Reading from the primary of the ignition coil into your 8051 coil is a scarry condition. If you use o-scope to look at the ignition coil primary you will see why!!. I suggest that you use a circuit with an opto coupler to isolate the coil from the 8051 circuit. The output of the opto coupler will still need to be filtered becasue there will most likely be many transition of the opto output as the ignition coil voltage rings. This filtering can be done in the 8051 software if you wish, in which case it would be inappropriate to use the an interrupt edge to capture the signal, by using a software sampling filter from a simple port pin. The signal can also be filtered at the output of the opto coupler with an RC circuit which in turn drives a schmidt trigger CMOS inverter. The output of the inverter can then drive your port pin or the INT/ pin of your processor. Once again do not under-estimate the value of an o-scope to show you what is going on in the circuit.

Note that reading the spark control signal in this manner will require a scaling to arrive at the actual engine RPM depending on the number of cylingers in the engine and the number of revolutions per spark cycle for each cylinder.
Good Luck
Mike Karas