I know this should work- I should be able to wrap uninsulated wire around a spark plug wire and detect the pulses in an engine.
However, I don't know what the "output" of said wire would be- I assume it would be a very small DC voltage spike? Does anyone know if it would be at levels an Arduino could read, or can give me pointers?
I'm sure this has been done before (maybe not with Arduino), but my googling just shows lots of automotive pages, not information on building a circuit to do this.
It's more for temporary use, so the Ardiuno can be moved from one engine to the next without re-engineering for each engine. (in this application, spark plug wires are much more accessible than the coils)
Though I suppose I could use the signal off the coils..
Also check TinyTach ( http://www.tinytach.com/tinytach/index.php ) for prebuilt tachometers that work the same way. I know several folks who use these on scooters (1 cylinder, 4 stroke, lost spark) but they have other cylinder/stroke arrangements available.