>Does anyone know how to convert a TR7 tach to work with a v8? I have read
>several books on Z-car/V-8 conversions, which state that a simple resistor
>(of the proper size) is inserted into the wire driving the tach, thus giving
>an accurate reading.
Sounds very unlikely. Fitting a resistor will merely reduce the voltage
and/or current to the tacho. Some Smiths tachos use a current loop through an
inductive pickup (e.g., like the one in my Hillman Hunter GT). Putting a
resistor in won't work.
What the tacho reads is frequency. In some cases it is the frequency of the
ignition pulses which means that a tach calibrated for a 4 cylinder engine
will read double the RPM with an 8-cyl engine.
Other tachos are connected to one of the three AC phases of the alternator.
As the engine speed changes, so does the rotational speed of the alternator
which, in turn, changes the frequency of the AC supplied by the alternator
(before rectification to DC by the diode pack). One advantage of this system
is that it is easy to fit a tacho to a diesel.
The problem with fitting an alternator-driven tacho is that the pulley sizes
between motor and alternator have to be right to calibrate the tacho.
Regards,
Ron Beckett (electronics technician for the past 35 years)
'86 Range Rover 4.6L
Webmaster & Database Manager Hillman Owners Club of Australia
Editor & Webmaster Land Rover Owners Club of Australia (Sydney Branch)
check my web site http//www.users.bigpond.com/hillman
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