Ron,
Thanks for responding. I understand what you are saying. Is it that most
"modern" tachometers use a different type of circuitry that measures the
pulses from (or to) the ignition coil, and then converts it to a signal
usable by the tach INSIDE the tach itself? How is it that you can convert a
280 Z tach (a 6 cylinder) to work with a v-8, by simply adding a resistor?
And how is it that one can purchase a cheap parts house tach, that can be
used on a 4, 6, or 8 cylinder by simply the movement of a switch on the
tach?
Inquiring minds want to know!
I could just use a "speed shop" tach, but I'd rather keep mine in the dash.
I have also had more than one shop offer to send my TR7 tach out to be
"modified" to work with the 8, for about $75.00
Let me Know! :)
Perry
> -----Original Message-----
> From: The Becketts [SMTP:hillman@bigpond.com]
> Sent: Friday, March 26, 1999 2:51 AM
> To: Perry Robinson; buick-rover-v8@autox.team.net
> Subject: Re: TR7 to V-8 Tach modification
>
>
>
>
> >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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