Same type of project I am working on but I am using an 89C52.
I use a comparator for my zero crossing.
Signal feeds into a full wave recifier I made of small signal diodes, output from this feeds a 4.7v zener diode which is the input to my comparator. I get a good signal from this.

Idea is the reluctor produces a postive and a negative pulse, the full wave rectifier turns it into two positive pulses that I can measure time with (start pulse and stop pulse).

Not sure how practical my circuit is but it makes sense to me.

I believe Mr. Bull described how the process worked.
http://www.8052.com/forum/read.phtml?id=89695


One thing I did figure out is some hysteresis needs to be introduced to the circuit to counteract noise which will produce false triggers.

So far I am not sure if it works as the starter in my bike is failing but I know I am getting closer,
persistance is the key and lots of querying databases ie google.

Regards

James Krushlucki