wvmarle:
It's probably your 555 circuit that's limiting you. 100 nF and 18kΩ means a pulse length of 2 ms. I suppose you need at least that much time before you can have the next pulse (so the cap can discharge sufficiently - I don't know enough about the 555 monostable but you definitely must allow the cap to discharge enough before it can be triggered again). 2 ms pulse + 2 ms idle = 250 pulses per second or 15,000 per minute.
Actually, the 555 circuit requires virtually no idle time. As long as the input pulses are > 2 ms apart, it detects them and outputs the right thing. For less than 2 ms pulses, you get a solid DC signal since the square waves are smashed together.
I'm going to add in an optocoupler. I haven't dealt with this before, so any recommendations on a specific chip to use?
I'm guessing putting the optocoupler on the Arduino shield with the rest of my circuitry would be pointless since noise would get in, even tho the spark signal is separated from everything else by the optocoupler.
If the optocoupler is 6-12 inches away from Arduino and circuitry, do I need to worry about shielding the spark signal and grounding everything else? I don't have a grasp on how noise gets around. How far from the spark signal cable would it carry?
Thanks for all the help.