Tach input from spark voltage (idea)

[Grant Beattie]

I've been working on various inductive tach schemes with some success.  I 
want to try a new approach which would be to take the input to the tach 
from the "points" or "tach" signal from a car distributor (or whatever).

In case my description is lacking, from the input or low voltage side of 
the coil (or cap or whatever).  What voltage range should I expect this 
signal to be in?  Are we talking tens of volts or hundreds?

I understand that the spark plug sees kV, but the input to the spark 
circuit should be quite a bit lower, right?

My application is data acquisition for cars/karts/motorcycles.

Thanks,
GB
http://ourwolrd.compuserve.com/homepages/grantb