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General Information => General => Topic started by: TylerDurden on May 17, 2014, 05:37:17 pm

Title: Validate a tachometer acoustically (?)
Post by: TylerDurden on May 17, 2014, 05:37:17 pm

All this chatter about trannys and rpm got me wondering if my stock tach was accurate. After all, it is driven by the W-terminal of the alternator and a v-belt.


Pulling a page from Mark the Limey's playbook, I recorded the engine with my digital camera and brought the sound files into Audacity, a free sound editing program.

(https://lh3.googleusercontent.com/-hPOBR_CPxKQ/U3fuACLrf6I/AAAAAAAABeI/-yqSlsBi80o/s800/TachAudio.png)

As you can see, the clacks are .010sec apart, and I measure every other clack.

1 / .02 = 50 (rps)
50 * 60 = 3000 (rpm)

That's exactly what my tach was reading during the above recording.

Ok, you might wonder by looking at the waveform, why I'm skipping a clack... well, the engine fires every 180 degrees.

I checked at 1000rpm, 2000rpm and 3000rpm, the recording numbers pretty much add up to what the tach reads:  .061s, .030s, .020s.

So, it seems my stock tach is fairly accurate.

Title: Re: Validate a tachometer acoustically (?)
Post by: ORCoaster on May 17, 2014, 09:30:04 pm

I love that program.  Good to see it used in a very different validation method.  NICELY thought out and very creative.  Kudos.

Title: Re: Validate a tachometer acoustically
Post by: TylerDurden on May 19, 2014, 05:24:23 am

I rekon most folks with smartphones can easily do this with an audio app. (I have a stupidphone, so an extra step with Audacity. )

Title: Re: Validate a tachometer acoustically (?)
Post by: 745 turbogreasel on May 19, 2014, 02:25:05 pm

I'm fine with my $13 optical tach, but it's good to have options.

Title: Re: Validate a tachometer acoustically (?)
Post by: bajacalal on May 19, 2014, 11:18:47 pm

I wonder if somebody could make an app to gauge the rpm of any engine acoustically.

Title: Re: Validate a tachometer acoustically (?)
Post by: ORCoaster on May 20, 2014, 08:05:49 pm

Somehow you need to find a reference point.  The click and clack of injectors might work on about any car but diesel does it best.  That is what he was able to do with the program and then it was math after that. 

Title: Re: Validate a tachometer acoustically (?)
Post by: 745 turbogreasel on May 21, 2014, 12:40:09 am

check the app store for tachometer, there are  a few acoustic apps.

Title: Re: Re: Validate a tachometer acoustically (?)
Post by: TimpanogosSlim on May 21, 2014, 08:56:01 am

I've seen amateur watchmakers measure the beat accuracy (not the same thing as timing by a long shot) of a mechanical watch movement with audacity.


. :Sent by pneumatic tubes

(Edit for explanation: 'beat' accuracy on a mechanical watch is, basically, whether the time between "tick" and "tock" is exactly the same as the time lag to the next "tick". A watch that is out of beat will never run reliably no matter how much you fiddle with it. You need a precise 50/50. 60/40 and it will have issues. 70/30 and it won't even stay running.)