"You can also "sanity-check" your data, knowing that the engine can't
instantly change its speed by a large amount."

Maybe this engine cannot, but others can. =20

Audi's actual 3.0 TFSI engine takes 2.5 seconds to speed up from 750 RPM to
6500 rpm under load.=20
On engine dynamometer, it takes less than 0.5 second for this speed change
(less than 0.1 second for a delta of 1000 rpm).

But measuring coil signals, is only an estimation anyway.
Even with older engines without full electronic coil system, the ignition
point is not fixed to crank position. It is dynamic, controlled by speed of
the crank (centrifugal force) and engine load (vacuum actuator). With to
influences, the ignition point may change much faster as the speed of the
crank itself. The ignition point may vary from +6=B0 up to +45=B0 relative =
to
crank (Golf GTI from early 80's). And with older engines, speed limiter is
done with ignition breaks (no ignition pulses, so this measurement would
show 0 rpm at real 6000+ rpm ;-) and even when speeding down, some engines
stop ignition pulses (older Golf/Rabbit engines have this feature to save
fuel). Newer engines may stop complete cylinders from working (fuel saving)=
,
the first BMW V12 in the late 80's (still without computer controlled
ignition) was able to stop a complete bank of 6 cylinders from working. Not
to forget multipoint injection and ignition systems ...

I think the best way would be (as all modern engines are doing now) to have
a separate hall sensor on the crank itself. Nearby all engines have a
visible hole in the clutch housing for adjusting ignition points during
maintenance work. I have seen some years ago solutions with plug in hall
sensors, using the marking on the clutch pressure plate to add electronic
ignition and injection system to older engines.=20

This would work even with engines without ignition system (Diesel) or multi
coil ignition system (like the first 2 generations from BMW M5 engine, one
coil per spark direct build-in in spark-holder).

Just my 2 cents

Jens


--=20
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