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No Tach; How can you check rpm's?
Topic Started: Jul 10 2016, 11:46 AM (716 Views)
freegeo
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If you own a car with no tach and are wondering how to check the rpm of the engine, you may want to get one of these. It is a digital laser photo tachometer. Model DT-2234C. Here is a picture of what it looks like. They are available on ebay and other websites. They cost around $10

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Here is the basics of how to use it.

The tach does not come with a 9 volt battery so you will have to get one and install it before it can be used.

Included are some strips of reflective tape. Cut off about a 1/2" long piece. Peel off the back of it to expose the adhesive. Place the reflective tape to the crank pulley(or whatever rotating component you want to check) securely. I just used the inside lip of the pulley for the A/C belt. Just make sure where ever you put it the laser can be aimed at it to take a reading.

It will look like this once applied.

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Start the engine. Press and hold the TEST button on the digital laser photo tachometer and a red laser light will appear. Aim the laser at the reflective tape and hold it steady on that spot. The digital laser photo tachometer will display the rpm number. If you miss what the reading was. Once done press and hold the MEM button on the digital laser photo tachometer and it will display the value.

I checked the digital laser photo tachometer I got while cranking the engine over and recorded the cranking rpm's with my oscilloscope. The results where impressive.


Reading on the digital laser photo tachometer

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Oscilloscope results. 50 CMP signals / 3 = 16.67 camshaft rotations x 2 (the cam rotates at 1/2 the speed of the crank) = 33.34 x 12 (the 50 cmp signals was in 5 seconds) = 400.08 rpm.

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MarkyMayhem


400 RPM? whatchoo doing that slow?
Cool tool!
-M
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myredvert
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myredvert

Quote:
 
I checked the digital laser photo tachometer I got while cranking the engine over and recorded the cranking rpm's with my oscilloscope.
Quote:
 
400 RPM? whatchoo doing that slow?
I think he was cranking the engine over and measuring cranking rpms? :whistle

I have used mine to measure the rpms of our weedwhacker, chainsaw, tractor engine, and pretty much anything that rotates. These things are so cool I'm thinking about putting a strip of reflective on a turd and measuring the rpm of a toilet flush. :D
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suzukitom
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Tom

myredvert
Nov 17 2016, 12:54 PM


I have used mine to measure the rpms of our weedwhacker, chainsaw, tractor engine, and pretty much anything that rotates. These things are so cool I'm thinking about putting a strip of reflective on a turd and measuring the rpm of a toilet flush. :D


Make sure to have your turd balanced and blueprinted before you do this test. :shit
Edited by suzukitom, Nov 17 2016, 01:14 PM.
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GTluver
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The MN welder.

myredvert
Nov 17 2016, 12:54 PM

These things are so cool I'm thinking about putting a strip of reflective on a turd and measuring the rpm of a toilet flush. :D
:rofl :rofl :rofl
Edited by GTluver, Nov 17 2016, 01:18 PM.
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1DCGUY
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Don't be a "Richard"

suzukitom
Nov 17 2016, 01:13 PM
Make sure to have your turd balanced and blueprinted before you do this test. :shit
I'm not getting anywhere near that Geo Glenn spit shine!!! :O :O

:rofl :rofl :rofl :rofl :rofl :rofl :rofl
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GTluver
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The MN welder.

1DCGUY
Nov 17 2016, 06:05 PM
suzukitom
Nov 17 2016, 01:13 PM
Make sure to have your turd balanced and blueprinted before you do this test. :shit
I'm not getting anywhere near that Geo Glenn spit shine!!! :O :O

:rofl :rofl :rofl :rofl :rofl :rofl :rofl
According to this you can't get enough of it LOL!! :D :D :D
Spit Shine video!!
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Murf 59
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Glow shift sells a great little tach that works with our G10 engines for about $45
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cwatkin


I think you mean "Geo Jeff spit shine."

Either way this is cool and could apply to all kinds of things such as lawnmower RPM, etc.

Conor
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freegeo
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Murf 59
Nov 17 2016, 07:51 PM
Glow shift sells a great little tach that works with our G10 engines for about $45
I tired a glow shift oil temp gauge and it wasn't very accurate at all. Not sure if the tach would be much better.

While the hand held unit can't be used while driving for $10 you can use it on a lot of other stuff.
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myredvert
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myredvert

Quote:
 
Glow shift sells a great little tach that works with our G10 engines for about $45
Quote:
 
I tired a glow shift oil temp gauge and it wasn't very accurate at all. Not sure if the tach would be much better.
The key is understanding the difference between a calibrated reading and an indication that can be (and often is) affected by both sensor errors (e.g., overall accuracy and lag) and gauge errors. The Glowshift products, like most affordable gauges typically don't use the most accurate type of sensors ($$) and can have indications that shouldn't be trusted for anything except the most general observations.

In the case of the glowshift oil/coolant temperature gauge freegeo mentioned, the error was not only considerable, it was the worst error you can have, i.e., the indicated temperature was as much as 20F lower than actual at the high end of the operating ranges. To put it simply, what you see (indication) is not necessarily what you got (calibrated).

For rpm measurement these inexpensive and extremely accurate handheld tachometers are fantastic to use when it comes to any process that involves verifying or setting idle speed, and also would allow someone with an aftermarket tachometer installed to check its accuracy across its indicating range.

Another option I recently found that has the ability to directly and easily measure rpm as well as having several other features that would allow measuring some more detailed engine operating parameters is the Innova 3340 digital multi meter. It's a bit pricey, but for roughly $80 or so (on Amazon) you can get an instrument that can perform the functions of quite a few different instruments combined into one.

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I can't wait til Christmas. :D
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