Analog Tachometer In Testing

GoldenMotor.com

MikeJ

New Member
May 3, 2009
82
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0
Colorado Springs
Hi Everyone -

I think an analog tachometer for a Chinese engine can be done using off-the-shelf parts. I will spare everyone the long, gory details, but the bottom line is you have to use that one controversial component of every Chinese two-stroke...

The White Wire.

That's right, the White Wire. Disregard trying to use the spark plug wire if you want to use an analog tach. You need an analog signal, specifically the sine wave voltage that appears on the white wire. The sine wave frequency and voltage varies with engine RPM. Those are the keys to using an analog tach.

Get the tach of choice at the local auto supply store. The tachometer must say to connect it to the distributor or coil of the car; most tachs do. I purchased one that would look good on a bicycle for about $32 from an auto parts chain store and I was out the door. It even has a light to illuminate the face of the meter for night riding.

The provided diagrams should be fairly self-explanatory. The sine wave voltage coming out of the magneto gets rectified into TWO positive-going pulses. The tach has a selector switch for 8, 6, and 4-cylinder engines. Set the switch to only "4". This allows the tach to sense the pulses fed into it and display RPM directly onto the meter face. There is no other alteration to the purchased tachometer.

The battery is a 3.3 Amp-hour battery from a local hardware store. A charger had to be purchased as well. If you have a 5000 cranking amp 12 volt truck battery, you can use it. The tach draws only 35 milliamps; it won't hurt either battery if wired correctly.

My test frequency of commercial electricity is available at 60 Hz (common knowledge). That is also the same frequency as a Chinese engine magneto output at exactly 3600 rpm. By using this, I proved that the circuit you see worked at this one frequency, simulating an engine spinning at 3600 rpm. The voltage being fed to the rectifier was 10 volts AC, right in the middle of the magnitude range produced by the magneto at operating rpm.

There is still testing to be done, to include with a real engine. Just because it works on a test bench does not guarantee success in the field. But it is a good indication of success.

If you are interested in pursuing this, please do so. Share your findings with the rest of us; I probably will need a few weeks or so of more time for testing.

MikeJ
 

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DuctTapedGoat

Active Member
Dec 20, 2010
1,179
10
38
38
Nampa Idaho
Umm. For a digital analog tach, wouldn't you just hook it up to the sparkplug wire?


Though, when I think "analog", what I'm really thinking is "not digital".

For a cabled tach, I would go off the flower nut (which is threaded in the center) and gear it to match - or off the smaller bevel wheel, and just drill a small hole in the center of the bolt to mount to, which is really the true RPM of the motor.

Or is this not right?

Found em : "Inductive Tachometers" or "Inductive Hour Meter" is what they're called, they do exist, and there's no work to be done, as it just counts a spark as a rotation.

Unless there's a reason that lots of work and more cost trumps a product that's out there, does it, and is cheap.
 

The_Aleman

Active Member
Jul 31, 2008
2,653
4
38
el People's Republik de Kalifornistan
He mentioned the "white wire", which means he's talking about the 2-stroke chinagirl engine.

so did this work??? and are you using a 2 or 4 stroke engine?
I looked into running an analog tach back when I converted my MaB from 2-stroke to 4-stroke. What I found was that Summit sells a few "universal" tachs that work on single cylinder engines, 2- or 4-stroke, that run off an inductive lead from the spark plug wire. Also requires a 12v source. The problem is, they are over $100.

I ended up using a Trail Tech TTO instead.
 
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decoherence

New Member
Aug 23, 2010
476
2
0
sebring,fl
tt151...you can install it in less than 5 minutes.



How would that work? Blowing the wire? laff
you don't hook it up to the blue or white wire.
you hook it to the magic wire.
you can't see it because it is magic but, it's there.
don't worry, just because it is a wire doesn't mean it sends electrical pulses.
i only sends MAGIC!
there is a hack somewhere that shows how to hook the magic wire to a blue tooth. then you can have the RPMs sent to your earpiece.
 

Ernst

New Member
May 28, 2011
363
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0
Turlock Ca
Why? No reason..

I would suspect that we want to measure the cam speed right?

I guess that electronic one where it uses spark timing to determine RPMs is not as exact as the electronic one but isn't it a lot neater?

BTW what kind of Jerk am I ... I would like to be a Soda Jerk actually..

Aleman would be an Ale Jerk I would guess...

Doesn't a scoop of vanilla ice cream floating in an old fashioned hand mixed cherry coke sound good?

I remember real soda jerks and having that after school treat!
 

Ernst

New Member
May 28, 2011
363
0
0
Turlock Ca
Well yeah, no one is suggesting that an analog tach isn't a good thing.

What seems simpler is the spark pulse as a timing signal...

So how goes it?

My fav was root beer and vanilla ice cream. I actually remember a Soda Fountain and how my older Sister was all about being there. I got to have a treat while she did the older sister thing :)
.
 

greaser_monkey_87

New Member
Mar 30, 2014
397
0
0
USA
It is actually possible to calculate rpms in your head, if so inclined. Tachs are not terribly expensive, but doing math in your head is free. First, determine your top speed. I normally do this with a gps speedo for best accuracy. Then use a gear ratio calculator. There used to be one stickied around here, but the link appears broken. Anyway, put all your info in, for the china girls enter 20, 80, 10 and 44. Then enter different rpms until you find one that matches your top speed. For example, if your top speed is 35, enter something like 6000 and if the top speed doesn't come out to 35 at that rpm, adjust accordingly. Once you find the rpm that matches your top speed, divide the rpm by the top speed. For example, if you're hitting 7k at 35, divide 7k by 35. This will tell you how many rpms you are spinning per mph. Then you can use a speedo to determine rpms by multiplying any given speed by the number you got when dividing top speed by rpm. For example, my rpm per mph is 162.5. If I were to multiply that by say, 25mph, I would get 4062.5. So at 25mph, I'm spinning around 4062 rpm. Not everyone is good at doing math in their head, I just happen to be. If you can learn to do it, you can easily calculate rpm without a tach. I suppose it is easier if you have a tach though. But this is good for giving yourself an idea.
 

Davezilla

New Member
Mar 15, 2014
2,705
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San Antonio Texas
That's how I get my rpm's now by using a GPR reading and recording my max speed then using the formula above to get rpm. It would be much easier to have a gauge on the bike to look at while riding, even tho it's not really needed on a single gear bike other than to find out what your max rpm is, and you could reverse the formula to estimate speed.
One like this http://www.ebay.com/itm/NEW-CHROME-...Parts_Accessories&hash=item4a9c71753a&vxp=mtr with a small 12 volt battery would work on a single cylinder 2 stroke but since it only reads up to 8k rpm it wouldn't be practical for anyone who has an engine that's proted and tuned to run at higher rpm.
I put one of these on my Harley before but the vibrations from the engine made the gauge dial come off after only a few thousand miles.
Maybe later on I'll cary the torch on this project and get one of these tachs working for these engines... especially if I can find a cheap analog one that reads up to at least 10k or so.
 

Davezilla

New Member
Mar 15, 2014
2,705
10
0
San Antonio Texas
Ok... Now we're talking... just gotta find out if it can read a single fire twin or a wasted spark single cylinder 4 stroke, if it can it'll work. It'll still need a 12v power supply, but a super capacitor (about the same size as a C battery) will power something like this for hours off a single charge, and some of these will still read accurately off a 9 volt battery.
http://www.ebay.com/itm/Motorcycle-...Parts_Accessories&hash=item27d211fbba&vxp=mtr

I'll give this a go and post the results.
 

SuperDave

Member
Sep 24, 2011
179
0
16
Panama City Beach, Fl. USA
An 8 grand tach is sufficient for my needs, I built mine for torque, not speed. Long header on the expansion pipe & intake an extension keep my power band peak around 6500. Although my bike will easily exceed 30, its not something I like doing for long on a rigid frame & unsprung banana seat.

I have a Sun Tune mini tach, and a 12v power source, what I want to know is do I coil an inch of wire around the plug wire for inductance field pickup and tie that into the trigger wire, or tie it into the kill wire? Will I need a capacitor or resistors? I have a SuperRat with the 1 piece magneto cdi.