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A friend of mine is hopefully going to come up with a numerical LED display tachometer for my 38. Going to be a tiny display. If it works would anybody else be interested?

Have you tried one?

I believe that that tach would only give a true reading on a 2 stroke engine (one plug firing per rev.)

If it works off spark plug inductance, (which I think it does), then even a 4 stroke motor would work, as the a single plug only fires once per revolution. Similar to how a timing light works.

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Kev, The distributer turns half as fast as the crank. Your timing light illuminates the timing mark on every other rev of the crank.

Also the circuit for an analog tach is very simple and can be modified or calibrated for an engine with any number of cylinders. The problem I see with the digital one is that it probably needs the 12 volts to operate at all.

Of course you are right. I was thinking spark-plug firing is irrelevant on 4 stroke vs 2 stroke as it's one firing her entire cycle. But of course on a 4 stoke the crank is making 2 revolutions to ever 1 one of the distributor. If the Tach works of of inductance, then you could also split the sensor wire so that is attached to two separate plug wires. Lets say the first plug to fire and the 5th plug in the firing sequence (on an 8 cyl) to keep the pulses even. This would give you two even pulses per distributor revolution, or 1 pulse per crank revolution. Not sure if you would need to have a couple of diodes or something to prevent any feedback between the plugs.

The one wire tachs are available for 2 or 4 stroke. Contain their own battery that is something like a hearing aid battery or watch battery. I've heard they are short life tachs. I don't know. TinyTach is the brand name of one such tach. Cruise eBay to see others and for more details. Usualy cheap in the $25-$50 dollar range.

I've used 12v tachs on my 6v vehilces without any problems.