# What RPM tachometer should I buy



## unknown1 (Dec 13, 2015)

I am thinking of getting a tachometer so that I can test engine RPMs.

I don't mean a permanently fixed unit for measuring hours or for constantly monitoring RPM.... I just mean a test unit for diagnosing how an engine is operating from time to time.

Can you offer any low-cost but reliable suggestions? Preferably with a link.

I believe there are some where you hook a wire close to the spark cable and others that are non-contact laser point-and-click.

I am looking for reliability, longevity and accuracy over convenience or gimmickry. I don't mind wrapping a wire if that's what it takes to get the job done and if those models are simpler and therefore more reliable (I'm guessing they might be).

Any thoughts?


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## Kiss4aFrog (Nov 3, 2013)

If you have a smart phone they have apps for that.

Download and view Acoustic Tachometer (RPM) for Android | AppDownloader

Harbor freight: LCD Automotive Multimeter with Tachometer Kit

Digital Photo Sensor Tachometer


inductive timing light | eBay


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## Spectrum (Jan 6, 2013)

I just use a Tiny Tach and wrap it around the spark plug wire as you describe and clip it. It looks like the models have been updated since I got mine. It is tach only, most seem to include hours too now. I forget how long the battery is supposed to last, I better test it!


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## unknown1 (Dec 13, 2015)

Kiss4aFrog said:


> If you have a smart phone they have apps for that.
> 
> Download and view Acoustic Tachometer (RPM) for Android | AppDownloader
> 
> ...


Thanks so much! Wow a smart app.... interesting.... probably does some kind of fourier analysis of the sound and reports peak frequency.

Hmmm. I guess you get what you pay for (sometimes). Do you have any opinion on this much cheaper thing I found on eBay? I don't really need the multimeter capabilities (although that's nice) I already have a multimeter.

New Digital Laser Photo Tachometer Tach Meter Motor Speed Gauge 2 5 99 999rpm US | eBay


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## dbert (Aug 25, 2013)

That should work if you have something to point it at. A small piece of reflective tape is installed and it times the reflections. With the belt cover off and a piece of tape on the edge of the pulley say.


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## unknown1 (Dec 13, 2015)

dbert said:


> That should work if you have something to point it at. A small piece of reflective tape is installed and it times the reflections. With the belt cover off and a piece of tape on the edge of the pulley say.


Oh I see... that's not very convenient then is it? Mirrors on pulleys and taking off the belt cover.... I assumed you could just point it at the engine and it would detect micro-vibrations and report that as a frequency. (I guess that could work).

So in that case, I am thinking that the wired version seems much easier.
I assume you just hook that clamp thing around the spark cable and it uses electromagnetic induction to figure out the number of sparks per minute.

If that's true I think I'll go with the wired clamp thing.

I'm sure the user manual will explain all this but.... you probably have to multiply the number of sparks per minute by 2 to get RPM ... right?
If that's true.. hopefully those meters have a selector where you tell it if you have a 2-stroke or 4-stroke engine. They must do.


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## unknown1 (Dec 13, 2015)

Just found the manual... it says it's for multi-cylinder engines (3 and up)
That means having to do some arithmetic to get the correct value for a single cylinder 4-stroke snowblower engine.
Also, it's probably different arithmetic for a 2-stroke single cylinder. (Additional factor of 2).

http://manuals.harborfreight.com/manuals/95000-95999/95670.pdf

In fact, I just found a thread discussing these issues..
http://www.diygokarts.com/vb/showthread.php?t=10545

*So it sounds like Tiny Tach is the only one that "just works" out of the box without mental arithmetic.*
So I think that's my answer!

Tiny Tach TT2A $41.99 here:
http://www.ebay.com/itm/Tiny-Tach-H...846883?hash=item4627776563:g:n0AAAOSwgQ9VlV~M

Tiny Tach TT2A Manual:
http://www.papteam.com/PDFs/TT2A.pdf


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## classiccat (Mar 1, 2014)

Hardline makes a similar tach. Wrap the plug wire & fire away.


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## ELaw (Feb 4, 2015)

stuart80112 said:


> Just found the manual... it says it's for multi-cylinder engines (3 and up)
> That means having to do some arithmetic to get the correct value for a single cylinder 4-stroke snowblower engine.
> Also, it's probably different arithmetic for a 2-stroke single cylinder. (Additional factor of 2).
> 
> http://manuals.harborfreight.com/manuals/95000-95999/95670.pdf


Yeah, the design of that meter is rather silly.

It's apparently designed to measure the pulses from the coil in a coil + distributor setup as found on (older) automotive and similar engines. For a fixed tach like the one in the dashboard that makes sense as it's driven off the coil primary. But for a portable tach that clamps onto a high-tension wire, it would have been much easier to skip the "number of cylinders" selection and associated circuitry, and just have you clamp it onto a wire going to one cylinder. Then the only variation is between 4-stroke which fires once per every two revolutions and 2-stroke which fires once per revolution.

With OPE (outdoor power equipment) engines it's even easier, as both 4-strokes and 2-strokes fire every revolution. So a tach that's designed for such an application and reads the signal from the spark plug wire should never require any math.


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## Kiss4aFrog (Nov 3, 2013)

Instead of spending 42 bucks on a "Tiny Tach" why not just buy the generic tach/hour-meters that you install the same way for $10 on Ebay ??
And when I say install I'm talking about wrapping it's lead around the spark plug wire but not bolting it to the machine so you can use it on different machines as a portable tach.










It's not as handy as an inductive pickup meter but it functions the same as the Tiny Tach for 30 bucks less.


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## ELaw (Feb 4, 2015)

^ ^ I'm with this guy!

I've got one of those meters and it works just fine, and cost under $10 if I recall correctly.

As with many Chinese products the documentation is laughable, but if you randomly press the buttons until it displays "RPM" and a plausible number*, it functions well. And is dirt cheap!

*Seriously, one of the buttons switches between "2 stroke" and "4 stroke" mode. With engines like ours that fire on every revolution, one of those modes will display the correct RPM and the other will display 1/2 the correct RPM. So press the button until you see the higher number and that's the correct RPM.


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## unknown1 (Dec 13, 2015)

Kiss4aFrog said:


> Instead of spending 42 bucks on a "Tiny Tach" why not just buy the generic tach/hour-meters that you install the same way for $10 on Ebay ??
> And when I say install I'm talking about wrapping it's lead around the spark plug wire but not bolting it to the machine so you can use it on different machines as a portable tach.
> 
> 
> It's not as handy as an inductive pickup meter but it functions the same as the Tiny Tach for 30 bucks less.


Thank you... I managed to cancel my Tiny Tach order and saved $30

Looks like they are..

$7.14 shipped from Hong Kong...

Digital Engine Tach Tachometer Hour Meter Inductive for Motorcycle Motor FC0 | eBay

$9.35 shipped from California...

Digital Tach Hour Meter Tachometer Gauge Spark Plugs Gas Engine Motocycle ATV | eBay

I'm guessing they probably both source them from the same supplier in Asia..... It's just that one will arrive a little quicker.


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## Waterlooboy2hp (Nov 29, 2011)

I got the same tach from China. Found that the wire on it, does not wrap very well, around the plug wire.

I took a length solid 14 gauge wire, out of some Romex . Wrapped 4 turns around the plug wire and stripped about 1/4" of insolation off one end of the 4 turn coil. It stays on the plug wire full time. ---- Then put a small gator clip, on the end of the tach wire. You can then just clip to the 4 loop coil on each of you engines. -----John 
==========================================



And when I say install I'm talking about wrapping it's lead around the spark plug wire but not bolting it to the machine so you can use it on different machines as a portable tach.
[IMG said:


> https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQm5oggplRy8Pkizr4lvhKTTK5fgdZktwXeZJh9nCBK_PycXq4g[/IMG]


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## unknown1 (Dec 13, 2015)

ELaw said:


> Yeah, the design of that meter is rather silly.
> 
> With OPE (outdoor power equipment) engines it's even easier, as both 4-strokes and 2-strokes fire every revolution. So a tach that's designed for such an application and reads the signal from the spark plug wire should never require any math.


There's so much I have to learn as a newbie!
A couple of basic questions here...

I'm a bit confused when you say OPE engines fire every revolution.

I thought the classic 4-stroke engine was..

1) Intake
2) Compression
<<<< SPARK >>>
3) Power
4) Exhaust

....That would be one spark for every 2 revolutions

Are you saying that OPE engines do this...

1) Intake
2) Compression
<<<< SPARK >>>
3) Power
4) Exhaust
<<<< ANOTHER SPARK >>>>

... if so.. what's the second spark for?


Also... I was looking at the rockers yesterday to adjust clearances.
That was the first time ever looking hands-on inside an engine for me!

As I cranked the engine by hand it looked like the valves were doing this..

1) Right hand valve fully raised then closes (assume this is intake)
2) Left hand valve partial raised then closes
3) Left hand valve fully raised then closes

What's happening with the double raise on (what seems to be) the exhaust? I don't think I imagined it.

Does anyone know a good introductory web reference that describes these "double sparks" and "double exhausts"? I'm intrigued.


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## ELaw (Feb 4, 2015)

stuart80112 said:


> I thought the classic 4-stroke engine was..
> 
> 1) Intake
> 2) Compression
> ...


Yep!



stuart80112 said:


> ... if so.. what's the second spark for?


For no reason other than ease of design. The magneto/points or ignition module are driven from the flywheel, which of course turns with the crankshaft and thus makes two revolutions and thus two sparks per cycle. The spark between compression and power is used to ignite the mixture, the other one does nothing. The term for this is "wasted spark".

It's really just done because it's more convenient to mount the ignition components on the flywheel rather than the camshaft which would yield one spark per cycle. One problem being that the magnet must move past the magneto fairly fast, and the flywheel's large diameter provides that needed velocity easily. Also the flywheel is mounted on the outside of the engine which is an easier (and more service-friendly) place to put the magneto than inside the engine.


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## unknown1 (Dec 13, 2015)

ELaw said:


> ...For no reason other than ease of design....


Ah of course... I get it now... and that's why the Tacho RPM calculation doesn't need the extra factor of 2...... Thanks!

Any thoughts on the "double valve" motion I think I saw?
I've been looking at web images of a cam shaft to see if I can spot a "double bulge" on the exhaust cam... I'm not convinced I see it.


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## ELaw (Feb 4, 2015)

stuart80112 said:


> Any thoughts on the "double valve" motion I think I saw?
> I've been looking at web images of a cam shaft to see if I can spot a "double bulge" on the exhaust cam... I'm not convinced I see it.


I'm... not sure!

Some snowblower engines have a "compression release" which is a device that I think kicks the exhaust valve open momentarily during the compression stroke when the engine is at very low RPM. The idea being to release some of the pressure generated during the compression stroke while you're pulling the engine to start it, so it's easier to pull over.

But I'm not sure that quite fits with the sequence you saw?

Edit: I found a youtube video that illustrates this pretty well:


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## unknown1 (Dec 13, 2015)

ELaw said:


> I'm... not sure!
> 
> Some snowblower engines have a "compression release" .......
> 
> Edit: I found a youtube video that illustrates this pretty well: https://www.youtube.com/watch?v=9Yjudea7tmw



That's exactly what I was seeing! Thanks... I wasn't hallucinating after all!

That may also explain what the Canadian-cemetery-guy was struggling to understand here on this video at 11:04
Comments under that video said it was a low RPM counterweight... but that may be the same thing.... perhaps (??)
Low RPM you want the compression release to work... High RPM you don't.
My brain is slowly beginning to understand this stuff.

https://youtu.be/CP6cNHrFyAg?t=664


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## unknown1 (Dec 13, 2015)

OMG.. I see I'm now a "senior member"
Must be because of 100 posts.
Hahahaha... There's NO WAY I'm anything but a newbie!!
I just ramble on a lot and ask lots of questions.... ;-)


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## bwdbrn1 (Nov 24, 2010)

Hey, being a senior gets you a discount at the SBF cafeteria!


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## Grunt (Nov 11, 2013)

bwdbrn1 said:


> Hey, being a senior gets you a discount at the SBF cafeteria!


I don't think so-


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## unknown1 (Dec 13, 2015)

Kiss4aFrog said:


> If you have a smart phone they have apps for that.
> Download and view Acoustic Tachometer (RPM) for Android | AppDownloader



Holy Moly! I thought about playing with the smartphone app but rapidly decided against it. This video shows what it wants to do on installation..

Download Acoustic Tachometer (RPM) APK | AppDownloader

It basically wants to have total control of your phone! 

Why??? All it needs is to record sound from the microphone... why the heck does it need to add accounts and see your location,access WiFi etc etc etc.
Maybe the developer just took the "easy option" ...or maybe something a bit more sinister.... or maybe I'm paranoid.

Use it at your own risk.. and avoid internet banking on that phone!


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## Kiss4aFrog (Nov 3, 2013)

EVERYTHING you do on your phone or computer you're doing at your own risk. Listening to any of us you do at your own risk and that might be even scarier !!










When you talk about the "classic" 4 cycle that's what it is.
The second spark isn't used for anything it's just there because it's easier than trying to prevent it and since you're coming up on exhaust when it's firing a bit before TDC it's not doing anything to the incoming "intake" fuel air mixture. Almost all new distributor-less vehicles run this way. Since the late 80's I think.


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## ELaw (Feb 4, 2015)

Kiss4aFrog said:


> Almost all new distributor-less vehicles run this way. Since the late 80's I think.


Actually that was true a while back, but the latest coolness is "coil on plug" - a small coil sitting on top of each spark plug. Or near the plug, as on the GM LS series engines.

You also won't see wasted-spark used on engines with an odd number of cylinders, like my 5-cylinder Audi. It's very difficult to fire cylinders in pairs on such an engine... :icon_whistling:


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