I was trying to build a digital tachometer readout and took a signal off the low tension line of the distributor through a 24k resistor to a digital input with an 11k pull down (odd values because they were what I had to hand):
Low tension from distributor >---[24k]---o---> input 2
|
11K
|
0v
I used the interrupt to count the pulses and then divided them by 2 (2 pulses per revolution).
I only tried it once on an old 4 cylinder VW engine and didn't blow anything up, I'm sure there are better ways to protect the input but it worked for me!
Good luck.