding people close to me. From what people who have it worse than me say, it can really drive you nuts.

Exactly my issues!! Maybe she does believe me and is just yanking my chain haha. I have to work today, running a big hoe with hydro hammer (349 Cat) so I think I might just make sure I have them in my ears

 

[Knots]

The wind can mess with me too. Probably from motorcycles and driving cars cross-country with the windows down. Hearing loss is often about the duration of the noise - not necessarily the magnitude (dB).

Sorry for the thread hijack - I appreciate your ingenuity in approaching your rpm measurement.

 

[Fifelaker]

I always tune by ear. Make the high side rich enough to hear it "Miss, four stroke, burble" out of the wood free reving and clean up when it is in the wood. While cutting lift the bar and it should instantly four stroke. If you are not sure Madsons has a good wav file to show you what it should sound like. I can hear it better with ear protection on also. All of my saws are muffler moded so tach's are worthless. I have never burned up a saw in 35 years doing it this way. I saved a cylinder on my MS440 once, because I didn't hear it four stroking. I shut it down. When I started looking the impulse line had a pinhole sucking air. I replaced the impluse line, fuel line and carb boot while I was in it. If you become used to the sound they are supposed to make, it can save you a headache later. The only time this method dosen't work is if the coil is rev limited or the carb is governed. Then you either tune with a tach or tune in the wood not free reving. If the saw is modded in any way the tach is worthless again so back to the wood and your ear.

 

[GeHmTS]

How do you know to multiply by 60?

 

[saskwoodburner]

No offense but seems like a s.w.a.g. to me, unless you can show with math that the x 60 works across the whole range in a linear fashion. And even then, you'd need an rpm gauge to prove it. I don't claim to be a genius, or smart even, but some things just can't be figured out by making them overly complicated.

Set your saw to burble at wot, and it won't spin to max rpm, and won't be running lean.

I'm all for discussion, and have never minded being proved wrong, so don't think I'm shouting you down or poo-ing your ideas.

 

[ewdudley]

No offense but seems like a s.w.a.g. to me, unless you can show with math that the x 60 works across the whole range in a linear fashion

Sorry, but your ignorance is in of and of itself offensive.

 

[saskwoodburner]

Sorry, but your ignorance is in of and of itself offensive.

How so? Convince me I'm wrong rather than call me ignorant. I'm not scared to apologize or admit it when proven wrong.

 

[Fifelaker]

How so? Convince me I'm wrong rather than call me ignorant. I'm not scared to apologize or admit it when proven wrong.

Not ignorant by any means! This is the way 2 strokes have been tuned since they were invented. How long have we had tachs that were affordable? Since somebody has made them cheap enough that most anybody can afford one all too many people can't live without one. Now the EPA comes into play and a manufacture has to meet a standard weather that saw is lean or fat. Who cares how long a saw will live being tuned too lean by the affordable tach and an EPA standard.

 

[GeHmTS]

I'm curious as to what is the signal to noise ratio and the dB falloff. To me it seems you would need to use some kind of doppler effect to understand the rpm of the equipment using sound.

 

[saskwoodburner]

I will concede this, there is in fact, something to frequency analysis with engines. That's what google tells me. So while I sit here and eat crow, I say "Sorry for poo poo-ing the idea". In the first place, I understand that things will make a certain frequency at a certain rpm. and it might stand to reason that doubling the speed of the motor would perhaps double the frequency?

What I don't understand now, is how you would know you're using the right frequency? Do you just grab the highest one, and that x 60 and you're there in terms of rpm? Does it take into account any resonant frequencies or exaggerated kind of frequencies?

 

[GeHmTS]

I know that when you're talking about speed of something using sound, you need to consider the doppler effect. For example, the doppler effect is used in calculating blood flow through an artery or vein using ultrasound. You would not be able to calculate blood flow without this principle. That's why I think the OP analysis is somewhat flawed.

However the OP haven't responded yet, so it's hard to understand his argument. At this point, I wouldn't try a method like this to understand rpm unless there is much more information on the technique. It's unclear why 182Hz and 60rpm are used in the calculation.

 

[Oldman47]

How do you know to multiply by 60?

Frequency is expressed using seconds. RPM is minutes. 60 is the ratio. Not trying to be a smart ass, just trying to answer your question.

 

[GeHmTS]

Since I'm a nice guy. Thank you