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wiring in a tach?

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

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is it possible to wire in a tach via the coils, like on an automotive application? my tach cable isnt long enough to put the factory tach where i want it. thanks in advance~
 
No it is not because the firing rate of the coils is twice that of a auto 4 cylinder. You would read twice the engine speed, 4000 rpm when it is really going 2000 rpm. Unless your math is real good I would find a way to make the mechanical tach work.
 
yes, you can get a longer cable.

And yes, an electrinic tach can work. Most tachs even have a swich for 2 stroke or 4 stroke, which will compensante for the fireing rate.
 
There is one way to do this.

Open an electronic tach that works for 4,6 and 8 cylinder engines and notice there are three input wires. 8 cyl, 6 cyl, and 4 cyl. On the circuit board there is are going to be three small capacitors near where the input wires connect.

These three capacitors "tune" the tach. Read the value of the 8, and the 4 cylinder capacitors. You will notice they are multiples of each other, the 4 cylinder one is probably twice as large in value as the 8. Try replacing the 6 cylinder capacitor with that is around 4 times as large, since you have a twin. depending on the 8 and 4 cylinder caps.

Basically it is possible to tune a tach, by looking at how they tuned it for 8 and 4 cylinders. Experiment with different values, comparing the electronic to the mechanical until you get it right.
 
duaneage said:
No it is not because the firing rate of the coils is twice that of a auto 4 cylinder. You would read twice the engine speed, 4000 rpm when it is really going 2000 rpm. Unless your math is real good I would find a way to make the mechanical tach work.
Click to expand...
If it's an auto tach and it's reading 2x, try setting the tach to 8 cylinder mode if it has that option.

It's also faily easy to rig up a divide by 2 counter to halve the pulses if that's what needs to be done. I'm planning on using a 95 gixxer tach on my project and it's electronic.
 
Swanny said:
It's also faily easy to rig up a divide by 2 counter to halve the pulses if that's what needs to be done. I'm planning on using a 95 gixxer tach on my project and it's electronic.
Click to expand...

Any schematics for this please Swanny!!, It'll save me the trouble of doing it myself for my GS550 with the boyer ignition :D
 
Big N Daft said:
Swanny said:
It's also faily easy to rig up a divide by 2 counter to halve the pulses if that's what needs to be done. I'm planning on using a 95 gixxer tach on my project and it's electronic.
Click to expand...

Any schematics for this please Swanny!!, It'll save me the trouble of doing it myself for my GS550 with the boyer ignition :D
Click to expand...
If you need a schematic I can get you one, but it's pretty simple actually.

One method is to use a D-Flip Flop. Wire the Qbar (Q not) output to the D input. Your input signal is wired to CLK. Your output is Q. Wire both Set and Reset to ground. Power is from battery and ground is to ground.

I'd put a small cap across the power and ground terminals to filter out noise. You can mount this just about anywhere, but the closer to your tack the better. If there is space inside the case, better yet.

You should be able to get a D Flip Flop from any electronics store. They usually come in dual packages.
 
Sounds simple enough, I'll give it a try. I did have doubts about something like this setup as I wasn't sure what kind of output I'd be able to generate from an I/C and whether it could drive the tach, also I was unsure of exactly how the tach operated as it just seemed to be a current measuring device and not a pulse counter. However, your example would only cost pennies so I'll give it a go!
 
Big N Daft said:
Sounds simple enough, I'll give it a try. I did have doubts about something like this setup as I wasn't sure what kind of output I'd be able to generate from an I/C and whether it could drive the tach, also I was unsure of exactly how the tach operated as it just seemed to be a current measuring device and not a pulse counter. However, your example would only cost pennies so I'll give it a go!
Click to expand...
I have a 95 GSXR tach that I jsut pulled apart to have a look inside. I believe you are correct in that it doesn't actually count the pulses. If it did, there would be more circuitry there. I'm going to try to find the chip schematic or data sheet somewhere to get an idea of what they are doing.

I can't guarantee that my example circuit will do what you want, unless you just want to gt f/2. The output will be a frequency with a 50% duty cycle. The tach may not read right.

On the other hand, the input to the tach may not be duty cycle sensitive. If the chip is just using the edges and then generating a pulse of known width for current averaging purposes, then it may work.

I'll add a bit more here. Giving this some more thought, the chip may be just looking at edges. Consider that the pulse width AND frequency AND amplitude of the input is changing. The frequency is changing because the RPM's are changing. The pulse width is changing because the pickup is near the magnet a shorter period of time at higher RPM's. The amplitude is changing due to battery charge.

If the chip is only sensing edges, it can then focus only on them and ignore the amplitude (regulate it) and pulse width variations.

The way this circuit is built I'm not concerned about driving the tach input. It's probably a very high impedance and draws little current. The Flip Flop will have a 12v (battery) output level with a more than capable current sourcing.

If you decide to try this out, let me know if it works. If it doesn't I'll try to follow up with something that will.
 
Tach

Tach

With an electric tachometer, lets say a small automotive one from canadian tire made to mount up on the pillar of a car..and its electric..if it were in a car how would it read the rpm of the engine..by the spark out of one of the cylinders? if i connected to one of the coils on my bike why woudnt it be accurate? does it matter if the car and bike have different lengths of strokes in the piston, i would think not because the longer the stroke the slower it will run thats all and the tach would show that...thats why our bikes can rev so high because of there short strokes right ?
 
Because that coil only deals with two of the cylinders, so it would get half the pulse count that a car tach would when reading from a coil handling all four cylinders. However, if you set it for an 8 cylinder motor it should read fine (unless I'm just having a brainfart).
 
You should be able to make an automotive tach work. They get the signal from the coil just like your bike. The big disadvantage that I see is weatherproofing it.
 
Swanny,

It'll be a few weeks before I can get round to doing this, I'm currently laid up with a torn knee cartilage thats just doing wonders for the osteoarthritis in that joint. Got loads of work lined up to do on 4 of the bikes and loads of time to do it in, I just can't physically do it!
 
One tip, give the tach its own ground wire. I wired in a set of analog GSXR guages and found the tach needed a dedicated ground to work right.
 
Wait... if it's getting half the pulse count, wouldn't you need the tach set for half the cylinders, not double? If so, an automotive tach wouldn't work.
 
fastpakr said:
Wait... if it's getting half the pulse count, wouldn't you need the tach set for half the cylinders, not double? If so, an automotive tach wouldn't work.
Click to expand...
The GS's give 2x the pulse counts that a 4 cylinder car generates. If you used a tach set for an 8 cylinder car, then it should give the correct RPM reading.
 
If im not mistaken dont four cylinder engines fire 2 cylinders at a time? same as a four cylinder on a gs motorcycle right?

So if this is true wouldnt the tach get the same reading in a car or a bike.

And with this topic comes a question..Does the size of the stroke influence the rpm reading or it doesnt matter, cuz not all engines have the same crank and stroke size so is a tach specially made for each engine or it doesnt matter as long as it knows when the spark is?
 
Swanny said:
fastpakr said:
Wait... if it's getting half the pulse count, wouldn't you need the tach set for half the cylinders, not double? If so, an automotive tach wouldn't work.
Click to expand...
The GS's give 2x the pulse counts that a 4 cylinder car generates. If you used a tach set for an 8 cylinder car, then it should give the correct RPM reading.
Click to expand...
8 cylinder = 4 pulses /rpm
6 cylinder = 3 pulses /rpm
4 cylinder = 2 pulses /rpm
2 cylinder = 1 pulse/rpm

Look at the crankshafts of these engines and it becomes apparent.
 
If I may offer a suggestion. While everyone seems to have a grasp on the number of pulses per revolution, you only need to monitor One coil to get the tach to work properly.

So, if you hook the tach to the 1-4 or 2-3 coil and set the tach to 4 cylinders, it should read correctly.

I will hook my dwell/tach tester (Yes, I am an old fart and still have one) to my 1100 this afternoon to verify.

I will let you know how it turns out.
 
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