WolfByu
Member
I know you can get the digital tachs for these engines but I cant find anything on a analog tach for a HT... has anyone tried this or know where a person could buy one?
Analog Tach for HT?
- Thread starter WolfByu
- Start date
Wheres my dog
New Member
In years of MB'iing now myself, I have never heard of or even seen one ever, analog style that is
I would think analog would have a mechanical fitting at the end that mates with the engine, such a specialized device may not exist for an HT
I like the thought of it for a vintage look on an MB
I would think analog would have a mechanical fitting at the end that mates with the engine, such a specialized device may not exist for an HT
I like the thought of it for a vintage look on an MB
WolfByu
Member
OK what about a electric analog tach? I mean one where the the gauge still has a needle dial but you have wires that hook it up. One ground, one 12v, and a switched 12v for light
Wheres my dog
New Member
Can't say I've ever seen one of those either...
My next question is how much would that thing cost???
My next question is how much would that thing cost???
WolfByu
Member
Wheres my dog
New Member
I kinda knew that already, but there is most likely no way to modify that without knowing electronics and circuitry
WolfByu
Member
yea your right about that. I wonder about a scooter tach? I will do some checking..
loquin
Well-Known Member
Typically, with an aftermarket analog tach, the input comes from a wire coil wrapped so that a spark plug wire goes through the coil. The coil picks up enough voltage to to be used as the signal input.
That input pulse is fed through a shaper circuit, to limit the voltage to safe levels and polarity. Then, the shaped pulse is used to trigger a fixed width pulse. (The width of the pulse is the critical thing you would need to change.)
After this, the fixed width pulse is fed through a signal diode, and used to charge a Capacitor/Resistor filter circuit, which averages the voltage, so that the voltage across the capacitor is what is displayed on the analog volt meter.
Suppose you have a 10 volt analog meter. (and 12 volts available on your vehicle.)
The 12 volts would probably be regulated to 10 volts for the tach. If you want a tach to display up to 10000 RPM, that means that you need to supply 1 volt for each 1000 RPM.
A HT engine fires on each revolution, so 10000 revolutions in one minute is the same as 166.667 revolutions in one second. If you divide one second by 166.667 revolutions, the result (.006 seconds, or 6 milliseconds) is the time that each pulse needs to be, for a 10K RPM to equal 10 Volts.
If the motor is running at less than 10000 rpm (let's assume 6000, for giggles,) you would have 6000 RPM/60 Secs/Min pulses, or 100 pulses every second. Since each pulse is .006 seconds long, the total time in one second that there is a pulse is 100 times .006 seconds, or 0.600 seconds total, and that is 60 percent of the possible time. Since the 10 volt pulses are fed through an averaging circuit (diode/capacitor/resistor) you would have 10 volts * 60 percent, or 6 volts across the capacitor, and that is the scaled voltage that would cause your 10V meter to show 6. If you've changed the meter face to be labeled 'RPM x 1000' rather than 'Volts' you have a working, scaled tach.
In the after-market tach, for a V8, you would have 8 pulses every two rotations, or 4 per rotation, rather than 1. If you had the same 10V meter we talked about earlier, the pulse width you would need is 1/4th of what was needed for 1 pulse/Rev, so the pulse width would be 0.0015 seconds (1.5 mSec)
To modify the tach, you would need to get someone with an oscilloscope to locate the pulses on its circuit board and verify pulse width. And, you'll need to locate the Pulse circuit (it's a 1-shot, also known as a monostable multivibrator circuit) and adjust the R and C values for that circuit so as to make the pulse 4 times longer. (3 times longer for a 6 cylinder tach/2 times longer for a 4-cylinder tach.)
I've thrown together a block diagram of an analog tach, below, to help explain what's going on inside.
That input pulse is fed through a shaper circuit, to limit the voltage to safe levels and polarity. Then, the shaped pulse is used to trigger a fixed width pulse. (The width of the pulse is the critical thing you would need to change.)
After this, the fixed width pulse is fed through a signal diode, and used to charge a Capacitor/Resistor filter circuit, which averages the voltage, so that the voltage across the capacitor is what is displayed on the analog volt meter.
Suppose you have a 10 volt analog meter. (and 12 volts available on your vehicle.)
The 12 volts would probably be regulated to 10 volts for the tach. If you want a tach to display up to 10000 RPM, that means that you need to supply 1 volt for each 1000 RPM.
A HT engine fires on each revolution, so 10000 revolutions in one minute is the same as 166.667 revolutions in one second. If you divide one second by 166.667 revolutions, the result (.006 seconds, or 6 milliseconds) is the time that each pulse needs to be, for a 10K RPM to equal 10 Volts.
If the motor is running at less than 10000 rpm (let's assume 6000, for giggles,) you would have 6000 RPM/60 Secs/Min pulses, or 100 pulses every second. Since each pulse is .006 seconds long, the total time in one second that there is a pulse is 100 times .006 seconds, or 0.600 seconds total, and that is 60 percent of the possible time. Since the 10 volt pulses are fed through an averaging circuit (diode/capacitor/resistor) you would have 10 volts * 60 percent, or 6 volts across the capacitor, and that is the scaled voltage that would cause your 10V meter to show 6. If you've changed the meter face to be labeled 'RPM x 1000' rather than 'Volts' you have a working, scaled tach.
In the after-market tach, for a V8, you would have 8 pulses every two rotations, or 4 per rotation, rather than 1. If you had the same 10V meter we talked about earlier, the pulse width you would need is 1/4th of what was needed for 1 pulse/Rev, so the pulse width would be 0.0015 seconds (1.5 mSec)
To modify the tach, you would need to get someone with an oscilloscope to locate the pulses on its circuit board and verify pulse width. And, you'll need to locate the Pulse circuit (it's a 1-shot, also known as a monostable multivibrator circuit) and adjust the R and C values for that circuit so as to make the pulse 4 times longer. (3 times longer for a 6 cylinder tach/2 times longer for a 4-cylinder tach.)
I've thrown together a block diagram of an analog tach, below, to help explain what's going on inside.
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WolfByu
Member
wow you really do know your stuff! Maybe you should start making and selling them? Seems like a untapped market lol.
