Tach for 5 cyl

[raf40@oes.amdahl.com (Richard Funnell)]

Racers,

There has been a lot of discussion lately about finding a racing tach that
will work with 5 cyl.  It seems to me that a fairly simple electronic
circuit could be built to count ignition pulses being input to a 4 cyl
tach, and squelch every 5th one.  The input from the 5 would then occur at
the same frequency for a given RPM as the input from a 4.  This would
produce some jitter into the tach, but my guess is that the tach wouldn't
mind and the driver wouldn't see it.  The circuit could live in the
passenger compartment so mil spec components wouldn't be necessary.

The circuit would basically convert the ignition pulses to logic level and
feed a binary counter.  The counter output could be used in various ways to
squelch the 5th pulse.  The safest would likely be to feed the tach through
a resistor to allow the input to be shorted, blocking the 5th pulse.  If
the circuit failed, a switch could disconnect it and at least the tach
would continue to work but would read high.  Cautious people could even
have a second circuit to be switched in if the first one failed.

Has anyone tried this?

Regards,

Richard Funnell, OES
raf40@oes.amdahl.com
tel. (408) 992-2265
fax. (408) 746-7633