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Serious Thumper
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Savage heart of darkness
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Cartersville, GA
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Actually it's not quite that simple. One cylinder engines CAN fire every revolution; once on the compression stroke and again on the exhaust stroke, as has been pointed out. The exhaust stroke spark is wasted because the engine is already exhausting burning fuel. The only downside is the spark plug wears out twice as fast. However, one spark per rev is not always the case because the timing can also be derived from the camshaft which operates at half crankshaft speed. This would fire the plug every other rev and a tach designed for it would indicate twice the RPM on a Savage. Therefore, since a single cylinder engine only has one spark plug you only need one coil, but that coil may fire once per rev, or once every other rev.
With 360 degree side-by-side twins (both pistons are at TDC simultaneously, but fire alternately) you can also use a single coil and fire both spark plugs every rev. Again, the cylinder that is exhausting won't matter. With 180 degree side-by-side twins it gets a little more complicated, and some engines will actually fire twice per rev to allow single coil/two cylinder operation.
Further, single coil is also permissible for BMW & URAL boxer engines (opposed cylinders) because, like a side-by-side 360 twin, both pistons are at TDC simultaneously (one on compression, one on exhaust).
V-twins, however, are a little more complicated because the 2 cylinders are NOT crankshaft symmetrical. By design, the ignition timing for each cylinder must be separate, and will, in fact, will be offset by the exact crankshaft angle (delta) between the cylinders. This presents a problem; i.e., either you must use a separate coil and timing trigger for each cylinder (called single fire ignition), or you can use a single coil fired at different crankshaft degrees and distribute the spark to the appropriate individual plug. In the latter case, the the coil dwell time will be different, but with only 2 cylinders, more than sufficient ( a relatively long time between firing). All this can be even more problematic in multi cylinder engines which can use a number of variations, ranging from one coil per cylinder (single fire) to one coil for all.
Anyway, the problem for "universal" tachometers is to be able to take in these variations and produce an accurate reading. This is reasonably easy to do at the design stage, but harder as an afterthought when trying to adapt a tach to a situation is was not intended for. The tach internal design is usually done with "divide by" counters, the count of which is generally switch operated and selected by the consumer.
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