So I don’t have a tachometer, but I do have a guitar tuner, and I need to adjust the idle speed on my scooter. The engine is a single-cylinder 50cc, and the idle speed needs to be around 2,000 RPM +/-100. I have this crazy idea that if I figured out which notes correspond to 2,000 hertz, I can use a guitar tuner in lieu of a tach.
Would this work, or am I forgetting something really asinine here?
Sure…Guitar tuners, AM radios tuned to static. All kinds of things work as long as they can somehow measure the changes, there’s even an iPhone app for it http://tunelab-world.com/tachometer.html
Might just be easier to buy a laser tach (the cheapest are like 20 bucks) or ask the local garage if you can borrow theirs.
It appears you’re forgetting that hertz are cycles per second. 2000 rpm = 33.3 rps. I’m pretty sure that’s below the range of guitar tuners.
I doubt it would actually work. Even ignoring the important detail that 33 Hz is a good bit lower than a tuner would expect (low E is 82.4 Hz), tuners aren’t precision instruments. As they must response to a range of frequencies, they aren’t band-pass filtered, which is fine when there is a loud vibrating string (acoustic) or a clean input signal (electric) feeding the tuner. If you’ve ever tried to use a tuner on an acoustic guitar in a noisy environment, you know it can freak out and given useless information. I suspect that this would be the case with your scooter.
If I had to do this MacGyver style, I would find a piano (possibly a fake online one) and try to match the left-most C on the keyboard. You’d need an okay ear, but not a spectacular one, to get to with the 5% you need. Alternatively, you could play a C on a guitar and tune to that – it would just be a different octave.
2,000 RPM IDLE? :dubious:
You sure about that number? I have never seen a gas engine that idles that high, from the itty tiny motor on my string trimmer to any car truck engine you can name.
With experience you can set idle by ear. After all of my years of working on cars, I can usually gauge the idle speed on a car to within 50 RPM, many times within 25.
Oh, so I forgot to convert minutes to seconds. D’oh!
It’s a 50cc scooter. A 2003 Honda Ruckus, to be exact. That little engine idles that high, and redlines around 8,500 RPM.
The low E on a bass would be 41.2 Hz, right? So it might be that a lot of tuners out there handle this.
Yeah lots of tuners do bass and guitar. They also need to be able to handle a range either side of the normal highest and lowest notes to cope with people using alternate tunings, drop D being a typical one where the low E is tuned down to D.
If it’s a two-stroke engine (one combustion event per crankshaft revolution), this is correct. This is probably the case if it’s a <50cc scooter; for something larger (like a Burgman), the engine is likely a four-stroke (one combustion event per two crank revs), and so the target frequency is lower by half.
Either frequency might be too low for a tuner, but you could find an appropriate multiple (octave) and hit that note instead, and tune your idle speed to match.
Start with A440. Cut the frequency in half for each octave down. so 220 Hz is one octave lower; 110 Hz is two octaves lower; 55 Hz is three octaves lower. Can’t go one octave lower, so now we have to go down by chromatic half-steps. 12 half-steps in an octave, so for each half-step down, you divide the frequency by 2^(1/12). So
A55: 55 Hz
G#: 51.91 Hz
G: 48.99 Hz
F#: 46.25 Hz
F: 43.65 Hz
E: 41.2 Hz
D# 38.89 Hz
D: 36.71 Hz
C#: 34.65 Hz
C: 32.7 Hz
SO, you’re looking for a note somewhere between C and C#. If you have a bass guitar or a piano you can hit that low frequency exactly, bit that may be inconvenient out in the garage. Instead, hit a note a few octaves higher on a more portable instrument (trumpet, harmonica, etc.), and let that be your reference.
Let’s back up a step. I don’t think there’s any way on God’s green earth that a guitar tuner, piano, tone generator, or anything similar is going to be helpful here regardless of its range.
Check out the link in GHO57’s post (#2). The description there says “The RPM value is calculated from the peaks in the frequency spectrum.” That’s not the same as sensing a single tone.
Engines make a bunch of noise, and it’s at numerous frequencies. I have doubts that a tuner would register a note, and even if it did how you know that the note registered corresponds to what you’re trying to measure? If you’ve ever tried to use a (non clip-on) tuner in a noisy room, you’ve seen that it’s overwhelmed by the audial chaos. And while a clip-on tuner will get a strong signal from the body of the guitar while not sensing the ambient noise, clipping on to an engine means it’s being directly fed all that unwanted noise. I’d be very surprised if a tuner was useful in this situation.
Exactly what I was thinking Gary. Now, what we do know is that little scooter engines are kind of loud, and they do seem to emit a tone of sorts as they rev up and down (picture a little kid making motorcycle noises). If you knew what tone the thing overall should be making at your desired idle speed, you might be able to get it reasonably close with something like a normal tuner; but I think it would be significant to note that the sound is more a function of the exhaust system and other components rather than the rotation speed of the engine itself. The rotation frequency of the engine is not the same as the frequency of the vibrations in air that the engine as a whole generates. And as Gary T noted, even if the tuner did register a note the sound of the exhaust for some other reason may not properly correspond to engine speed. And even if it did, you’d still have to have a measurement of the pitch of a properly tuned one because that’s what you’d have to match rather than the rotation frequency of the engine.
The exhaust system may alter the timbre of the sound by attenuating the amplitude of specific harmonics, but it does not alter the frequencies themselves. For example, if the engine is spitting out 33.3 combustion events per second, you will not hear the exhaust vibrating at 38 or 40 Hz.
Well a guitar doesn’t make a single note either. Pick the A string and there’ll be all sorts of harmonics in addition to the primary note. The question with an engine is whether the primary engine note is strong enough for the tuner to work with. We’ll never know until someone tries it.
I was vaguely envisioning picking up the spark electromagenetically rather than acoustically.